Hot Rod Roy

What is the difference between the '75-'81 HEI coil, which has red/yellow primary wires, and the '82-'91 HEI coil, which has red/white primary wires? They look to be physically interchangeable, but there must be something different.
My '84 has a weak spark, and I have a spare early HEI coil. Can I use it?

Cliff Harris

I heard on the Forum (so you can take that as gospel) that the colors must be matched to the ignition module or something vaguely bad will happen. How you know which ignition module to use was not specified.

I think the turns ratio and/or wire gauge caused the old coils to draw too much current for the modern ignition modules. Should be OK for a short term replacement.

WVZR-1

The magnetic polarity of the coils is different and it's generally assumed cranking and lower RPM issues by mismatching. I believe the distributor internal components were different also, there were different colored connector colors used internally to ID p/u coils for modules etc. There's some interesting reads that honestly don't make sense much sense but are entertaining. I don't doubt the facts though but a person would have to have several various (CPOBK) distributors to confirm.

Maybe all of the reads are relevant for pre ECU changes that were made in maybe '81 to all GM but seen in '82 production for CFI and some others TBI products. The older coil red/yellow was used through maybe '84 or so other applications.

Maybe just get a correct coil? Borrow the correct coil?

**The only quick comparison for coils that I could think of using the (CPOBK) applications was RWD G-body cars. A 231A motor used a red/white (all years), 229 & 267 used red/yellow beyond '82. Starter on different side of block so maybe there is fact in some of the reads that mention it being important.

WW7

The Accel coil I had on my 89 when I bought it was the wrong one with the red/yellow wires , I
didn't know this and ran the car for 3 years before deciding to replace it. I never had any running problem, but after changing to the right coil I did notice the car started much faster..What I found out is like WVZR-1 mentioned above, the polarity of the 2 coils were opposite from each other...The wrong one will work , but starting the car will be harder....WW

cardo0

Whats your plug gap. To wide plug gap will weaken spark. HEI likes just 0.035" gap. Also check your plug wire resistance. Should be just a few hundred ohms max. Heck check the cap & rotor to.

Cliff Harris

Actually it's the opposite. It takes MORE voltage to jump a wider gap. Once the spark is generated the resistance in the arc gets very low.

Now that reminds me of what the deal was with the two versions of the HEI coil. The original gap specified was .045" but they reduced the turns ratio for less voltage and decreased the gap to .035".

I still don't remember what the symptoms were that caused GM to make the change... Carbon tracking inside the cap? There was a post on this that explained the whole thing.

69427

I haven't heard definitively what may have changed with the coils over that time span, but just two observations:
Reducing the turns ratio won't change the secondary voltage. The plug gap size and cylinder pressure are the prime determinants of the actual secondary peak voltage level seen on an engine.
If you reduce the turns ratio it usually increases the secondary current (ampere-turns kinda thing initiated by the primary current magnitude).

If someone does know the actual technical reason for the change and could post it I would greatly appreciate it (just engineering curiosity). The model year timing of the change was right in that timeframe when the engines were stopping use of the HEI distributor and were going with the EST distributor.

cardo0

Well its my guesstimate the change coincided with the reduction in the plug gap. The larger gap required more spark energy to get the spark started and left less energy to add heat to initiate combustion. The advantage the HEI has is at lower rpm where it can provide a longer spark producing more ignition heat and less chance of having it blown out.

As for the difference between the 2 HEI coils i dont know. I wouldnt know where to find their specs either.

Hope this helps.

Cliff Harris

That's why I think it's funny when I see the "super coils" with 50,000 volt secondaries. Guess what? You'll never actually see that much voltage on a actual engine unless there is an open circuit.

Does anyone remember the old Sun ignition scopes where they would disconnect one spark plug wire to see how much voltage the system could generate?

I think it's interesting that nobody considers the gap between the rotor and distributor cap posts...

Tom400CFI

That is b/c that gap is relatively insignificant since it is at atmospheric pressure. 69427 is right that cylinder pressure and plug gap are the meaningful drivers of voltage requirement. Assuming the same gap, the higher the cylinder pressure, the more voltage required. Under atmospheric conditions, most HEI coils will throw a spark over an inch gap.


Yes, I played w/a scope years ago. Great teaching and diagnosing device.

.

Cliff Harris

Considering how useful they are I think it's weird that you don't see them in the repair shops any more.

Tom400CFI

I completely Scan tools have taken over as "gamers" have entered the tech profession. People don't know it but a scope can tell you a LOT about and engine's condition, when used thoughtfully. If they weren't so HUGE, they'd be a cool tool to pick up used, for the home garage.

cardo0

Ive been holding out buying an o'scope thinking i will come across a bench scope that fits my usage and budget. But i dont want spend for a 1Ghz scope for the little use it would get. So now im leaning to a PC/USB scope which is mostly an D/A converter/adapter but with program software. U can find them under $100 but mostly in the low range like 20Mhz. Ya know a lot of tuners are using those with success as the ECM's/PCM's are fairly slow too. And who the heck is gonna look at the code signal anyways?
The PC/USB adapters are small enough to go anywhere the PC goes. Its more a matter of taking the time to shop for a PC/USB scope - as there are so many - than anything else. Just been on my long list for months now.

Cliff Harris

Me. To do that what you really need is a logic analyzer:

Cliff Harris

I know a guy who bought one from his employer when they upgraded. He started his own tuneup business out of his garage.

cardo0

Well i recall clocking registers with as many channels as the scope has. And to look at a packet of serial data i think they (not me) use software. But yes u cant look at a 8 or 16 bit parallel input w/only 2 channels. Seems i can recall using an 8 input scope though just for training and i think i remember this because it wasnt a logic analyzer which surprised me as the instructor made us think to make it work.

69427

(just keep the shaving cream off the bare boards )