Don Terrill’s Speed-Talk

Bill Jenkins said in his book that he found 15 hp in one of his pro stock engine by changing the plug gap from .035 to .060 anyone else found power in the plug gap.

On my 421 pulling truck engine I had started it at home before taking it to the dyno, and had just stuck the plugs in without checking the gap,...planning on going over things once more before dynoing, but plugs stayed in.

After some break in pulls and a few tuning pulls I went over things and caught the plug gap. They were at .015" and I thought this will be some power,...but opening them up to .035" netted nothing. Same power at .015" or .035".
It was with a MSD 7al box.

Randy

I read the same Jenkins tidbit. Was curious and dyno tested the theory up to .080 gap. Unless there is more to the story than plug gap, I haven't seen any power.

Back in the 80's my friend had a Firebird with a very low compression ,7.7 to 1 stock, 400. The factory gap was .060". It didn't really run better or worse at .035 but if the .060 opened up just a little it would start missing. At .035 it would run a long time before you had to mess with it.

No doubt there is an optimum plug gap for any particular engine. Ignition system energy, voltage potential and insulation will all play a part.

The Jenkins story simply shows that one of the engines he built worked best with a 0.060" plug gap. Insufficient gap can cause a misfire but in less severe circumstances a slow initial burn (flame kernel formation) which has the same effect as retarding the spark for that particular cylinder/cycle.

One thing for sure, with big gaps and big CR it's a lot harder to keep the sparks where they belong...

I believe Vertex Magneto co. Said less gap it more desirable, plus or minus a little depending on conditions plug is firing under. Mark H.

MadBill wrote: ↑Thu Nov 29, 2018 4:01 pm One thing for sure, with big gaps and big CR it's a lot harder to keep the sparks where they belong...

YEP!

swampbuggy wrote: ↑Thu Nov 29, 2018 6:17 pm I believe Vertex Magneto co. Said less gap it more desirable, plus or minus a little depending on conditions plug is firing under. Mark H.

I think it is .020 on the positive side and .018 on the negative. Might be wrong it has been a few beers ago!

tresi wrote: ↑Wed Nov 28, 2018 5:05 pm Back in the 80's my friend had a Firebird with a very low compression ,7.7 to 1 stock, 400. The factory gap was .060". It didn't really run better or worse at .035 but if the .060 opened up just a little it would start missing. At .035 it would run a long time before you had to mess with it.

...and some Oldsmobeaters had .080 recommended gap. GM revised that figure after a while, went down to .060 or thereabouts.

New vehicles can/could get briefly fired 40 times or more before they hit the dealership. In the carburetor days plug fouling was a major issue in cold winters with the giant gaps. The truck drivers used to pop the plug harness loose and lift it up ~ 1/4" from the HEI cap to create an auxiliary air gap to get them started...

swampbuggy wrote: ↑Thu Nov 29, 2018 6:17 pm I believe Vertex Magneto co. Said less gap it more desirable, plus or minus a little depending on conditions plug is firing under. Mark H.

If I remember right, I'm pretty sure their rationale is that gaps higher than that would produce high enough arc over voltages that it could reduce the life of the mag. That might be a wives tale though..Some of then may not even be able to fire a large gap at low rpm.

I personally have never seen any dyno gains from spark gap or indexing plugs. The bigger the gap the more energy it takes to fire the plug. The electricity has to ionize the air to carry the spark. Change gaps and put an ignition scope on it and observe the firing lines and burn time at the plug. As turbulence in the cylinder and compression increases it takes more energy to fire as well. I have settled on .030 on most normally aspirated engines for gap, and .020 on super charged engines. Newer cars with the long 100,000 mile plug change develop huge gaps, and it stresses the coils trying to fire the plug. Most high mileage cars that I get that have burnt out a coil, almost always have huge plug gaps when you check.

Schurkey wrote: ↑Thu Nov 29, 2018 7:48 pm

tresi wrote: ↑Wed Nov 28, 2018 5:05 pm Back in the 80's my friend had a Firebird with a very low compression ,7.7 to 1 stock, 400. The factory gap was .060". It didn't really run better or worse at .035 but if the .060 opened up just a little it would start missing. At .035 it would run a long time before you had to mess with it.

...and some Oldsmobeaters had .080 recommended gap. GM revised that figure after a while, went down to .060 or thereabouts.

that was because the spark was arcing thru the wires where they touched metal and they would not start in very cold weather

Out of interest from this thread, I experimented yesterday with a wider gap. Bear in mind this was one test, on one engine only, so the result is hardly statistically strong, but nevertheless, here's my finding.

The engine is a single cylinder air cooled vintage bike motor, 38ci/620cc. Modified factory aluminium head, aftermarket crank rod and piston, 10.5:1 SCR, good intake and exhaust, MSD Streetfire ign and tailored advance curve, 4000-7000 power band, and approx 49hp.

I opened my normal 40th gap on an NGK B7ES to 60th. Under load in higher gears, the bike was noticeable flatter, nosed over early, and on one occasion had a brief moment of just detectable ping/detonation. Acceleration in the lower gears seemed unaffected.

Changed the gap back to 40th, and the performance returned. No changes to fuel or timing over the test.

My experience- your result may vary.
Regards
Paul