Ignition ECU operation, HEI

[Bitsyncmaster]

One of my customers sent me a photo of his GM HEI 4 pin module he plans to use in his DeLorean. That got me thinking so I took some measurements of the stock ignition voltage on the negative side of the ignition coil. Correct me if I say something wrong to my thinking.

FYI:
You drive current into the ignition coil to saturate the core (iron) magnetically. So more current or keeping the current on for longer time will only waste power and increase heating of the coil. The coil would take a fixed time of current to saturate it (lets say it takes 2.0 ms to saturate). So it would be best to have that timing at all engine RPM. So voltage on that negative coil side should be zero volts for only 2 ms and +12 volts for the rest of the time.

When current flow is stopped, the magnetic field collapses and that creates the high voltage spark.

So I measured the dwell on that negative coil signal. With the meter set on 4 cylinder dwell at 850 RPM I read 62 deg and at 2500 RPM I read 60 deg. Checking that signal on my oscilloscope, the power on time (zero volts) was 11 ms and 21 ms (you get two timings because of uneven fire PRV) at 850 RPM. At 2500 RPM I got 4 ms and 7 ms.

So it looks like the stock ignition ECU has no dwell change to speak off. The GM HEI claims to have a dwell change which would keep the ignition coil running cooler and therefor not require dropping resistors if the correct coil is chosen. So I'm going to do some playing with the GM HEI 4 pin module. I know Bill uses the Ford Duraspark unit with success but the Ford unit holds the dwell constant.

[content22207_2]

That's an Internet rumor that is untrue. I accepted it myself until I hooked a dwell meter up to my car. Duraspark dwell is unique in that it *DECREASES* as RPM's go up. Near as I can figure it's part & parcel with large diameter Duraspark distributor caps and wide rotor blades -- increases the length of spark duration.

I measured a stock Bosch module at 54 degrees while cranking, 44 degrees at idle, and 46 degrees at 3,000 RPM.

Duraspark is 56 degrees while cranking, 50 degrees at idle, 44 degrees at 3,000 RPM.

6 cylinder Duraspark cap and rotor versus DeLorean (that Duraspark needs a larger cap to avoid cross firing is another Internet rumor that's untrue -- cap is bigger because the rotor blade is wider):

41946 41947

Note the absence of a resistor in the Duraspark rotor button -- to the best of my knowledge no American car has ever had a resistor in its rotor.

Bill Robertson
#5939

[Bitsyncmaster]

That's an Internet rumor that is untrue. I accepted it myself until I hooked a dwell meter up to my car. Duraspark dwell is unique in that it *DECREASES* as RPM's go up. Near as I can figure it's part & parcel with large diameter Duraspark distributor caps and wide rotor blades -- increases the length of spark duration.
#5939

The dwell reading is the primary current nothing to do with the high voltage currents.

[content22207_2]

The only dwell that doesn't change is breaker points. That's one advantage of electronic ignition: the ability to change dwell as RPM's change.

Bill Robertson
#5939

[content22207_2]

The dwell reading is the primary current nothing to do with the high voltage currents.

High voltage depends upon dwell. If the coil can't recharge fast enough, secondary voltage will drop. That's one of the limiting factors high rev'ing race engines deal with.

Bill Robertson
#5939

[Bitsyncmaster]

The only dwell that doesn't change is breaker points. That's one advantage of electronic ignition: the ability to change dwell as RPM's change.

Bill Robertson
#5939

True. I would not consider a few degree change to be very valuable info. The meter accuracy with the shorter timing of higher RPM probably suffers. Our PRV is even harder on dwell meters accuracy with it's uneven fire engine.

[content22207_2]

I suspect an analog dwell meter can handle the uneven firing order better than a digital meter -- needle just can't respond fast enough. Kind of like averaging.

Bill Robertson
#5939

[Bitsyncmaster]

I suspect an analog dwell meter can handle the uneven firing order better than a digital meter -- needle just can't respond fast enough. Kind of like averaging.

Bill Robertson
#5939

Your probably correct but my Snap On digital dwell meter holds pretty steady values.

[content22207_2]

See if this helps: http://www.bgsoflex.com/igncoil.html

Bill Robertson
#5939

[content22207_2]

This morning I brought my school bus back to the house to get it ready for a field trip tomorrow. It's an old gas burner that runs as well as a 2016 Honda, partly due to its HEI (factory original Duraspark module -- I just swapped out the coil and wired it to charging voltage):

41954

(Autolite carburetor, circa 1960's, in the background also helps -- Holley monstrosity the bus came with didn't work worth 2 cents).

Bill Robertson
#5939

[Bitsyncmaster]

Here is my home brew unit ready to test but I am waiting till after Saturday because I'm driving to a tech session. This is the cheapest GM module I could find on Ebay so I will verify it does the dwell change. Some info on the internet says some of the cheap modules don't do the dwell change and are not good.

I also ordered a new timing light. My old Sears light died. I wanted to check the timing does not change.

[content22207_2]

Why not buy a module from Autozone? It should match OEM specs, and it comes with lifetime free replacement warranty if you accidentally fry the thing: http://www.autozone.com/ignition-tune-up-and-routine-maintenance/ignition-control-module/duralast-ignition-control-module/chevrolet/c10-1-2-ton-p-u-2wd/1980/8-cylinders-l-5-7l-4bl-ohv/115808_728936_0/

Bill Robertson
#5939

[Bitsyncmaster]

Why not buy a module from Autozone? It should match OEM specs, and it comes with lifetime free replacement warranty if you accidentally fry the thing: http://www.autozone.com/ignition-tune-up-and-routine-maintenance/ignition-control-module/duralast-ignition-control-module/chevrolet/c10-1-2-ton-p-u-2wd/1980/8-cylinders-l-5-7l-4bl-ohv/115808_728936_0/

Bill Robertson
#5939

That's an option. They are all priced between $10 and $50. The higher priced units have a name brand on them.

[content22207_2]

"Duralast" is Autozone's house brand. They stick it on everything: brake pads, starter motors, radiator hoses, etc.

Slightly off topic, but I've been surprised at the number of things Autozone sells that are marked Made in USA (assuming the label is correct).

Bill Robertson
#5939

[dustybarn]

I use a GM 4-pin in my car. They are sensitive to coil primary impedance, which can cause truly bizarre RPM-related dropouts if the wrong coil is used. IIRC the GM module needs around a .7 ohm or less primary (I'm using an Accel 8145).



