GM DIS Upgrade using HEI Modules

[Pinhead]

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.

[TOOLS1]

Welcome to the forum. Glad you found it. Hope you did not get overheated out there today. I think I lost a few pounds
Hope to see you guys at the meetings, and some more of the events we have planned.
TOOLS

[Re-run]

Sounds like this is something that, while points may work ok for, a cdi ignition would be best since those use low ohm coils already. I think points might take a beating with something like this. But I could be wrong.
Nice to see you still on your 650! I left that other site quite a while ago and came here.

[Pinhead]

Actually, using points to trigger the HEI modules, which then trigger the coils, reduces the strain on the points since there will be virtually no current flowing through the points. However, note that using a points system to trigger the HEI modules does NOT allow you to use the GM DIS coils; the dwell would be entirely too long for the 0.4 ohm primary winding.

But that's only on the pre-'79 Hondas; the stuff after '79 came with electronic ignition from the factory, and the reluctors in the stock system work perfectly with the HEI modules and DIS coils, producing nearly the perfect dwell throughout the rev range.

[TOOLS1]

I tried to understand the diagram, for the point triggered set up, but I need some one to translate GEEK to CURMUDGEON.
I did not see Ben at School today. Think he started the B-day celebration early?
TOOLS

[Re-run]

So booya on points but sounds like cdi is better. Might have to rob a dohc.

[Pinhead]

So booya on points but sounds like cdi is better. Might have to rob a dohc.

The ignition system on the DOHC bikes (and the CB650) isn't actually CDI, though that's what my Clymer and Haynes manuals calls it. It is actually a TCI ignition.

TCI is fundamentally and functionally similar to a points system; they're both based on the Kettering ignition.

Which means there is one simple way to differentiate between the two: If the coils are supplied battery voltage at all times while the engine is running, it is a TCI ignition. If, on the other hand, the coils are "pulsed" voltage from some type of high voltage source (ignition box, etc) than it is CDI.

But that's just terminology. You're right that the DOHC ignition is far ahead of points. And if you replace the factory components with HEI modules and Cav coils, it's in a league of it's own.

[shinyribs]

Just a little bit of info in the HEI modules.I race Chevy engines and will not use any aftermarket distributors.I swear by the HEI.  On the later model modules,5 and 7 pins, GM introduced automatic ignition retard into the module itself.Hence to need for the extra pins.It was because of the "smog" motors at that time that would run extremely high combustion chamber temperatures to promote cleaner emissions.They used the automatic retard in the 5 & 7 pin modules to help combat detonation at upper rpms.The retard begins at 4500 rpm,if i remember correctly. I don't know of any difference between the 5&7 pin,personally.There may be some,I just don't know.But i do know that the good ol' 4 pin has no timing retard.So if you are looking for max performance you may want to consider the four pin. GM introduced the HEI in 1974,so if you go to an auto parts store and ask for an ignition module for that year model vehicle you should be able to acquire one with no problem.Small block and big block chevy's use the same distributor,and six cylinders used a different dist. but same module so any engine will have the correct 4 pin module.

[Daddyman]

I always carried an extra module in my glove box,  That was the only thing that ever went out on my Truck.

[Cricket]

Hi,

I'm trying to figure out how to do this. Does anyone have photos of the HEI installed/mounted? I have an 81 750k with CDI/TCI ignition. How do the HEI modules wire to the rotor?

Thanks,
Jim

[Re-run]

I wish i had a chance to see this too. Pinhead seems to be rocking this thing pretty good.
I suggest emailing him directly to see if he can get you some pics or if he can post some here.

[Field]

This looks great, but I want to see pictures too...

[icerigger]

Since i'm doing a minor teardown this winter, this looks like something I really want to try. I really gotta smooth out the bottom end on this thing as it's hard to manoeuvre at low speed when you can't count on low rev control.

+1 on pictures on anyone's build.

 Anybody use this with iridium plugs?

Inquiring minds need to know.

[sgtslag]

I had stock coils, but the Iridium plugs made a night/day difference -- tremendous improvement.  I installed the Iridium plugs in my '93 Voyager touring bike:  no discernable improvement, but I do know that they will last 50,000-100,000 miles.  The Voyager's coils are much higher quality, no weakness in them at all.  The real improvement on that bike was longevity.

If you plan to upgrade the coils on your 750, the Iridium plugs will gain you longevity, but not much else (they overcome the weaknesses of the OEM coils).  If you have them already, fine; if not, the standard NGK plugs will be good.  To see the improvements they offer, with the new coils, would require an exhaust gas analyzer -- that's about the only improvement you will be able to quantify, is lowered emissions.  Not facts, just my edu-macated guesses.    Cheers!

[icerigger]

Thanks Sarge