Ignition ECU operation, HEI

[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:

DwellMeter.jpg

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:

DurasparkAdapted.jpg

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.