One of my customers sent me a photo of his GM HEI 4 pin module he plans to use in his DeLorean. That got me thinking so I took some measurements of the stock ignition voltage on the negative side of the ignition coil. Correct me if I say something wrong to my thinking.

FYI:
You drive current into the ignition coil to saturate the core (iron) magnetically. So more current or keeping the current on for longer time will only waste power and increase heating of the coil. The coil would take a fixed time of current to saturate it (lets say it takes 2.0 ms to saturate). So it would be best to have that timing at all engine RPM. So voltage on that negative coil side should be zero volts for only 2 ms and +12 volts for the rest of the time.

When current flow is stopped, the magnetic field collapses and that creates the high voltage spark.

So I measured the dwell on that negative coil signal. With the meter set on 4 cylinder dwell at 850 RPM I read 62 deg and at 2500 RPM I read 60 deg. Checking that signal on my oscilloscope, the power on time (zero volts) was 11 ms and 21 ms (you get two timings because of uneven fire PRV) at 850 RPM. At 2500 RPM I got 4 ms and 7 ms.

So it looks like the stock ignition ECU has no dwell change to speak off. The GM HEI claims to have a dwell change which would keep the ignition coil running cooler and therefor not require dropping resistors if the correct coil is chosen. So I'm going to do some playing with the GM HEI 4 pin module. I know Bill uses the Ford Duraspark unit with success but the Ford unit holds the dwell constant.

[content22207_2]

Guess that's one advantage of a Duraspark ECU -- they don't care what coil they're attached to. I'm running Pertronix 1.5 ohm. I'm pretty sure Steve Rice runs MSD (don't know the resistance). Byrne Heninger and Jim in Louisiana are running stock Bosch coils (with ballast resistors). Chad Krause is running Duraspark as well, but I don't know what coil.

Bill Robertson
#5939

[Bitsyncmaster]

I use a GM 4-pin in my car. They are sensitive to coil primary impedance, which can cause truly bizarre RPM-related dropouts if the wrong coil is used. IIRC the GM module needs around a .7 ohm or less primary (I'm using an Accel 8145).

Thanks for that info. I think the stock coil is one ohm but I can check that later. Did you bypass both resistors? The GM module says to run it without any resistors.

[DMCMW Dave]

"Duralast" is Autozone's house brand. They stick it on everything: brake pads, starter motors, radiator hoses, etc.

Slightly off topic, but I've been surprised at the number of things Autozone sells that are marked Made in USA (assuming the label is correct).

Bill Robertson
#5939

Sometimes "made" = "inserted into this package"

[content22207_2]

So I measured the dwell on that negative coil signal. With the meter set on 4 cylinder dwell at 850 RPM I read 62 deg and at 2500 RPM I read 60 deg....

So it looks like the stock ignition ECU has no dwell change to speak off....

On a 6 cylinder scale, my analog meter read Bosch ignition ECU at 44 degrees at idle, which roughly corresponds with your digital reading (67% of 62 = 41.5).

However, my meter read slightly *INCREASED* dwell at 3,000 RPM, not decreased (46 degrees). Still 2 degrees total change, just in the other direction.

That said, I do agree the Bosch module has no significant dwell change to speak of. In that respect it offers no real advantage over breaker points.

If I send you an Autozone Duraspark ECU, would you be willing to test it as well? On a 6 cylinder scale, my meter read 50 degrees at idle, dropping to 44 degrees at 3,000 RPM.

You and I both read 3-4% dwell change on the Bosch ECU (albeit in different directions). I read 13% dwell change on an aftermarket Duraspark ECU -- it would be nice to double check that.

Bill Robertson
#5939

[content22207_2]

Just measured my Autozone ECU again, and a factory original Motorcraft ECU:

Autozone: 49 degrees @ 750 RPM, 43 degrees @ 3,000 RPM
Motorcraft: 46 degrees @ 750 RPM, 40 degrees @ 3,000 RPM

I am using the dash tach for my RPM readings (my meter only goes to 2,500 RPM anyway).

Autozone's aftermarket ECU appears to be a reasonable replacement for Ford original.

Bill Robertson
#5939

[content22207_2]

Just remeasured my Bosch ECU: 43 degrees at 750 RPM, 45 degrees @ 3,000 RPM.

One thing I've noticed is the needle jumps very unsteadily with the Bosch ECU as I'm opening the throttle plates (turning the curb idle screw). You have to wait a second or two for it to stop quivering. Don't know if all Bosch ECU's do that, or if #5939's is sick.

Bill Robertson
#5939

[content22207_2]

Just tested the Bosch module from #2508. Got the same readings: 44 degrees at 750 RPM, 46 degrees at 3,000 RPM.

Needle jumps around with #2508's module too. Interestingly it's basically 42-43 degrees while turning the curb idle screw (quivers noticeably between them), then settles down at 46 when you stop turning the screw. Both Duraspark modules gave smooth consistently dropping readings while rev'ing up the engine.

Bill Robertson
#5939

[content22207_2]

This is the ultra precision meter I am using:

41978

Dave's digital meter is probably a bazillion times more accurate.

Bill Robertson
#5939

[Bitsyncmaster]

On a 6 cylinder scale, my analog meter read Bosch ignition ECU at 44 degrees at idle, which roughly corresponds with your digital reading (67% of 62 = 41.5).

However, my meter read slightly *INCREASED* dwell at 3,000 RPM, not decreased (46 degrees). Still 2 degrees total change, just in the other direction.

That said, I do agree the Bosch module has no significant dwell change to speak of. In that respect it offers no real advantage over breaker points.

If I send you an Autozone Duraspark ECU, would you be willing to test it as well? On a 6 cylinder scale, my meter read 50 degrees at idle, dropping to 44 degrees at 3,000 RPM.

You and I both read 3-4% dwell change on the Bosch ECU (albeit in different directions). I read 13% dwell change on an aftermarket Duraspark ECU -- it would be nice to double check that.

Bill Robertson
#5939

Of course send me a loaner Duraspark to test. I plan to do a lot of component testing with this project. My timing light came in today. I drove the D today for about 6 hours so to tired to do anything tonight.

I also ordered one of those 40,000 volt 0.70 ohm coil (old oil can type). People say the "square" core type coil are more efficient which I would believe is true. All the transformers use a closed core around the coils.

