GM DIS Upgrade using HEI Modules

Posted by Pinhead on Jul 11, 2011; 4:36am
URL: http://vintage-and-classic-hondas.81.s1.nabble.com/GM-DIS-Upgrade-using-HEI-Modules-tp3158051.html

Hi guys, first post here. I was at the VJMC meeting today at the Denny's in KC and was directed to this site. I've done a few mods to my engine so I thought I'd share. :) This one's the easiest and cheapest.

http://home.comcast.net/~loudgpz/GPZweb/Ignition/CBheiModCavCoil.html

This project is an ignition module design to replace the stock unit for a 1980 Honda CB 750 C motorcycle using aftermarket GM ignition coils commonly found on Chevy Cavaliers and many other GM cars instead of the stock coils. It also uses two GM HEI igniter modules. The parts for this project should not cost much more than $35, (not including the coils which are about $20 each). This module is designed to work with the stock inductive pickups. This project will also work as a replacement for the many other early 1980's Honda CB motorcycles that use the same pickups, rotor, as the 1980 CB 750 C. It uses a mechanical advancer unit. This project is not compatible with electronic-advance type ignitions.

http://home.comcast.net/~loudgpz/GPZweb/Ignition/CbHeiModCavCoil.gif

I had a hard time finding 4-pin HEI modules so I'm using the 7-pin modules.

The idle is absolutely amazing. The bike starts up extremely easily. Very little choke is needed (impressively so, considering the generally "cold blooded" nature of the CB650). The bike can take off in 1st gear at idle without dying. The low-end (1000 to 2500 rpm) is as smooth as bikes with CV carbs.

The wiring had to be slightly altered, however, to use the 7-pin moduels. The attached diagram shows how to wire the system when using 7-pin HEI Modules and Chevy Cavalier coils.

http://img148.imageshack.us/img148/7584/7pinhei.jpg

When I was trying to figure this project out, I sent a few emails to the author of the mod and here is some background information:

The 4-pin has internal dwell compensation.
The 7-pin has no internal dwell compensation circuit. The 7-pin is just a two-mode driver.
B stands for bypass. You are using it in bypass mode when B, R, and E are unconnected.
R stands for reference. That is the signal from the module to the car's ECM. It can be used for a tach signal, which is how some people probably use it, but that is not the real purpose of R. The ECM uses the reference signal to determine dwell and timing. That ECM signal then returns to the module through pin E. E stands for ECM.
 
N on the 7-pin correlates to W on the 4-pin.
P on the 7-pin correlates to G on the 4-pin.
 
N and W are what I refer to as the negative side of the reluctor pickup (positive and negative are arbitrary labels, but I use them consistently, which is what matters, other sources may refer to them opposite from what i do).  The main difference between the 4-pin and 7-pin is that the bias voltage on N is relatively steady, while the voltage on W changes greatly with RPM (based on reluctor signal strength and output's current limiter usage). (My full notes on the 4-pin are available from that link you referenced.)
 
If you are using Cavalier coils, (as that is what the link refers to, note there is a different page for using stock spark coils), then the W-pin does not get used, so the issue between W and N is irrelevant.

However, P and G also have a slight difference in their behavior. The thresholds are slightly higher on the 7-pin module.  So there may need to be a slight compensation on the negative terminal of the pickups. The difference is about 1 volt. A 1-volt reference voltage should be applied to the negative sides of the reluctor pickups.  That is the blue wire with white band, and yellow wire with white band.  On the 4-pin page, those wires are just tied to ground. To use it with the 7-pin module, you will likely have to go with a simple voltage provider like that used on the Kawasaki version:
http://home.comcast.net/~loudgpz/GPZweb/Ignition/GPZgmHEImod.html

Use P in place of G. The white band wires would be the negative pickup wires, but try both polarities in case the timing seems off. This is in case someone swapped the white bands. Double check the timing with a strobe to make sure the polarity is correct (wrong polarity alters the timing).

If the polarity seems reversed, maybe there is a chance someone put the white band on the wrong wire.

The HEI ignitions' plug gaps were originally set at 0.080" but later turned back to 0.060" because the distributors and wires weren't lasting long enough. We don't have a distributor to arc over or wires overlapping to cause cross-talk which are the two of the HEI's problems. The GM DIS ignitions (where we get the Cav coils) are factory-gapped at 0.060 inches so the plugs can last 100,000 miles. By the time they're changed I've seen the gaps as wide as 0.100 inches and still firing.

There was a study by a prominent ignition company that said 0.100" is the optimum plug gap for kernel expansion if enough spark energy is available. We probably won't be able to get quite that far, but the closer the better. :)

I'm running 0.080" on my 11:1 CR CB650. Lower compression bikes can likely go further. I'd start at 0.060" and open from there; high compression and lean mixtures are more difficult to fire than low(er) CR and stoich mixtures.

0.080" gap seems to be the limit on my engine with DIY Accel wires and factory plug caps; 85 thou causes spark scatter when the engine is under load.

Any questions, don't hesitate to ask.