spark plug gap
 

looking for suggestions on the best plug gap for my 467 E-head stroker & for the average lower power street engine thats maybe 100-150hp above stock.

ive read most people use ~.045 that was a reduced gap size from GMs original ~.060 for hei's & ive been using that for 20+ years with no noticeable issues, but then i read this from NGK that talks about reducing the gap .004 for every 50hp above stock, which is kinda confusing considering the range of hp increases on pontiacs today... if an engine is say 250hp over stock they want you to reduce the gap .020?? never really heard of .025-.030 gap sizes on hei engines.

Reference: From NGK's FAQ: Spark Plug Gap -
"Another consideration that should be taken into account is the extent of any modifications that you may have made to the engine. As an example, when you raise compression or add forced induction (a turbo system, nitrous or supercharger kit) you must reduce the gap (about .004" for every 50 hp you add). However, when you add a high power ignition system (such as those offered by MSD, Crane, Nology) you can open the gap from .002-.005"."


heres some general specs of the 467.
RPM round port 72cc heads. 10.7 comp.
NGK plugs
original OF cam, 1.5 HS rockers.
SD ported HO intake & cliff built Q-jet.
sun tuned prepped HEI with accel in cap super coil & "MSD HEAT" in cap module but also have run the stock GM module & will again if/when the MSD dies.
i use 91 straight fuel but occasionally will use 93 10% ethanol.
let me know if any other details are needed.

im mainly looking for what most guys with similar ~500hp combos are using for gap size with HEIs as well as mild street engines in the 350hp range using a stock AC delco r45 type plug.

Yes that's pretty much the general consensus as HP increases you want to tighten the gap.

Some of the big HP stuff running around, especially power adder stuff, will run a very tight .020 gap or there about. One reason is to keep from blowing the spark out, on 1000+ HP stuff. Some of the more popular power adder engine builders have talked about that in detail.

I've always kept mine around .035 even on the 300-400hp stuff whether they have a big whiz bang ignition or not. On the 600-700hp stuff I stick with .035 as well.

I've never found any gain on a dyno or any noticeable difference in drivability by opening the gap just because it had a zapper ignition. In fact Smokey Yunick decades ago showed that it really taxes the coil in doing so and found no other benefit.

So on the MSD stuff here or any other style electronic ignition, I stick with .035 and find no need to change it, at least up to the 750 HP level.

A question: do you need to advance the timing when going to a tighter gap to compensate for firing faster ? Just wondering.

Timing is determined on the dyno when best power is achieved.

Or at the track with back to back testing.

Never found anything playing with the gap.

Similar engine combo here with OEM HEI, MSD "street fire" box ( i know, pretty high end stuff lol) and standard oem coil. I have run .035 to .045 , but have stuck with .035 due to the reasons mentioned below. All seems well with 91 non ethanol and 93 10%, which is what I mostly run.

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I would , and do, keep the gap at 45. As it wears, it gets bigger. This will not be desirable on a engine above stock.
Want CHEAP improvement? Put in a low resistance "carbon" distribution button in the cap.. pic below.

thanks for the replies so far, im aware of the boosted engine gaps being lower but for a NA ~500hp or the average mild build of 300-400hp ive always heard .045 is the best general range & that a hotter/bigger spark was better most the time.

i do use the low resistance button in the cap for the super coil.

i have always set all my plugs to .045 maybe .048 due to a cheap gap gauge & never had any issues, but after reading that NGK FAQ i may try & lower it to .035. i wouldnt really consider the super coil or MSD module being "wizz bang" stuff, so maybe a little less gap is worth trying.

only thing i noticed on gap sizes as i've tried narrow to wide is when i would go past 40 on the gaps i would get break up at higher rpms under load (runs)
almost like rev limiter kicked in
bring the gaps down, and no issues
had this experience with various cars/toys too (i know... just my luck)
and i've had this with various ignition setups and various MSD boxes/coils despite the MSD instructions on the wide gaps
latest test was with the MSD7a and MSD8 boxes with the red and blue HVCII coils
oddly the only one without an issue with over 50 on the gap was HEI....

just my experience

right now using Brisk LGS-T plugs so i dont worry about gaps/indexing
screw-em in and go....

Sorry I should have been more clear. When I mentioned the MSD I was referring to the digital boxes and multispark stuff with hotter coils that I use here. Gap is still .035 with that stuff.

I've run all kinds of SCRs, heads, cams, distributors, ignition boxes, etc, and in my engines, I've always used .035-.040.

On my last iron head 400 which was about 9.95 SCR, it would det at anything above .035.

On spray, I never went over .035.

Bigger gaps also would give me a hotter operating temp. Between .045-.060, I could see as much as a 10-15 degree increase in water temps.

Wider gaps also wore caps & rotors faster, though never really saw plug life impacted, but I did change them fairly frequently.

Even now, with aluminum heads, and reasonable SCRs, I still run between .035-.040. Some plugs come pre-gapped, otherwise I would just run .035 on everything. I pretty much try to run plugs that allow for gap changes lately, some either can't be or have issues when trying to change the gap. (like some of the special tips ones).

Gaps can change timing requirements too, it was explained to me at one point, but foggy at the moment on the topic, mainly because at one point I just decided to stick with .035. Flame front I believe was the subject, and placement of the tip in the head.