[content22207_2]

PM me a mailing address.

This is the schematic:

41989

Will you be cutting the harness connectors off and soldering the wires to your Bosch tray? Note that Duraspark accepts a starting signal, but several owners are running their modules just fine without it. I measured dwell at 56 degrees while cranking with that signal attached, 58 degrees with it disconnected.

I measured the Bosch module at 54 degrees while cranking, dropping to 44 degrees after the engine starts.

Also, could you double check that your Bosch module decreases dwell as RPM's increase. I checked two modules today, and both of them increased dwell. Dwell dropped (unsteadily) while I was opening the throttle plates, but after the engine reached 3,000 RPM it stabilized higher.

Bill Robertson
#5939

[content22207_2]

If you need to replicate my ignition: I am running full charging voltage into a 1.5 ohm coil (Pertronix). Plug gap is either .044" or .046" (can't remember which). Spark plugs are standard single electrode. I've still got the resistor in the rotor button.

Bill Robertson
#5939

[Bitsyncmaster]

Dwell is really the duty cycle of the power consumption but calibrated in degrees. So values you read will be double set to a 4 cylinder scale rather than using 8 cylinder. Using a 6 cylinder scale is between the two. Not sure if that would affect real accuracy of the readings on a digital meter but most likely would on older analog meters.

What we are really looking for is a constant power on time. But how would the unit know when to turn the power on since turning the power off fires the plug. So a constant power on time would read a very low dwell at low RPMs and dwell numbers would increase as RPM increase.

I don't think plug gap would affect the primary current draw on the coil but maybe it does. My only consern using a wider gap is getting cross firing with our stock distributor cap. I would probably go with 0.030 to 0.035 when I am using the new GM module.

[content22207_2]

That is why I was initially surprised to see Duraspark dwell decrease as RPM's go up. But it clearly does, very dramatically -- 13% *SHORTER* recharge time. But in context of the bigger Duraspark cap and wider Duraspark rotor blade that makes perfect sense: Ford obviously wanted to maximize spark exposure. At higher RPM's it is harder to ignite the mixture, so coil discharge time lengthens (dwell decreases) as RPM's go up. Since Duraspark dwell is so long at low RPM's there shouldn't be potential discharge throughout the entire Duraspark blade width at lower RPM's -- only higher RPM's.

Obviously a Duraspark ECU is not designed for our smaller diameter cap and narrower blade rotor. But since it ultimately drops dwell basically to Bosch ECU's dwell (43/44 degrees versus 45/46) that doesn't matter -- both basically perform the same at higher RPM's. Only difference is lower RPM. At lower RPM's the Duraspark ECU definitely leaves the coil saturated longer. I know two owners running stock Bosch coils with Duraspark ECU's, so longer saturation at lower RPM's appears to have no effect on their health. I've been running a Pertronix coil with Duraspark for tens of thousands of miles -- longer low RPM saturation definitely has no effect on it. I suppose if your coil is sketchy or dying (having trouble recharging) Duraspark's longer low RPM dwell could actually be a benefit.

Irrespective, Internet rumors that Duraspark is fixed dwell are very much false. I believed them myself until putting a meter on my own car.

Bill Robertson
#5939

[Bitsyncmaster]

Here is my stock ignition ECU data (base line).

I measured the current on the white/slate wire right near the ECU (unwrapped my harness) with my Fluke DVM and the Fluke clamp on current probe set at 20 amps full scale.

0 RPM = 7.1 amps. Current will slowly drop as the coil heats up.
850 RPM = 5.2 amps.
2500 RPM = 4.1 amps.

Measured ignition timing with my new Actron CP7527 timing light which works very good (nice cheap timing light). I get a solid 10 deg. BTC at 850 RPM. Not sure if my scale is calibrate since I removed it to clean and paint. Next time my intake is off I will need to do that calibration.

[Bitsyncmaster]

Replaced the stock ignition ECU with my GM ignition module all else stock (resistors and coil stock).

At zero RPM there is no current draw but my fuel pump keeps priming. So there must be some noise on the ignition line with the key on and engine not running.

850 RPM = 2.0 amps and 10.5 deg dwell on 4 cylinder scale.
2500 RPM = 2.8 amps and 29 deg dwell on 4 cylinder scale.

Engine ran fine when testing but this does suggest the resistors should be removed even with the stock coil and the GM module.

Ignition timing increased to 12 deg. BTC.

[Bitsyncmaster]

Stock coil resistance is 0.387 ohms.

To measure resistance accurately, drive DC voltage into the coil from a bench supply until 1.00 amps is flowing. Then measure the voltage on the two coil terminal studs. 1.00 volts would = 1.00 ohms resistance. I read 0.387 volts.

Now we know why voltage reads so low with the stock setup. Even with that 3 or 4 volts on the coil that results in 5 amps of current.

[content22207_2]

The coil will draw however many amps it wants (enough to saturate the core) -- 3-5 amps. But that isn't what ignites the gasoline. Secondary windings don't have any amperage to speak of. They are very high voltage at very low amperage (milliamps). Anyone who has shocked himself with a bad spark plug wire knows there is no amperage on the secondary side.

The coil is a backwards transformer -- it multiplies voltage, not amperage. Too little voltage into the coil and the secondary windings simply won't put out anything usable (anything that can jump the spark plug gap).

Bill Robertson
#5939

[Bitsyncmaster]

The GM unit keeps the coil unpowered until it sees the distributor signal. I really like that function so you don't keep powering that 7 amps if you are testing something turning the key to on. But I will need to change my RPM relay software so it won't keep priming in that that condition. My software just assumed it was running the stock ignition ECU and was looking for no ground on the ignition coil to determine the engine was running.

So the stock ignition unit keeps the white/slate wire grounded with the key on and engine not running. The GM unit keeps it floating so you will see +12 volts on that white/slate wire.

[content22207_2]

I've always assumed Duraspark does the same thing (energizes the coil whenever the car is on). I unplug the coil if I'm going to energize the car for an extended period of time without the engine spinning to avoid overheating the coil.

Bill Robertson
#5939

[Bitsyncmaster]

Got my new ACCEL 8145 coil. Primary resistance measures 0.740 ohms.

[content22207_2]

Ignition module is in the mail to Dave. Not quite a Ford versus General Motors smackdown, especially since we're dealing with aftermarket ECU's rather than factory originals, but close enough for DMCTalk purposes.

