smoothsedan

im having my b18a rebuilt and ive been looking at some pistons but im not sure what to get.....im hopeing to boost it in the future and i just want to know a little about compression in turbo and non turbo engines.......like some of the ones im looking at are 84.5mm, 9.0:1 and 85 mm 9.7:1 could anyone tell me what these numbers mean and how you would use each one??

thanks

z6hatchboy

The 85 and 84.5mm is probably reffering to the bore size (width) that the piston is. Compression in turbo engines will almost always be lower than in naturally aspirated (n/a) engines. This is because to much pressure period will lead to detonation and since turbo will increase pressure its better to have lower compression. Fuel in the right way when combusted in a cylinder is a slow burning process, when you have too much pressure (in the cylinder) the fuel becomes unstable (much like in a diesel engine) and explodes and thats what detonation is and thats what usually blows motors/blocks in high horsepower applications. Your numbers (9.0:1 and 9.7:1) are the compression ratios and you can be explained like this. We'll use the 9.0:1, with the piston at bottom dead center (all the way at the bottom), theres a certain amount of distance to the top of its bore. The distance is basically divided in 9 parts and when the piston reaches the top of its stroke it will be 1/9th of the way from top of its bore. So when fully compressed your cylinder will be 1/9th of that from being fully decompressed, hence 9.0:1. Lastly as long as you have the right amount of octance (a burn rate almost) you can do anything, you could run a turbo with 11.0:1 if you wanted, but most people dont have that and its not practical so that a major reason people use low comp pistons on turbo apps.

smoothsedan

thanks man, what about the bore?? is larger better for lower or higher compression??

smoothsedan

and what about dome/flat/inverted dome pistons??

z6hatchboy

One bore size wont really be better for low or high compression. A pistons compression number is set at the factory and not by bore size.

z6hatchboy

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by smoothsedan &raquo;</TD></TR><TR><TD CLASS="quote">and what about dome/flat/inverted dome pistons?? </TD></TR></TABLE>

The way a piston looks often can describe its compression ratio. A dome will be high comp (11.0:1+), flat is a moderate comp (around 10.0:1), and inverted dome or dish will be low comp (usually 9.0:1 and lower). Number are an educated estimate, lol.

smoothsedan

lol, ok so piston size doesn't really have an effect?

z6hatchboy

Not really, of course the bigger (diameter) the piston the more displacement you'll have (just for the ones who might want to argue that one, lol). But anything under .030 over standard size would just be cleaning the walls basically.