choosing a compression ratio

[andrewgordnier]

hi all,

i have a '98 323is (m52b25) that i have already stripped down and plan to turbo. since this is my first time buying my own parts (i've helped my friends in the past and would ask them for advice but i moved away), i'm having issues deciding on a compression ratio. as of right now, here are the specs i have that matter:

85mm bore
97mm stroke
forged bottom end
garrett gt4049r
athena cut ring head gasket + ces copper spacer
full arp hardware (main and head studs)

i plan to run e85 and am shooting for around 700whp. with that turbo i believe that means around 20-25psi, which is a good amount of boost and i haven't seen a compression ratio on this forum from the people who run those numbers. i've seen 9.0:1 and 9.5:1 thrown around for lower boost builds, but these people are also running pump gas. i also know that stock cr is 10.2:1 and 9.0:1 with a copper spacer seems low even for a high boost build on pump gas. any help from anybody more experienced than me would be appreciated.

[PiepPiep]

9:1 can do it all.
Skip the spacer, when you can buy forged pistons with the right compression.
If this is E85 only, you can go 10-11:1 no problem, but you would be very restricted on normal pump fuel.

[E30s50dude]

9:1 can do it all.
Skip the spacer, when you can buy forged pistons with the right compression.
If this is E85 only, you can go 10-11:1 no problem, but you would be very restricted on normal pump fuel.

Agreed I'm using 9:1 pistons with no complaints here, running up to 32psi on a 6466 on e85. I am curious on what the difference in power curve would be with 10.5:1-11:1 comp on e85. Also if your doing main studs you'll need to have the block align honed before you go back together.

[Sio2crew]

E85 only. Keep it 10 to 1. If your ever running pump, 9 to 1. Fwiw I was going to go cut ring and spacer. But my iwis race chains wouldent fit. So I got j.e. 9 to 1 . ( flex fuel/pump, dual maps ) Im so happy I did. Everyone I personaly know with a stock bottom end breaks ringlands sooner or later. Usually much later on e85, but still broken in the end.

If I was buying today, I would shop the alien engineering sight. Perry the owner (someguy325 on here) has built low and high compression turbo builds. I think he built a turbo set up over 12 to 1. He spins them real high too. He knows all the pros and cons.