Bill Robertson
#5939

[Bitsyncmaster]

Ignition module is in the mail to Dave. Not quite a Ford versus General Motors smackdown, especially since we're dealing with aftermarket ECU's rather than factory originals, but close enough for DMCTalk purposes.

Bill Robertson
#5939

It will be nice to have the measurements done the same way with the same test equipment.

I'm thinking running that new 0.74 ohm coil without any resistors and the GM module should about equal the power that the stock set uses. Since the GM module uses shorter power pulses (lower dwell numbers) that 0.74 ohms as opposed to the stock 1.38 ohm (coil and resistors) the average current should be close. But in reality, your putting all the power into the coil where as the stock system only put about one third of the power into the coil.

[content22207_2]

the stock system only put about one third of the power into the coil.


I have no idea what Bosch was trying to accomplish with its secondary ignition. Even with a high winding coil and no ballast resistors, Bosch's small plug gaps severely limit secondary voltage. Makes sense with breaker points (avoids arc'ing damage to the points), but there's no advantage with electronic ignition. Smaller plug gaps also reduce the flame front. Throughout the 1970's American manufacturers pursued larger plug gaps, not smaller (by 1980 General Motors was already running .060" in its small cars such as the Malibu and Citation). Same with Japanese manufacturers (1981 Datsun 200 Series was .050", 1981 Honda Civic was .044", Toyota Corolla and Tercel were .044" as well). Europeans were the only ones still languishing with plug gaps less than .030".

I've been running .044-.046" gaps for more than a decade with excellent results. Originally I ran HEI with the Bosch ECU, but about 5 years ago I switch to Duraspark. Only within the past year have I been using full charging voltage.

Bill Robertson
#5939

[Bitsyncmaster]

In the early years the problem with spark plugs was fouling caused by the leaded gas. So they developed capacitive discharge ignition which really put high voltage and high currents into the spark plugs.

I have not really read up much on the performance improvements of wider spark plug gaps but yes, all of today's cars use 0.050" or more.

Aircraft engines use two spark plugs for each cylinder. That is for safety redundancy but when you do your preflight runup check, you turn one of the plug banks off to verify the other is working (no fouled plugs). With both banks running you do get a few hundred RPM increase as opposed to one bank. So it must be the two plugs fire the mixture better.

[dustybarn]

It will be nice to have the measurements done the same way with the same test equipment.

I'm thinking running that new 0.74 ohm coil without any resistors and the GM module should about equal the power that the stock set uses. Since the GM module uses shorter power pulses (lower dwell numbers) that 0.74 ohms as opposed to the stock 1.38 ohm (coil and resistors) the average current should be close. But in reality, your putting all the power into the coil where as the stock system only put about one third of the power into the coil.

The GM module requires no ballast resistors, and doesn't use them in the original GM/Jaguar applications. It performs the current limiting internally.

[Bitsyncmaster]

The GM module requires no ballast resistors, and doesn't use them in the original GM/Jaguar applications. It performs the current limiting internally.

I was looking for that info about current limiting but never found it. Can you show a link to that info so I can devise a test to check that it works?

[Bitsyncmaster]

Ignition module is in the mail to Dave. Not quite a Ford versus General Motors smackdown, especially since we're dealing with aftermarket ECU's rather than factory originals, but close enough for DMCTalk purposes.

Bill Robertson
#5939

Thanks Bill. The Ford unit arrived today. I hope to get it wired to another spare connector I have in a day or two. We had a break of two days without rain and I have to get the grass cut before the rain again on Saturday. Grass is two foot high in areas and turning to seed.

I was going to keep that spare stock ECU as is but thinking I will probably use the GM unit permanently so I will make another GM unit to carry as my spare. Doing that would give me another ECU also which is the one still bolted in the car. I think I will buy another GM module from another vender just to test if it works the same as that $10 unit I already bought.

I bought the GM module at autozone.

[content22207_2]

Perhaps you should hold off putting all your eggs in the General Motors basket until you've had a change to test a (faux) Ford unit?

Bill Robertson
#5939

[Bitsyncmaster]

Perhaps you should hold off putting all your eggs in the General Motors basket until you've had a change to test a (faux) Ford unit?

Bill Robertson
#5939

Good idea. Now I'm anxious to test it.

[content22207_2]

Once you go Ford you never go back.

Bill Robertson
#5939

[Bitsyncmaster]

Well I could not wait. So hooked up the Ford unit and ran my tests. One thing, the wiring diagram showed the distributor sensor signal and ground terminals reversed (my engine barely started until I reversed those wires).

Ignition timing = 13 deg BTC
0 RPM coil current = 7.0 amps
850 RPM coil current = 5.6 amps and 70 deg dwell on 4 cylinder scale
2500 RPM coil current = 4.1 amps and 65 deg dwell on 4 cylinder scale

So it does seem like the Ford ECU does not change dwell much with RPM increase. Also works like the stock unit and draws power with the key on and engine not running.

[content22207_2]

I measured about 6 degree dwell decrease idle to 3,000 RPM on an analog 6 cylinder scale (both factory original module and an aftermarket module).

What is unique is dwell decrease as RPM's increase. Most module increase dwell as RPM's go up, to give the coil a better opportunity to recharge (I measured 2 degree dwell increase idle to 3,000 RPM for a Bosch module).

Bill Robertson
#5939

[Bitsyncmaster]

So it looks like the Ford unit works pretty much the same as the stock Bosch unit. The currents and dwell readings are pretty close. Now the question is how much power on time and current will saturate the ignition coil. I would need to somehow measure the high voltage output (duration, current and voltage) running through a spark gap (spark plug) to determine saturation of each coil tested.

[Bitsyncmaster]

Here is a great article on ignition dwell and ignition coil theory and testing. Probably more info than most want to read but to really sum things up, looks like 7 amps is the current limit and up to 5 ms is typical for coils to saturate.

[Bitsyncmaster]

Replaced the stock ignition ECU with my GM ignition module all else stock (resistors and coil stock).

At zero RPM there is no current draw but my fuel pump keeps priming. So there must be some noise on the ignition line with the key on and engine not running.

850 RPM = 2.0 amps and 10.5 deg dwell on 4 cylinder scale.
2500 RPM = 2.8 amps and 29 deg dwell on 4 cylinder scale.

Engine ran fine when testing but this does suggest the resistors should be removed even with the stock coil and the GM module.

