View Full Version : Compression question

08-09-2002, 09:39 AM
This post is not 240 or even Nissan only -- but I like the people in this forum so I figured maybe I could find an answer.

I know that the base compression of the SR20DET is 8.5:1. What I've been wondering is, when you upgrade the internals (pistons specifically) of a low compression engine such as the SR20DET, what specifically would I be trying to accomplish? If I put higher compression pistons in it, I wouldn't be able to boost as much, and if I did lower compression, I would be able to boost slightly more but the lack of base compression would mean I'd really have the same amount of power. Or do people usually only upgrade to stronger materials? Like staying at 8.5:1 but with forged pistons...a friend of mine and I were BSing last night about rebuilding an SR and this question came up. I bumped into a guy I used to race with and he told me he's putting GTI-R 8.2:1 pistons in his engine and selling it. Explanation would be greatly appreciated.


08-09-2002, 12:15 PM
i think for the majority of a pump gs mobile stay with 8.5 unless you want super high boost, then go 8.0. or if you are a race gas ***** go 9.0.

But 8.5 is the c.r you can live with
unless youre like me and have 7.5 :-)

08-09-2002, 10:34 PM
Well. I will give you my explanantion from what I've learned.

Basics: The piston compression(8.5:1) is the static compression, it is the compression that stays constant. When you boost you alter the compression, changing it from static to it's effective compression. So, you can boost more to raise the compression, or start off with a higher static compression(high compression pistons). Basically, if you want to be able to switch on the power(ie. floor it and the turbo comes on) then lower/stock compression is for you. However, if you want more power down low for coming out of turns and more torque, then higher static compression is what you need.

Now, more compression will make more power, but obviously there are limits on pump gas. So, if you want a turbo that needs to hit 20+ psi to make its real power, you should get lower compression pistons. Obviously, lower compression pistons do give you more of a safety net for tuning when it comes to detonation as pre-ignition is more likely to occur in a high compression motor(all other factors being the same). You can go with a thicker head gasket to slightly lower compression to help out, also.

So basically as I see it....10 psi on a t25 will make XXXhp on a 8.5:1 motor, and it will make more power on a 9.1:1 motor at 10psi, but you will find the limits of pump gas a lot quicker. You just have to figure out if you actually need that extra compression, unless it's a dedicated race car, most likely the answer is no.
Hope this helps.

08-10-2002, 12:47 PM
in case anyone cares... 10psi on 9.5:1 is = to 15.9:1 and 13psi on 8.5:1 is = to 16.2:1 so that will allow you to boost alsmost an aditional 3 psi and keep the same effective compression ratio. just some random info

08-10-2002, 03:32 PM
Forgive me if im totally wrong but have any of you guys read any of T.O.O's articles on theoldone.com? I remember one of his articles saying that static compression like isn't worth jack or something, that dynamic compression is far more important. Could anyone elaborate on this?

08-10-2002, 06:22 PM
Probably means that you can run a really high duration high overlap cam with a turbo system and high comp pstons and you wouldnt detonate as much as if you hgad a boost cam. I guess what Im trying to say is that it depends on the way you built your motor, you can have 20 psi and whatevers comp and things will be different by the way your cylndrs fill. Make sense? I dont know.

08-10-2002, 10:02 PM
yeh thats exactly wut i was thinking. But then how do people calculate how much boost they can run just knowing the static compression? would it be different for every car and cam profile?

08-11-2002, 05:41 PM
Yes, it would be different for every car and cam combo...but for certain combinations, there are numbers that are known to work.