Spark Plug Gap

[RocketFox650]

So our boat has a slight miss, and I brought up plugs to my dad multiple times and got the "they're gapped at the factory" deal. Even arguing "that doesn't mean they didn't get bumped shipping" didn't work. We all know the don't come pregapped. I had to do plugs today. Well guess what.... 4 plugs with .13+ variation across them! Showed him that, so hopefully that'll solve our miss.

My question- most online resources are .20 or .21-.24, (one site .43?? Wtf!?). I know the theory on gapping with land vehicles. What should I expect on a 787 motor and the 500 series motor from 96, if I play with gaps? I'm looking for more performance without doing damage. They're set at .205-.21, which is the minimum on my gap coin. Better to go middle ground maybe? Or will my performance goal, a better holeshot, be better served at the lower value? I don't care about top speed so long as I don't lose a lot. But if I'm already gonna be doing plugs, thought this might be an opportunity.

 

[amcreativ]

I'd say since the engineers that designed these say a certain tolerance for gap, I'd keep them there. No doubt they tested many variations and came up with the best performance. My 951 calls for NGK BR8ES gapped between 0.020 - 0.024 in. I gapped mine right in between, I have tried the lowest gap and highest with absolutely zero change in performance. Also, my NGK BR8ES are never pre-gapped, they are usually more than 0.024.

 

[mikidymac]

You will see no performance gain. Set them to the factory spec. Too large a gap is hard on the coil and could potentially not fire if the spark energy can't bridge the gap.

 

[AKnarrowback]

Never gapped a plug in the last 25 years unless it's been crushed or bent out. You aren't going to gain any performance with gap, you can index your plugs and that still won't gain any noticeable performance.

 

[GGuillot]

Never gapped a plug in the last 25 years unless it's been crushed or bent out.

You don't at least check? I'm not finding these 3961 NGK's to be the same out the box, and they're not within spec called for. I swapped them on my 97'GTX last weekend, and the were both about .30 and .31 I tapped them and brought them down to .22, which spec is between .20 to .24.

 

[amcreativ]

You don't at least check? I'm not finding these 3961 NGK's to be the same out the box, and they're not within spec called for. I swapped them on my 97'GTX last weekend, and the were both about .30 and .31 I tapped them and brought them down to .22, which spec is between .20 to .24.

My findings as well. They are usually around .030 out of the package

 

[AKnarrowback]

I don't measure them, I tap the gap down a hair a run them. Not proffesional I know.

 

[mikidymac]

I am one to admit my flaws... never gapped a set of BR8ES's in the last 25 years and never had a single issue. I did start doing it two weeks ago though. So I guess you can teach old dogs new tricks after all.