Ignition timing increased to 12 deg. BTC.

Tested the second GM module (bought from AutoZone) and results follow:

Ignition timing - 12 deg. BTC
0 RPM = 0 amps.
850 RPM = 1.9 amps and 9.2 deg dwell on 4 cylinder scale.
2500 RPM = 2.9 amps and 27.5 deg dwell on 4 cylinder scale.

Now that I have a backup GM unit, I plan to replace the coil and bypass the resistors. Will test again and then thinking I will regap my plugs to 0.035".

I was thinking that these "GM" units must be reading RPM via the amplitude or rise time of the distributor signal. If it were using frequency you would need a different unit for 4 cylinder, 6 cylinder and 8 cylinder engines.

[Bitsyncmaster]

I put the ACCEL coil in and bypassed the resistors. Using the AutoZone GM module.

Ignition timing = 12 deg BTC
0 RPM = 0 amps.
850 RPM = 1.6 amps and 14.8 deg dwell on 4 cylinder scale
2500 RPM = 2.8 amps and 40 deg dwell on 4 cylinder scale

I expected more amp draw and no dwell change but amps seem to have held low and dwell increased. I'm going to look at the current with my oscilloscope.

Also that HEI coil needs a larger boot on the HV terminal. I guess because of the higher voltages they when with a larger (more insulation) terminal.

[Bitsyncmaster]

Put my scope on the current probe.

Coil current rises to 5.9 amps in 1.8 ms. and current limits at 5.9 amps after that. That explains why my current did not change after I replaced the stock 1.4 ohms (coil and resistors) with the ACCEL 0.74 ohm coil.

So the GM module does have current limiting. I really like this GM unit. It will also save power and should run the unit and coil cooler.

[Bitsyncmaster]

Gapped the plugs to 0.040" and buttoned things up. The dwell keeps increasing as RPM increases. At 4000 RPM dwell is 50 deg. on 4 cylinder scale.

[content22207_2]

That is typical behavior for RPM based ignition modules. Duraspark is the first one I have seen that decreases dwell as RPM's increase (I know you measured an increase, but I encourage you to test it again -- I measured decreases on two different modules, one of them factory original, on two different days).

Bill Robertson
#5939

[Bitsyncmaster]

That is typical behavior for RPM based ignition modules. Duraspark is the first one I have seen that decreases dwell as RPM's increase (I know you measured an increase, but I encourage you to test it again -- I measured decreases on two different modules, one of them factory original, on two different days).

Bill Robertson
#5939

I measured the same decrease you saw on the Ford module. It's a very small number which pretty much will not change anything. But you definitely want higher dwell numbers at high RPM. Your Ford unit has high dwell numbers all the time. It just wastes electrical power and heat keeping those high numbers at lower RPM. Same goes for the stock unit.

[content22207_2]

I measured a 12% decrease in dwell -- not inconsequential.

The problem with increasing dwell as RPM's increase is you are shortening discharge time as the mixture is becoming harder to ignite. It's like giving yourself 2 seconds to light a candle in a still room versus 1 second to light a candle in a windy room.

Of course decreasing dwell means less time for the coil to recharge, but obviously Ford calculated increased discharge time to be more beneficial than decreased recharge time.

Bill Robertson
#5939

[Bitsyncmaster]

I measured a 12% decrease in dwell -- not inconsequential.

The problem with increasing dwell as RPM's increase is you are shortening discharge time as the mixture is becoming harder to ignite. It's like giving yourself 2 seconds to light a candle in a still room versus 1 second to light a candle in a windy room.

Of course decreasing dwell means less time for the coil to recharge, but obviously Ford calculated increased discharge time to be more beneficial than decreased recharge time.

Bill Robertson
#5939

I'm pretty sure discharge time is nill compared with charge time.

[content22207_2]

I'm sure both are measured in milliseconds.

How long does it take to charge a coil anyway? Any milliseconds more are simply wasted energy (infinitesimal energy -- the clock probably uses more).

For better or worse, there was method behind Ford's madness. Internet rumors that Duraspark is fixed dwell simply are not true. Ford stuck with Duraspark until the mid 80's, so obviously they found no fault with it. I'm sure you need a wider rotor blade and larger diameter distributor cap to get the full benefits Ford intended.

Bill Robertson
#5939

[Bitsyncmaster]

I'm sure both are measured in milliseconds.

How long does it take to charge a coil anyway? Any milliseconds more are simply wasted energy (infinitesimal energy -- the clock probably uses more).

For better or worse, there was method behind Ford's madness. Internet rumors that Duraspark is fixed dwell simply are not true. Ford stuck with Duraspark until the mid 80's, so obviously they found no fault with it. I'm sure you need a wider rotor blade and larger diameter distributor cap to get the full benefits Ford intended.

Bill Robertson
#5939

My new ACCEL coil takes 1.8 ms for the current to hit the current limit of 5.9 amps.

[Bitsyncmaster]

That article I posted states spark duration has to be at least 0.6 ms and it can be up to 1.4 ms.

At 6000 RPM you get 300 Hz or 3.33 ms. So with a dwell of 45 deg (4 cylinder scale) which is 50% duty cycle that would give you 1.66 ms to charge and 1.66 ms to fire the spark. You would want a higher dwell since that charge time is usally to short and you still have to much fire time.

[content22207_2]

I think both of us can agree that 1 or 2 milliseconds difference in an 8-13 volt 4-7 amp circuit is two steps beyond negligible. Kind of like replacing an incandescent bulb in the attic light fixture with an LED bulb to save electricity when you get the Christmas decorations down. Any differences have no real world effect.

There is of course a world of difference between HEI, however it is implemented, and lower voltage secondary ignition. Stock Bosch ignition can not reliably jump .040" or larger plug gaps. The Bosch module is perfectly content switching HEI however (several owners are running HEI with their Bosch modules, as I did for many years before making a plug & play Duraspark adapter).

Any ignition module that uses a hall effect signal from the distributor will work on a DeLorean (distributor itself takes care of odd fire sequencing). That is why there are so many different modules already in service. IIRC the user with the "do you know how awesome you were driving" avatar (Wolf?) is running an MSD module. You and Dustybarn are running GM. Steve and I are running Ford. And there may be still other modules out there (Mallory, Pertronix, other OEM).

