Does a higher comp piston require less fuel??

[wzombie03]

i have a hotcam stage 2 cam and 86mm 406 kit piston set 10:1 just installed, but now after rebuild spark plug black so im runing rich with mid throttle bogging. i already had the hotcam so only change is new piston, do i need a SMALLER main jet due to too much fuel. i guess higher compression requires less fuel?? HELP~

[STUNTIN400EX]

a higher comp. generally requires more fuel.

[wzombie03]

hmmmmmmmm im stumpped

[wzombie03]

thats what i was thinking too (more fuel), but not i am running rich for some reason.

[GPracer2500]

Why would higher compression need more fuel? If someone can explain that I'm all ears.

Higher compression sometimes requires leaner jetting specs. Often it's not a dramatic difference though.


It's not that higher compression actually uses less fuel. It's that the higher vacuum signal (created by the smaller TDC combustion chamber volume) pulls more fuel through the same sized jet. The amount of fuel delivered through a jet is not solely a function of the size of the jet. The amount of differential pressure seen by the jet also has a big impact. So with higher compression you sometimes need to use a smaller jet to deliver the same amount of fuel you were getting previously with a larger jet.

It's like sucking through a straw. If you suck hard through a thin straw you can get the same amount of liquid as you'd get by not sucking hard through a fat straw.


That said, I'd be surprised if a relatively small increase in compression suddenly made your engine way too rich. Especially with the additional 9cc of displacement. Make sure the carb and air filter(s) are clean. Spray every orifice (even the ones with no jets) with carb cleaner and then compressed air if you haven't already. But it's possible you where borderline rich before with a worn piston that wasn't pulling a heck of a lot of vacuum and now with the fresh, higher compression piston it's just pulling that much more fuel through your same sized jets.

$0.02

[boosted3g]

couldnt have explained it better myself. Perfect explination

[wzombie03]

ok sweet, thanks GPracer2500...im going to try adjusting my needle jet first and see if i can lean it up a little. can anyone help me on doing this????