Ignition ECU operation, HEI

[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.