One thing an owner should definitely consider is replaceability. A module hard wired in would be difficult to replace in case of failure, especially while traveling. Any ignition module can fail, including OEM Bosch on a stock system. That's how I got into the Duraspark business in the first place: Byrne Heninger's stock ignition module died, so I made him a plug & play adapter for a module he can buy over the counter at any parts house (then used his dead module to make my own adapter).

Bill Robertson
#5939

[Bitsyncmaster]

Note a hall effect sensor outputs a square wave 0 to 5 volt signal. Our distributor coils are just "generators" and out put a "sinusoidal" wave form which can approach 100 volts at high RPM. The GM module requires that sinusoidal signal to adjust the dwell as RPM changes.

[SS Spoiler]

huh?

[Bitsyncmaster]

huh?

Hall effect sensor is the wrong term for our distributor sensors.

[content22207_2]

Our distributor has the exact same style sensor as every other distributor of the late 70's/early 80's. That's why ignition modules are interchangeable.

Bill Robertson
#5939

[Raphael]

Hi
I'm the author of the initial post about HEI modification. ( http://dmctalk.org/showthread.php?5158-HEI-ignition-conversion ). I used Pertronix's module and MSD blaster coil without ballast resistor so the coil receives 14 volts. Plugs are gapped to 1mm ( 0.04" ).
I have been running this setup for 4 years now without any issue, everything is working smooth. I get perfect starts and idle, and more low-end torque. overall driveability is better, probably not something you can measure with numbers, but the engine surely feels different

[content22207_2]

Hi
I'm the author of the initial post about HEI modification.

Are you the owner who soldered across the resistor in his rotor button? That is a modification I have wanted to make for a long time but never have gotten around to. As this picture clearly shows, Ford did not use resistors in its rotors (I'm pretty sure General Motors didn't use them either):

42150

Bill Robertson
#5939

[Raphael]

no, I left the rotor untouched.

[content22207_2]

Now that I think about it I seem to remember the owner being in Italy.

Bill Robertson
#5939

[Bitsyncmaster]

Hi
I'm the author of the initial post about HEI modification. ( http://dmctalk.org/showthread.php?5158-HEI-ignition-conversion ). I used Pertronix's module and MSD blaster coil without ballast resistor so the coil receives 14 volts. Plugs are gapped to 1mm ( 0.04" ).
I have been running this setup for 4 years now without any issue, everything is working smooth. I get perfect starts and idle, and more low-end torque. overall driveability is better, probably not something you can measure with numbers, but the engine surely feels different

Forgot about that old post. I went back and read all the comments again. The two GM modules I tested do have current limit of 6 amps. The dwell at low RPM is very low so you should not get much heating of the coil or module at normal RPM. Most of the GM cars mount that module in the engine compartment and some even on the distributor so our cool mounting location should really keep unit temps well below failure levels.

I will road test my car with the resistors bypassed and my 0.7 ohm coil but really don't expect any problems. I will then remove the wiring and resistors. I have unwrapped and rewrapped my harness about 4 times already to remove the thermal time switch, WOT switch and idle switch wiring. I'm not running my engine compartment light (heat killed it) so I can remove the whole resistor mounting plate. I was going to install a mercury till switch for that engine light anyway.

[Raphael]

I used a heatsink but it never gets really hot. I'm sure the module can be mounted directly on the stock ecu frame, this would offer enough surface to dissipate heat. after all, i had the same module inside a GM HEI distributor (on a 327 Chevy) and there is no heatsink, just the distributor itself..
I got rid of the ugly mounting plate too, with this setup the coil receives full voltage anytime so you don't need the ballast nor the hot start relay. it really cleans the firewall. engine compartment light got a manual switch.

[Bitsyncmaster]

Test drive felt good but when the cooling fans went on I think the engine had a bad skip. I put the blue GM unit on and it does not have that problem. I wired both the same.

I put the timing light on and I do see a big skip or crossfire. Now my test blue unit is not mounted on the metal plate and the "bad" Autozone unit is mounted.

Edit: Pulled the "bad" unit off the metal plate and it still has the problem.

Edit again:
Must be one unit has more sensitivity on the signal from the distributor coil.

[content22207_2]

That Duraspark ECU is looking better all the time, eh?

General Motors was all about pushing the performance envelope. Ford was all about simplicity and reliability.

(Can I hear an "Amen" from anybody who has struggled with a cantankerous Quadrajet carburetor...).

Bill Robertson
#5939

[Bitsyncmaster]

I put my oscilloscope on the 12 volt power going to the GM ignition ECU. As with all car circuits that 12 volt power is very noisy. I added a 1000 uf. 16 volt cap soldered onto the the ignition connector and it just about cleared all that noise.

Two screen saves from my oscilloscope showing the improvement. I did not get the problem with the "bad" ECU on my cold engine this morning. I will do another test drive later to see if that capacitor fixes the "crossfire" I had seen.

In any case I suggest everybody add a large capacitor onto the GM module. It may be some of the modules have a large cap inside them. I know the stock ECU has cap filters as probably the Ford ECU do.

[Bitsyncmaster]

It looks like that cap would fix the AutoZone GM module. But I tested standby current just powering the ECU with all other pins floating.

Stock ECU draws 400 ma.
AutoZone draws 140 ma.
The Blue GM HEI unit draws 40 ma.

I think I will buy another one of those blue modules (only $14 shipped) and will scrap the AutoZone module.

[content22207_2]

Isn't saving 100 milliamps a bit like eating a Whopper rather than a Big Mac because it only has 995 calories rather than a thousand?

Bill Robertson
#5939

[Bitsyncmaster]

Isn't saving 100 milliamps a bit like eating a Whopper rather than a Big Mac because it only has 995 calories rather than a thousand?

Bill Robertson
#5939

The blue one has already proved it worked better with that voltage noise. So it even produces less heat and less power.

[dustybarn]

INTERESTING. I may add a cap to mine, just for grins.

Hmmmm.... The "Lucas" ignition module in my XJS actually has a 4-pin GM module inside it..... along with a big 'ol capacitor.

[nkemp]

This might be worth reading: http://www.jalopyjournal.com/forum/threads/ford-gm-hei-coil-question.617596/

A couple key points:
- The coil must not exceed .5 Ohms. Check out the pictures a ways down as to what happens when higher resistance coils are used. The stock coil may be the best bet.
- Either here or elsewhere there is discussion about using heat sink compound. Some suggest using dielectric grease. Dielectric grease is not a good heat sink compound. Real heat sink compound is cheap and works well. Heat kills these modules.
- Another coil that we may want to consider is the S10 ignition coil. But it may not fit where the existing coil is located.

