Bigger pistons, smaller main jets

[trbig]

Yup.. That is what many professionals say to do. I've read quite a few articles on this and many are saying that if you do a big bore, you should go with at least one smaller size main jet. More vacuum pulling too much fuel with existing main jet.

What's your thoughts?

[DiverRay]

I'd say in most cases it's probably true. You would need to measure the flow before and after to know for sure. An "old" engine rebuilt with a big bore you would need it. Just a big bore, depending on the percentage of change, you may not.

[Bonz]

When I put the header on my ZRX, Dale Walker (Holeshot Performance) said it may richen things up because it allows better "draw" than the stock headers. But that was changing diameter, and shape of the exhaust which changed outflow potential and dynamcs big-time. Gaining 20 Hp and 7 ft/lbs of torque over stock, over a wider rpm range, has to flow more as we all know. The only way to do that is add air and fuel on the front end, thus the increased power with a jet kit and better airflow in the form of pods or modified airbox, that kind of thing.

Makes similar sense with a big bore, however the limiting factor is still how fast/much exhaust gas can get out in a given time to allow fresh mixture to come in, and the exhaust system controls that. You won't re-jet on the front end if you can't flow more on the back end, at least that is how I found it to work.

My question is with the same valve sizes and not a change in the exhaust diameter/system, how is it gonna pull more on the front if it can't get more out the back? It has also been said on the forum that the big bore doesn't do much, if anything, for overall power that can be felt other than a fresh build feels better than the old tired one it replaced.

I would leave the jetting alone and if you need to dial back on the fuel screws a bit that would be the place to start and go from there.

[MPittma100]

Jets?

I'm thinking that the air flow is governed by the valve size and camshaft lift/duration. Don't think flow rate will change unless one or both of them are changed. Would start with existing jet and monitor plug coloring.