Wiseco 11:1 Piston

[smorris1]

I am thinkin about getting a 11:1 stock bore piston. I have a 155 main jet and a 40 pilot. Will I need a bigger main with the higher comp piston?

[drew416ex]

Yep, it will prob be around a 160 main. Not sure but you may have to bump up the pilot to a 42.

[GPracer2500]

You don't need to use richer jetting specs when raising compression (assuming all else is equal--bore, porting, etc). If anything, it would be more likely you'd need to use smaller jets. Sounds strange, I know. But with a smaller TDC combustion chamber volume (i.e. higher compression) the engine can potentially produce a stronger vacuum signal in the carb at the beginning of each intake stroke. This can push more fuel through the same sized jet, hence the need for "leaner" jetting. High compression doesn't require any more fuel than lower compression. Air/fuel ratios are about how much air and fuel the engine consumes with each cycle, not what the engine does with that mixture once it's trapped inside the cylinder.

Your actual air/fuel ratios won't change with this "leaner" jetting scenario. It's just that size X jet can flow different amounts depending on the vacuum applied to it. The greater the vacuum signal passing through the carb the smaller the jet you need to maintain the same fuel flow.




In my estimation, the most likely thing is you won't need to do anything. Even if your engine did want a smaller jets I'd guess your looking at maybe one jet size. The only way to know for sure is to test your jetting once your engine is built.