what are the differences between 8.5:1 vs 9.0:1 Pistons ? why do some modded evo 8 run 8.5 and some run 9.0 ?? can some one explain in detail please. thank you

 

CBRD, Buschur, AMS ??? anyones ?

 

the compression from the piston and the compression from the turbo are a trade off basically. The higher compression pistion you run, the less boost you will be able to run if you hold everything else equal.

a higher compression piston is going to provide better off boost response, but allow for less boost once it hits.

 

^ The above is sorta correct. Higher compression allows for less off boost power. And very slightly more power under boost, Its just higher compression creates more heat aswell. So Higher compression and boost will make the pre-detonation threshold come much sooner. Which usually isnt a problem because most of the companies run race gas anyways. Its all personal preference and tuning preference. More boost with lower octane requirements with lower compression, And little better off boost response with higher compression.

 

A benefit of Higher compression is that it can help to spool a turbo sooner.

 

more power on less boost basically, but you can no longer boost as high as 8.5:1 compression pistons

 

Go 8.5:1

 

lol. thanks guy. I have Arias 8.5:1 piston.

 

Actually, higher compression gives better thermal efficiency and creates less waste heat.

Actually, the effect of higher compression tends to be the opposite if anything.

 

I run a 2.2 stroker with JE pistons and my Comp ratio is 9.4:1

When I mapped it on the engine dyno I ran upto 2.5 bar on 97 octane with quite aggresive ignition, no det at all. Infact at one point a pipe to the Tial wastgate came off and boost rose to 3.2 bar midrange, with no det heard through the earphones.

I got 620BHP on 97oct at 2.2bar, 725 on race gas.

Comp ratio is only half the story, I also reprofiled the combustion chamber shape quite dramatically, and changed the width of the squish area. Squish dimension also changes things where det is involved.

So don't discount higher comp ratios, they can work very well in conjunction with the right engine spec.

 

I was talking about cyl heat, If im wrong thats ok too, I just think you mis understood me. Cyl temp rises and causes more detonation that way.

 

Ted B know his stuff. So from i gather is 8.5:1 spool faster than 9.0:1 Thanks everyone.


by the way what power steering fluid does our evo use?? DExtron II ??

 

This is pretty true. When I used to build turbo'd Honda's the best performing builds I had were a b16 (1.6 liter) that ran a 10.5:1 c/r. You weren't able to run butt-loads of pressure like with a low c/r motor, but you did end up with a decent torque curve due to the efficiency of the motor being better and thus a more advanced timing curve. Also a higher compression tends to move the combustion process further into the head where it belongs, and produces a cleaner and more complete burn (i.e. less waste heat) and aids in proper flame propogation which reduces detonation at more advanced timing levels. There is more to it, but take a quick look at most all-out forced induction drag cars...they usually run near the 11:1 c/r (and sometimes higher) and monstrous amounts of pressure. The key here is tuning, as a higher c/r you tend to be on the "edge", where as lower c/r's have more room for error.

 

well i do a lot of roadracing course. So i think 8.5:1 would be safe.

 

very much what he say. You can't really feel the different on half a point of compression. But if you go from 8.5 to 10.0 Cr pistons, your turbocharge will be very peppy. And your tune better be death on. Since higher compression means less tolerance for mistake.