[Bitsyncmaster]

I'm now running my GM HEI ignition with the ACCEL 0.7 ohm coil and 0.040" plug gap. I have the 1000 uf cap added and have a spare ECU I carry just incase.

I did find the GM module does current limit at 6 amps so I would think the stock coil would work fine. But since one user said the ACCEL coil was better, I use it also. Maybe the added capacitor would have cured his problem with the stock coil.

The GM unit will produce less heat than the OEM ECU. I used thermal grease "heat sink compound" which you can buy online or at radio shack.

I also read the GM module makes a one ms. minimum off time just to make sure the spark time is long enough to finish.

[nkemp]

Here is the schematic as modified for the current design.
43564

[dustybarn]

I'm now running my GM HEI ignition with the ACCEL 0.7 ohm coil and 0.040" plug gap. I have the 1000 uf cap added and have a spare ECU I carry just incase.

I did find the GM module does current limit at 6 amps so I would think the stock coil would work fine. But since one user said the ACCEL coil was better, I use it also. Maybe the added capacitor would have cured his problem with the stock coil.

The GM unit will produce less heat than the OEM ECU. I used thermal grease "heat sink compound" which you can buy online or at radio shack.

I also read the GM module makes a one ms. minimum off time just to make sure the spark time is long enough to finish.

Yeah, I can't say for sure that my problem was caused by the coil primary impedance, only that switching to a lower one fixed (or masked?) the problem. Adding a cap to clean up the module power supply honestly didn't occur to me.

[nkemp]

GM included a capacitor in their HEI distributors. I was trying to find the value but failed. Does anyone know or can measure the GM capacitor? From what I read, GM used it for radio noise suppression but maybe it provided other benefits.

As I was Googling to find the value when I found this "The two capacitors are optional but recommended. The 1.0 µF capacitor on the +12V (to ground) is helpful, it is similar to the one used on EDIS. What it does is provide brief energy storage for the discharge. The other capacitor will help eliminate back-fed noise to your MicroSquirt® controller. Use a 100 pF to 0.001 µF cap on the TTL trigger input wire to ground. What this does is shunt extremely fast noise spikes to ground and not let them feed back to the MicroSquirt® processor. The added capacitance is minimal - with the series resistance of 1,000 ohms (in the controller) and a 100 pF cap the RC 3dB time constant is 2 * π * R * C = 0.6 microseconds."

At MegaSquirt page.: http://www.useasydocs.com/details/coils.htm

[Bitsyncmaster]

I would guess my 1000 uf. cap is more than you really need. Maybe 100 uf or even 10 uf would have worked. The value depends on the wire resistance and inductance in the cars harness.

Normally cap filters use one large cap which is electrolytic and a smaller ceramic cap for higher frequency noise filtering. I used a 1000 uf. because I had one on hand. After I installed it my oscilloscope showed met that fixed the large voltage drops.

[Bitsyncmaster]

My car has been running fine but after doing a new engine harness, I've decided to try a higher resistance ignition coil just to reduce some heating of the ignition module. The module has current limiting of 6 to 7 amps. so when it limits that extra power is dissipated in the ignition module. I'm currently (no pun intended) running a 0.7 ohm coil so it will start current limiting above 5 volts on the coil. I have ordered a 1.5 ohm coil which will start current limiting above 10 volts on the coil.

So cranking a cold engine if the battery voltage drops to 10 volts, I still get full spark power. But when the engine is running I get 14.5 volts so it should keep the ignition module a lot cooler.

I have not done any testing of the heat of the ignition module, this is just my theory.

My new engine harness really shorted up the wiring to the ignition coil so the voltage is higher.

[Bitsyncmaster]

Got my new FLAME THROWER 1.5 ohm coil. I measured the primary and it is 1.23 ohms. Will install it tomorrow since it should be 65 deg. outside.

[nkemp]

Any recent update on this?

My car failed to start today and the ICU is in the suspect lineup. I smell gas and everything seems operational other than no ignition. Not sure how to test it other than to swap an ICU. Wiring up an HEI is relatively straight forward other than needing the connector. from the original in order to keep it PnP.

Anyway, I thought I'd reignite this thread (pun intended) to see what the latest info is.

[Bitsyncmaster]

I've not done any long distance driving but just the past few days have almost burnt a half tank of gas idling in my garage testing the temp gauge sensor. No problem with the ignition.

The GM module really runs a short duty cycle at low RPMs but it increases at higher RPM so I think it's best with the coil resistance of 1.5 ohms if your not using any resistors. Even though the GM module current limits (6 or 7 amps) I think the module would produce more heat when current limiting using a lower resistance coil.

[nkemp]

I believe that I have an original Bosch coil. Do you recall the resistance?

[nkemp]

another question ....

Does it make much difference which HEI module you get? It seems they should all be about the same.

[Bitsyncmaster]

I believe that I have an original Bosch coil. Do you recall the resistance?

My original Bosch coil is 0.40 ohms.

I measure real low resistance by pumping 1.00 amps into the resistance and measuring the voltage over the resistance. Hence I read 0.40 volts when I do that.

[Bitsyncmaster]

another question ....

Does it make much difference which HEI module you get? It seems they should all be about the same.

Bill insists his Ford unit is the best but I tested one and it is pretty much the same as our OEM unit.

The GM module has great dwell change, has current limiting and shuts down when the engine is not running. I put mine on an aluminum plate and mounted that plate in the OEM ignition box.

[Helirich]

I know this is an old thread, but I need some info. I understand I can get a different coil and not run a ballast resister. I don?t have a good understanding of our current system, but I like the idea of eliminating the resister. Something that is not there can?t fail. Also, I like the idea of running a coil that is more standard. Easier to find parts on a trip.

Can anybody hold my hand and give me step by step instructions? Starting with exactly what to order.

[Bitsyncmaster]

I know this is an old thread, but I need some info. I understand I can get a different coil and not run a ballast resister. I don?t have a good understanding of our current system, but I like the idea of eliminating the resister. Something that is not there can?t fail. Also, I like the idea of running a coil that is more standard. Easier to find parts on a trip.

Can anybody hold my hand and give me step by step instructions? Starting with exactly what to order.

