Switched to HEI

-
rednesss

rednesss

Well-Known Member
Joined
Mar 4, 2006
Messages
1,423
Reaction score
219
Location
Eugene, OR
So I finally took the plunge and switched over to the GM HEI module. I have to say, why didn't I do this earlier? I got the little aluminum adapter plate from Mopar HEI Conversion and an AC Delco D1906 module and a HEI e-core external coil and hooked it all up yesterday in about an hour after making some wiring harness modifications. So far, I'm very pleased. I didn't get around to re-gapping the plugs yet. Is it worth it to re-gap the plugs? Or even get different plugs? I'm running Champion RN12YC plugs right now in my SB 340. Just wondering what other people did who made the switch.
 
Last edited:
I wouldn't open the plug gap unless you have 8mm plug wires ,that's what I used when changing over because of the extra voltage required by opening the gap, and most gm hei,s came with 8mm wires on v8,s unless the plugs were gaped at .035
 
Opening the gap will give the full benefit of HEI. But if you go too far you will suffer mis-fire under WOT at low rpm (freeway ramps). I got that in my 2.4L at the factory 60 mil, so reduced to 50 mil and no more problems (Chrysler TSB for turbo version).

If you have a small block (nice to tell us), you can use wires for a Magnum V-8 engine. I grabbed some from the JY cheap. I think factory. They are silicone jacket, w/ each one labeled "cyl 1", ... MSD makes a set too. Of course, the distributor end is for a male terminal on the cap, but those who buy the Ready-to-run HEI distributor ($45 ebay) usually get that.
 
I kept my female wires, and just got a coil wire that had male on one end and female on the other.
 
ABSOLUTELY open the plug gap; one of the big points of the HEI is to be able to arc across that larger gap. The longer and hotter spark creates a larger spark kernal which initiates the combustion process better, faster, and more complete.... more power and efficiency and effectively a small timing advance. You're missing a good part of the benefit if you stick at .035. My son and I put his 10:1 SCR 340's plugs to .045"; might try even more at some point. Man, does it burn clean....
 
TrailBeast said:
X2
That larger gap makes a big difference in how the engine starts and runs.
Click to expand...

Look at some of the GM plug gaps from the 1970s. I think it was Buick that had some that specced at .120". LOL
 
RustyRatRod said:
Look at some of the GM plug gaps from the 1970s. I think it ... Buick that had some that specced at .120". LOL
Click to expand...
Buick,& some Olds V8's...(those wonderful carbed,non feedback years..). If you don't have a handle on a.f.r,& emissions controls,add a bigger spark ,lean the hell out of it,retard the timing... The three best ways to cheaply achieve emissions standards,& kill automotive customer quality,& engines...
 
Unless you want to spend a lot of time and money with a four gas exhaust analyzer and a dyno, the only thing you can do is experiment with gap settings that give you the performance you want. Different engines want different settings, an open chamber head may not want the same gap as a closed chamber high squish head. Cylinder pressure greatly affects the gap, the more pressure the less gap it'll tolerate. RustyRatRod is right, even GM spec'd their gaps from .025 to .120 depending on what worked best for each engine.
 
I've never seen a spec for anything near a 0.120" plug gap on any engine by GM or anyone else. Got a link?
 
I agree that 120 mil sounds like a really big spark gap. 60 mil is more typical for newer engines. But, Ford was working on a head gasket with integral "spark fingers" that would throw a spark across the whole cylinder (2000 mil?). Not sure how they would drive the spark. Advantages were a simpler engine, better lean ignition, no heat loss to a protruding plug, simpler wiring. Disadvantage was a "plug change" then required removing the head.
 
slantsixdan said:
I've never seen a spec for anything near a 0.120" plug gap on any engine by GM or anyone else. Got a link?
Click to expand...
The Shopkey tune up spec manuals sold on the Snap On trucks listed some extreme plug gaps in the 1970s when HEI first came out, but by the 1980s there were a lot of engines with .080 gaps as the max and by the 1990s the max gap was .060, so I guess GM decided large gaps were not the way to go.
 
Lemme try that again: I've never seen a factory spec from a reliable source for a 0.120" or 0.080" gap on any GM or other-brand engine. Aftermarket compendiums of tune-up specs are notoriously full of errors. I'm eager to be proved wrong on this, but for now I'm sticking with "Nope".
 
You must log in or register to reply here.
-

Back
Top