Inliners International Forums General Discussion Hi Performance Theory; lower compression

What would be benificial effects if any, from setting a flatop down in the bore (292 l6)turbocharged,10 psi,in order to lower the overall compression ratio. Bracket race applications. Would this be as effective as a reverse dome piston or as efficient. Any ideas/opinions? Thanks.

I have a stock compression 250 I'm planning to turbocharge right now. It's 8:1 compression and I intend to leave it there. With the lower compression the efficency goes down 3-5% (from 10:1 say) but I gain it back in what I can add in boost. I can't advise you on how to lower to compression (I don't know about the pistons all that much) but I can tell you that you'll be able to get more kick out of the engine with less compression (w/ turbo).

Just my opinion

[This message has been edited by Greg (edited 01-29-2004).]

Let's see if I can get my thoughts clear enough to type.

A flat top piston is better when it comes to proper flame front travel. I don't think dished pistons actually run into the same problems that domed pistons due concerning that though.

With the flat top pistons, how would you actually get the piston lower in the bore? You can raise the pin height, but that would mean a custom set of pistons. Not to mention moving the rings closer together. If you ran a flat top you could run a larger combustion chamber, or run thinker head gaskets.

I honestly don't think there would be any major or minor gains from running a flat top piston. You'd be better of getting a set of stock forged pistons. I'm not sure, but wasn't the stock CR 8.5-9.0 for the 292? You should be fine with a CR of 8.5:1 and running 10psi as long as you run a good intercooler setup, head studs, o-ringed head gasket from Fel-Pro, etc. Anyone correct me if I'm wrong. Hope this helps. John

Thanks for your inputs! It seems I try to make things difficult by trying to think too much. Simple is better, as 8.5 c.r. is max in this case, and juggling rod length/compression height/ring packages doesn't do much for my headache either. Thanks again.

The stock compression on the HD 292s can be pretty low i think ive read that it can get to the 7's pretty low, but its worth looking at in that kind of application

I think the one concern here would be using a forged piston and I really think the flat top would give the best burn-off situation. S/B 283 flat top pistons would increase the compression but if you used a 307 flat top piston you might get the advantage of the flat top a with compression closer the the stock low compression dish pistons.As always the lower the compression the more boost you can use. ---Paul

A TRUE flat top(not valve notched) of either pistion in the 292 will up the compression and Depending on the head thats used Or how much it may have been milled off it.Or angle milled can an will bring you anywhere from 10-1 to 13-1(plus?) You less you have the piston lower in the bore Our have a piston custom made. }[oooooo]

Thanks,certainly something to consider. I just recieved a set of Perfect Circle .060 8 to 1's from e-bay. forged 292 (not produced for 10 yrs now). They weigh a whopping 881.3 grams including the stock pin! They have the D shaped dish,which am assuming would provide some squish or quench. Compared to the cast piston these things are on steroids.