Spark plug gap is one of those things that some folks SWEAR makes a huge difference in power. It goes right along with the guys that think that indexing plugs is a big performance trick, but that’s another thing for another video. But does it really make a difference? Is there power to be had with wider gap? How about a smaller gap?
Now one thing we have to make sure you understand, is that we’re talking about naturally aspirated combos here. As Richard said in the video, the tight gap he had in the plugs to start this test were because it had been run under boost previously. Spark plug gap is something you have to pay attention to with a boosted combo because it can “blow out” the spark if the gap is too big.
But on a mile naturally aspirate combo does it care? Is there are reason for big gap?
Video Description:
DOES GREATER PLUG GAP PRODUCE POWER? HOW MUCH PLUG GAP SHOULD I RUN? CAN I ADD PLUG GAP TO ADD POWER? WILL LESS PLUG LOWER POWER? IS A CHANGE IN PLUG GAP WORTH ANY POWER? CHECK OUT THIS TEST WHERE I RAN A NATURALLY ASPIRATED 5.3L WITH A BTR RED HOT CAM. I MADE DYNO PULLS WITH THE PLUG GAP SET AS LOW AS .010 (TEN THOU) TO AS HIGH AS .100 (100 THOU). DID THE POWER CHANGE AT ALL? IS CHANGING THE PLUG GAP WORTH ANYTHING? CHECK OUT ALL THE RESULTS.
Back in around 1990, I used a .060” gap on a SBC which I knew was pushing the limits of my modified HEI.
I tightened the gap from .060” to .045”. Doing this increased the E.T. on my mid/high 12-second car by 0.15 seconds. I increased the gap by .05” and tested. I found that at each interval, my E.T. decreased proportionally until I went to .065” where I lost E.T. and had a little misfire so I backed it up eventually settling on .060”. On my combo, 15-Thousandths in gap was worth 15-hundredths in E.T.
It worked for me. Your results may vary…
If you add a little advance to the ignition will Probaldy get the same results
All you are doing is changing the size of the initial flame bubble. This effectively has the same effect as advancing the timing. If you adjusted the timing to optimize the power in each configuration, I suspect you would see no difference in power.
I would like to add an wxperience I had, and declare that changing spark plug gap also changes spark timing. Widening the gap delays the spark (effectively retarding the timing), and vice-versa. My proof of concept is the following. I have a Toro Wheel Horse lawn tractor with the 20hp Onan 2 cylinder engine. The Onan fires both plugs at the same time (because while one is on the power stroke, the other is on the exhaust stroke) so there is no distributor. Spark timing is done by a Hall-effect switch which is built-in to the flywheel, and (to my knowledge) is not adjustable. When my original ignition coil went bad I replaced it with 2 inexpensive automotive coils (because the Onan version was $180). The original recomended plug is .018″. My dual coil setup made a huge difference in how well the engine started and ran, but as soon as I started mowing grass, the engine almost died due to pre-ignition. The heavier the load on the engine, the worse it got. Since there is no timing adjustment (distributor) I theorized that the increased spark advance may be due to the fact that the more-powerful coils were able to “jump the gap” sooner than desired. So I increased the spark plug gaps and tried cutting grass again. When I had finally reached a gap of .060″ it had more power than when it was brand new, and no more pre-ignition. BTW, I tried this with the coils primarys wired in parallel, and then in series, and it worked fine both ways. I left them in series though, because it would be less of a load on the Hall-effect switch.