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ok. i have an MSD 6al box i have been considering trying with their separate "blaster" coil. in your opinion would that be worth using over the on the cap super coil & GM or MSD in cap module i have now?

i have held off on using the 6Al box due to some bad reviews & stories of being left stranded on the side of the road with some MSD stuff. & i only tried their in cap heat module for the built in rev limiter without having to hook up a separate box. im open to trying the box if its better than the stock GM module. its mostly a street car so reliability is a concern.

Wider gaps mean more rise time for coils too, and can add strain there.


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Thanks for all the good info here as there is alot of confusing info out there. Back in the '60's we gapped everything automotive at 035. Then the '70's came along and 045 was THE gap to use. After reading this thread, I'm going back to .035 on the Pontiac. The late model cars I can't decide on what to do (like my wife's 2018 Ford escape with direct FI). Manual spec is probably best.

Yeah some of the MSD reviews as of late haven't been that great. They are no longer made in Texas and I think that's when the troubles started.

My experience with them has been more good than bad, as far as the boxes go.
2 cars here with MSD boxes, coils, and billet distributors. Both of them have been on the cars for 25+ years, purchased back when the stuff was made in Texas. They have both been flawless for decades and thousands of miles.
Fast forward to today. I've bought 2 more MSD 6 boxes for other cars. One failed right out of the box. Exchanged no problem and it's been fine since. The other is in one of my daily drivers and for 2 years now has been fine. However I have that box running on the stock points distributor so that if it ever does fail, I can swap the coil wires around, bypassing the box, and run on points alone. I really do like this setup for simplicity and reliability on a daily driver. And the box has helped to keep the plugs a little cleaner with most of the 5000 foot elevation driving that I do and having that car run a little on the rich side anyway.

As far as MSD stuff for the HEI distributors, I've had less luck. I converted my 454 pickup to HEI module and coil with built in rev limiter on the original HEI distributor. The first setup popped and farted on occasion, then would clear up and be fine for a while. MSD wanted the whole setup back to test so I did. They found the module bad, but sent me a complete new module and coil setup to replace everything. That one worked fine for years and my wife daily drove the truck. I did not run a box on that one however.
That was the last HEI I ran, both with stock module and with MSD parts. Owned that truck 25 years, towed across country etc... and the stock HEI stuff never let me down. When I switched to MSD parts it was 50/50

Nothing against HEI's, I like them, I just don't use them anymore unless the vehicle was factory equipped, and I don't have a vehicle in the stable anymore that was factory HEI, hence the reason I don't use them now.

On the modern EFI stuff I follow the same procedure.

We were into LS engine stuff for years and had 3 SS camaros from 98-2002 as well as an LS swapped Blazer. Typical coil on plug high energy modern ignition system of the day. On all of those I switched them over to copper core plugs and .035 gap and daily drove that stuff for years and 100's of thousands of miles.
At that time many of the LS tuners and engine builders I was around were also doing the same thing, copper plugs and .035 gaps and sort of became a wide spread thing among the LS community at the time. Copper core plugs conduct electricity better and the tuners all seemed to like the tighter than factory gap. Haven't been into the LS stuff for years now but I doubt any of that has changed.

X2!!!
My engine builder (of course he is a Chevy guy) always used small gap plugs, gapped at .035, in ANY engine, he also liked the non projected tip plugs, when available per application. He liked to keep that small piece of nearly always, molten petal away from the piston top, just a little bit more...

That's where I differ a bit. I've always liked the projected tip plugs and relied more on my ability to provide a safe tune that won't melt the plug. Now if I were running a bunch of nitrous I'd rethink that but for N/A, projected tip is fine.

Most of the knowledgeable people I was around referred to the non projected tip plugs as "oil burnin plugs" LOL Kept the electrode out of the path so to speak. The projected tip got the spark out into the mixture where all the action was. So that's what I use on all the street cars here.

No. It won't affect the timing, but it will affect the ability of the spark to ignite the fuel

The photo is from the MSD Digital 6 Plus installation manual. Only interesting thing here is they only corelate compression levels to recommended spark plug gap. I keep my cast iron headed engines at 9.2 to 9.3 compression and find that .045" gap works best. Basically the wider the gap the hotter the spark and that helps in lower compression engines. My old drag engines in the 80's were over 13:1 and we kept the sparkplug gap at .035". High pressures and wide gaps don't work that well. Note that GM basically went to the .060" gap on their ridiculously low compression engines which were sometimes under 8:1.

Anyone remember the old Victor sparkplug cleaner and tester? The test part was really to sell customers new plugs. The coil in the tester was rather anemic and one could quickly see that the main difference between a plug firing and not firing was how high the adjustable air pressure was set. The mechanic would leave the gap huge on the old plug and would make sure the new plug was gapped tight. Old plug would fail to spark about half pressure while the new plug could fire with full air pressure. Of course you could take the old plug and put a tight gap on it and open the new plug up and get exactly the opposite result. I'm sure that machine sold thousands of new plugs whether they were needed or not.

Anyway, back to gapping plugs. There's something to be said for a wider gap cleanly firing off a low compression mixture -- and also something said about a high compression engine running race gas not needing or wanting a large gap.

Edit: Opps, Vixen and not Victor

Its hard to go wrong with a .040 plug gap.
Its only going to use the least amount of current to jump the gap anyway. A larger gap does not necessarily mean you will have a higher voltage across the gap.