The ignition coils normally need about 6 amps of current when running. The use of a resistor was the normal design so when cranking a cold engine you could get that 6 amps when the battery voltage is lower due to the 100 plus amps of starter current.

When engine is running you normally get about 14 volts battery voltage. Using ohms law (I = E/R) the resistance of the primary of the coil would need to be 2.3 ohms with 14 volts to get 6 amps of current. When cranking the engine you expect to have 11 volts of battery voltage which would result in 4.8 amps of coil power with that 2.3 ohms of primary resistance.

Our stock coil has 0.40 ohms of primary resistance. Add the two half ohm resistors and the wiring resistance in the harness and the result is approaching about 2 ohms. So if you use a 1.4 ohm primary coil and no resistors it would result in the same current with engine running. But less current when cranking the engine.

If your running the GM ignition module or most other newer then stock units they have built in current limiting around 6 amps. That current limiting result acts like a variable resistor (produces heat if active). So you could run the OEM coil without resistors but the ignition module would get as hot as the resistors. Using a 1.5 ohm coil will still give you 6 amps cranking and 6 amps running and keep the ignition module pretty cool.

[Helirich]

The ignition coils normally need about 6 amps of current when running. The use of a resistor was the normal design so when cranking a cold engine you could get that 6 amps when the battery voltage is lower due to the 100 plus amps of starter current.

When engine is running you normally get about 14 volts battery voltage. Using ohms law (I = E/R) the resistance of the primary of the coil would need to be 2.3 ohms with 14 volts to get 6 amps of current. When cranking the engine you expect to have 11 volts of battery voltage which would result in 4.8 amps of coil power with that 2.3 ohms of primary resistance.

Our stock coil has 0.40 ohms of primary resistance. Add the two half ohm resistors and the wiring resistance in the harness and the result is approaching about 2 ohms. So if you use a 1.4 ohm primary coil and no resistors it would result in the same current with engine running. But less current when cranking the engine.

If your running the GM ignition module or most other newer then stock units they have built in current limiting around 6 amps. That current limiting result acts like a variable resistor (produces heat if active). So you could run the OEM coil without resistors but the ignition module would get as hot as the resistors. Using a 1.5 ohm coil will still give you 6 amps cranking and 6 amps running and keep the ignition module pretty cool.

Dave, you are vary smart in this kind of stuff. I am not. I kind of get the basics. I?m looking for something like this.

1. Buy Summit Racing coil #44325
2. Install in normal coil location with same wiring.
3. Unhook all wires at ballast resistor and remove.
4. Join red wire to blue wire and tape off all others.
5. Drive your car.

I don?t know if it can be this simple, but I can dream. Are you saying I need to change the ignition module to change coil?

[Bitsyncmaster]

Dave, you are vary smart in this kind of stuff. I am not. I kind of get the basics. I?m looking for something like this.

1. Buy Summit Racing coil #44325
2. Install in normal coil location with same wiring.
3. Unhook all wires at ballast resistor and remove.
4. Join red wire to blue wire and tape off all others.
5. Drive your car.

I don?t know if it can be this simple, but I can dream. Are you saying I need to change the ignition module to change coil?

The white/yellow goes to the coil. The white wire gets 12 volts with the key on (from AUX relay). So you would connect the white wire to the white/yellow wire and tape off (or remove) all other wires. I don't have the relay bypass wiring diagram so that info is from the car schematic. You can try that now before changing the coil to see if it starts. (I like to change one thing at a time).

[Helirich]

The white/yellow goes to the coil. The white wire gets 12 volts with the key on (from AUX relay). So you would connect the white wire to the white/yellow wire and tape off (or remove) all other wires. I don't have the relay bypass wiring diagram so that info is from the car schematic. You can try that now before changing the coil to see if it starts. (I like to change one thing at a time).

Are you saying if I do that and don?t change the coil, it should start, but it will burn up the coil eventually?

[r00b]

Also, I like the idea of running a coil that is more standard. Easier to find parts on a trip.


I got a new coil a few years ago, Napa had it in stock. How often does the resistor fail? I don't think very often. If you're worried about not being able to find one, you could just carry a spare.

[Bitsyncmaster]

Are you saying if I do that and don?t change the coil, it should start, but it will burn up the coil eventually?

Yes. Don't keep running it like that with a 0.40 ohm primary coil.

[Helirich]

I got a new coil a few years ago, Napa had it in stock. How often does the resistor fail? I don't think very often. If you're worried about not being able to find one, you could just carry a spare.

That would mean caring a spare coil and resister. I think a standard coil is available anywhere and again, the resister can?t fail if you don?t have one. I already carry a spare alternator and starter and belts. I wouldn?t carry them if I knew I could get them at any auto parts store.

The only reason I can see for keeping the current system is to keep it stock. While I like the outside stock, I don?t have any qualms about making utility better. I already have non-stock tires, brakes, shocks, lights and exhaust. And, of course, a bunch of Dave?s relays.

[AugustneverEnds]

I don't know if it can be this simple, but I can dream. Are you saying I need to change the ignition module to change coil?

No module change is necessary.

[Bitsyncmaster]

No module change is necessary.

Agree. The GM module gives you better dwell (less power consumed at idle) and current limiting (6 amps max) but still can run OEM unit with about 1.5 ohm coil.

[AugustneverEnds]

Agree. The GM module gives you better dwell (less power consumed at idle) and current limiting (6 amps max) but still can run OEM unit with about 1.5 ohm coil.

If I wasn't so gunshy about making an adapter I would try running the GM module. I've been running the Pertronix 1.5 ohm coil for two years sans the ballast resistor without any trouble so I'm good for now.

[AugustneverEnds]

70428

For anyone curious, here's a non-OEM (Pertronix) coil installed, fits in the stock bracket just fine. (Sorry for the sideways view, I don't know how to rotate it)

[Sirolf]

70428

For anyone curious, here's a non-OEM (Pertronix) coil installed, fits in the stock bracket just fine. (Sorry for the sideways view, I don't know how to rotate it)i am running the exact same coil with a GM ignition module

Do you know how hot your coil gets?
Mine can get pretty hot uncooled

Verstuurd vanaf mijn SM-G996B met Tapatalk

[AugustneverEnds]

I'm not sure. Over the next few days I will try to get out to the garage and measure the coil's temperature. (The weather is very cold where I am right now so might be a few days before I can get to it)

[DMC VIN 6639]

Anyone interested in this Ignition ECU to convert to GM?
72036