Vintage and Classic Hondas

Is the GM HEI module necessary?

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Rodgil

Is the GM HEI module necessary?

I came across the GM coil mod on the DOHC site, and fitted two GM coils ($40) to my 1982 CB750FZ,
with the voltage to the coils coming off the battery, to the coils via a relay, which is now activated from the wire which originally went to the coil. I fitted better (electronic) universal coil leads, and opened up the plug gaps. Starts easier, runs smoother, and has worked well for thousands of miles. Why do some (not all) people also use the GM HEI module? Is that only for point-ignition bikes? Am I missing something?

TOOLS1

Re: Is the GM HEI module necessary?

Administrator

Good question. Maybe Pinhead will chime in, and explain this.
TOOLS

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1976 CB 750-A X 2
1977 CB 750-A X 4
1977 CB 750-K
1976 CB 750 F
1981 CB 750
1966 Kawasaki SG 250
1981 KZ 750 LTD
1973 CB 350
1979 CM 185 Twinstar
1982 Honda XL 80
South of Eden (Kansas City MO)

Re-run

Re: Is the GM HEI module necessary?

Administrator

In reply to this post by Rodgil

Yeah, you would need to talk to pinhead on this one. I never knew what gm coils to use but I would be pretty sure my dyna S might not like them is the ohms are too low.
I suppose the hei is able to jump the voltage even more.
What gaps are you running?

The ride IS the adventure. The destination is just to get gas!

sgtslag

Re: Is the GM HEI module necessary?

I looked into the GM Coil Mod, myself. I could not figure out exactly which coils to use, and after much consideration, I went with NGK Iridium plugs, instead. I know my coils are weak, and the plug wires could do with an upgrade, but for $32 ($8/plug), my ignition system now works flawlessly. For me, it was just easier, and an absolutely sure-thing, to replace the plugs (no chance of burning out the ignition system, which the wrong coils could have accomplished; the Iridiums will last 50k+ miles, so not a bad return on my investment). YMMV. Cheers!

1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII

Re-run

Re: Is the GM HEI module necessary?

Administrator

I am going to do those plugs too. I will also replace my coils too along with wires. Might as well do them all.

The ride IS the adventure. The destination is just to get gas!

TOOLS1

Re: Is the GM HEI module necessary?

Administrator

I installed them in the Toolmatic. Made a big difference, but also increased detonation, and pinging. Now I add octane booster. That helps starting even more then the plugs.
TOOLS

Re-run

Re: Is the GM HEI module necessary?

Administrator

What all did you install? Just the coils or the whole hei setup?

The ride IS the adventure. The destination is just to get gas!

Pinhead

Re: Is the GM HEI module necessary?

In reply to this post by Rodgil

Hmm very interesting. What coils are you using? What is the primary resistance of your GM coils? And what was the primary resistance of the stock coils?

Take a look at these charts:

Stock Coil, ~4.5 ohms
http://www.pcbwerks.com/pics/iggy14.jpg

GM DIS Coil, Wells C849, ~0.4 Ohms
http://www.pcbwerks.com/pics/iggy18.jpg

These show the current flowing into the coils. The point where the graph "flattens out" is where the coils are said to be fully saturated. Longer dwell and more current draw does nothing to increase spark energy and only unnecessarily heats the coils and drains the electrical system.

Notice how much longer it takes for the stock coil to saturate when compared to the GM coil.

Basically, I use the HEI modules because the Honda reluctor/HEI module combination produces the perfect dwell for the coils. Any extra dwell simply causes wasted heat and excess power draw from the electrical system. The spark isn't any hotter with extra dwell.

I assumed that the stock modules are optimized for the stock coils, thus having an unnecessarily long dwell time.

Rodgil

Re: Is the GM HEI module necessary?

The coils I used are from a Holden Commodore V6 (Australia) which used a slightly modified Buick V6 motor. Im told the Buick used the coils fro 1986 to 1995. These coils were also used in 4 cylinder Vauxhall (UK) and other GM cars. AC Delco P/no D555. Various GM P/nos 10482928, 10472401. GM quote the primary resistance (between the + and - terminals) as .3 to 1.5 ohms @ 20-30 degrees celcius, and the secondary resistance, between the HT terminals, as 5000 to 7000 ohms. The usual plug gap is .060. See info on www.cb750c.com forum topic GM Coil mod.