Hello,
I'm in the process of recovering from a fried ignition due to some questionable wiring and installation choices by the previous owner. The motor is a rebuilt 74 installed into a 72. The specifics of the build are unknown other than what I can see which is Flotek 180 aluminum heads, aluminum intake, roller rockers, 4 barrel Holley, and what seems a mildly aggressive cam. There is also a 3-core aluminum radiator. People with some experience are suggesting it makes around 375-400HP. I am replacing the burned out HEI ignition with an MSD Streetfire distributor and fitting MSD Super Conductor low resistance leads. Flotek recommend NKG R5671A non-resistor plugs but don't recommend a plug gap. The plugs come pre-gapped at .031. Multiple sources recommend that with the MSD ignition the gap should be wider (some say up .060) but I can not find any recommendation for Flotek heads AND MSD ignition, and not sure if the combination even matter.

I want to balance reliability with performance and not just err on peak performance considerations. Any advice on the right plug gap for this set up?

Thanks in advance.

 

Run the gap at the standard .035". The style of heads is irrelevant.

 

I like that idea. Any idea why Flotek are being specific about the type of plug they recommend?

 

I have the same heads on my car. The instructions specified Champion RC12YC plugs which I wasn’t able to find at any parts store and had to get online. I’m not sure why they’re recommended, but I later changed them out to some NGKs in the same heat range. Haven’t had any issues.

 

This is what Flotek is now recommending for my model heads. This is a non-resistor spark plug which suggests Flotek believe the heads will perform better with a more vigorous spark. I would have thought the gap would also matter but they don't offer that suggestion.

 

I saw an interesting Richard Holdener video where he tests different plug gaps on a dyno. What he found is that even a .090 difference makes less than a 1% difference in peak power. It seems that gap has a lot more to do with longevity than any sort of actual performance.
JUNKYARD LS PLUG GAP TEST. DOES MORE GAP EQUAL MORE POWER? HOW MUCH SHOULD I RUN? .010 VS .100 GAP

 

He kinda showed there's no free lunch in this world. Any ignition coil only has so much energy stored into it during its dwell time, and that's all you're going to get out of it. A large plug gap will require/show a large voltage when firing the gap, but the downside is a shorter arc time. A small gap requires a smaller voltage to arc over, but the up side is a longer arc duration. A coil and a bucket of water share a function. They both store the desired contents, and you can empty them fast or slow. It's your choice, but either way, you only get out what you put in there initially.

 

gab depends on coil rating.
before hei, your car, gaps were .035

hei, gaps went to .045

 

Yes I agree. I've always run .045 with HEI or most electronic ignitions.

 

Gaps were .035" before HEI. The regulations of the mid 70's caused GM to need to use LEANER mixtures which required a bigger spark gap to fire the plugs reliably. The bigger gaps required a bigger ignition system and fatter plug wires just to get the job done. That is why we got the HEI. Not the other way around.

If you are running a HOT ROD (non-emission style) engine calibration, you will not "need" the bigger gaps or the "HEI". You can just run .035".

 

You are correct. The plug gap is determined by what the cylinder needs to be able to get the mixture lit off reliably, and then the ignition system is then chosen to be able to supply the needed gap voltage and energy. As you said, not the other way around.