Correct plug gap
Thread Starter
Instructor
Joined: Sep 2019
Posts: 239
Likes: 31
From: Nashville, TN
Correct plug gap
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.
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.
Thread Starter
Instructor
Joined: Sep 2019
Posts: 239
Likes: 31
From: Nashville, TN
Melting Slicks
Joined: Apr 2021
Posts: 2,710
Likes: 2,156
From: Minnesota
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.
Thread Starter
Instructor
Joined: Sep 2019
Posts: 239
Likes: 31
From: Nashville, TN
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.
Melting Slicks
Joined: Apr 2021
Posts: 2,710
Likes: 2,156
From: Minnesota
JUNKYARD LS PLUG GAP TEST. DOES MORE GAP EQUAL MORE POWER? HOW MUCH SHOULD I RUN? .010 VS .100 GAP
Tech Contributor
Joined: Jun 2004
Posts: 20,905
Likes: 962
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
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
JUNKYARD LS PLUG GAP TEST. DOES MORE GAP EQUAL MORE POWER? HOW MUCH SHOULD I RUN? .010 VS .100 GAP
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Race Director
Joined: Nov 2006
Posts: 13,228
Likes: 7,825
From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
2024 C3 of the Year Finalist- Modified
2022 C3 of the Year Finalist - Modified
Cruise-In VIII Veteran
Yes I agree. I've always run .045 with HEI or most electronic ignitions.
Le Mans Master
Joined: Mar 2004
Posts: 7,493
Likes: 1,502
From: Little Rock AR
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".
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".
Tech Contributor
Joined: Jun 2004
Posts: 20,905
Likes: 962
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
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".
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".
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.
