Spark plug gap
 

My engine uses the external-coil distributor and the 8-pin hei icm. I'm running one of those aftermarket MSD 8226 coils. It is advertised as higher voltage than stock.

Should I open up the plug gap a bit or just leave it at stock spacing?

I'm running the stock Delco icm. Seems to be working fine. I've had a couple aftermarket icms literally melt and come to pieces, whereas the stock GM unit hangs in there. Is there a better alternative?

Re: Spark plug gap
 

When you replace the ICM do you make sure the thermal grease goes on clean and it snugs down onto the metal plate?

AN HEI on a stock LB9 I wouldn't think would do much. But you could open up that gap a bit I suppose.

Re: Spark plug gap
 

Yes of course I used the thermal grease and made sure there was no dirt or crap underneath before snugging it down. Two aftermarket icms literally melted down during summer driving. Engine just stops and won't restart. When I go to take out the bad icm, it come into pieces in my hand. The stock Delco unit does not, it hangs in there.

Re: Spark plug gap
 

I have the DUI distributor, ICM, and matching coil. They recommend Vortec plug gap - .060" for their system. But they have an ICM rated for this.

Higher voltage means more demand on the ICM. It will run hotter. Same with larger plug gap. Voltage is directly proportional to plug gap. Plug gap (and cylinder pressure) is actually *responsible* for the voltage. As voltage will build till it can jump the air gap. You can easily burn out ignition components with too much plug gap. The system will built voltage till the spark occurs - regardless of what the components are designed to handle.

"HEI" is "High Energy Ignition" and is a marketing term. All these cars had GM's "HEI" system stock. Really doesn't mean squat since the HEI's ran a pathetic .035" gap, and the Vortec ignition was .060" plug gap (a LOT more spark voltage).

GD

Re: Spark plug gap
 

I thought stock for these cars was 0.035, you can open it up to 0.040 or 0.045 but I wouldn't go bigger. GM toyed with opening up the plug gaps on the LS family engines and went up to 0.060 but they later put out a service bulletin saying to lower gaps back to the tried and true 0.04 range. Even the new engines like the LS7 are only calling for 0.040. Unless you're going to go with a big boy real external coil like the MSD hvc coil all of these little external coils are going to experience incomplete charging times above 3500 rpm. Some are going to be worse than others. DIY autotune sells possibly the best small external coil but you'll have to do a little wiring. AC Delco ignition control modules are the best ones out there and if you can find a NOS module that would be your four leaf clover.

Re: Spark plug gap
 

I have an older GM Delco icm in my dizzy now. it has been in there for probably 15 years. It is the 8-pin model. I was just curious if there was a better one in the aftermarket. I tired a couple, Pertronix I believe, and they just disintegrated from the heat. When they failed and I was removing them from the dizzy, they literally crumbled to pieces in my hand. I went back to the stock Delco and have had no problems since.

The MSD 8226 coil is a stock style replacement, mounts on the existing external bracket, and they say it puts out more voltage than the stock GM coil. I was just wondering if that means I should open the gap from the stock 0.035 to perhaps 0.040.

Re: Spark plug gap
 

You can try .040" and see if you like the idle better, etc. It's a matter of what the engine wants, but all other things being equal - if you enlarge the plug gap you will create a larger, but colder/weaker spark, and it requires a lot more spark energy - driving up heat in the module and coil. The higher you go in performance, the more spark energy you need, and very often this means closing the gap. Just as we are all aware that too large of a gap will cause misfiring, so will increasing the number of molecules between the electrodes - either with more fuel (richer AFR's), higher compression, forced induction, higher RPM's, etc. Typically decreasing the gap will actually provide a hotter, denser spark and will ignite richer mixtures at higher pressures. Also filing back the ground electrode till it only covers about half of the spark electrode will expose the air/fuel mix to the spark more easily and can help with smoothing the idle and may be good for a few HP also.

GD

Re: Spark plug gap
 

Have I missed the string of magazine tech articles and tech threads where folks have discovered HUGE GAINS doing this one simple trick to alter plug gap? Idk, but the only bits I remember reading were usually in reference to carbed engines, and included tricks like altering the plug gap and indexing the plugs, blah blah blah. Really I can't remember ever hearing anyone state they gained much of anything that couldn't be considered a placebo effect, by altering anything in an ignition system.

Can I go out on a limb and make the jump that maybe we're talking about enlarging the gap to take advantage of an aftermarket coil because the coil alone, didn't seem to make any quantifiable difference? :huh: I only mention it because it seems like most of the critical thinkers historically do a full ignition upgrade, and then want to know why it didn't do a thing. Certainly have found that sentiment to be more common than 'I gained two tenths by opening my plug gap .020"!'

Re: Spark plug gap
 

If you want to do that just switch to one of the Iridium plugs. The majority of brands for iridium use the needle point on the electrode and then grind the sides of the arm to make point. The autolite iridium on rock auto are a good price and there is a good mail in rebate bringing them down to I think $2 per plug.