Ignition ECU operation, HEI

[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-tun...5808_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-tun...5808_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