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Thread: From 8.5 to 9.0 compressionw while rebuilding?

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2011-10-26 15:17:30
From 8.5 to 9.0 compressionw while rebuilding?
-=S14 SR20DET / 2871 / 740cc / Z32 / JWT /264=-

Since I'm in for a rebuild (118-120 psi across all 4 cilinders...) I am tempted to up the c/r to 9.0 instead of 8.5 for a little extra respons and a few ponies.

Many people say you have to lower boost when going with higher compression, but I want to know if 20-22 psi is still ok with this setup and 9.0 c/r.

What is your experience with this kinda setup/boost and c/r's?

Thank you for your input.

2011-10-26 15:45:53
I don't see making much difference in maximum boost at all. You should be just fine assuming that you are talking about forged pistons. You will notice a slight improvement in response. Do they offer 9.5:1 forged pistons? I personally like the response that the little extra compression of a stock de+t over a 8.5:1 det and was wondering if there is an off the shelf forged 9.5:1 piston.
2011-10-26 16:04:10
I went from 9.3:1 to 8.5:1 and I didn't notice any difference. On the lower compression pistons I was able to run more boost and make more power on 91 pump. I didn't notice any loss of response or bottom end power at all.

If I were to build again, I would go for a 9:1 or even a 10:1 compression. I wouldn't need to run more than 10psi boost on the street, but I don't even feel the need now.

I'd say if you want to run that much boost, keep it low compression for safety. But these days you can run big compression and lots of boost with new tuning options. lower compression is sort of a bandaid.

All things said, I didn't notice any response difference. Running a 1.1 bar spring in the wastegate, I would get detonation on the 9.3:1 compression. Putting the 8.5:1 compression pistons made 18-19psi possible on pump gas. But I don't like to push it much further than that. Now I keep it at 15psi or less for pump gas.
2011-10-26 16:45:24
On a DET, and 8.5:1 CR being out of boost, is actually noticeably slower then 9.5. Coheed on a VE, with the small cams it probably isn't as noticeable .

I'm currently at 10.3:1, and on boost with a log manifold and 12 psi of boost it wasn't bad. At 15 psi of boost I couldn't retard the timing back enough to avoid detonation. Once again this is on log manifold though.

When I go to rebuild I'll do 9.3:1, that way I still have decent comp for cruising and can do 18-20 psi .
2011-10-26 16:59:58
I was talking off boost response as the difference in spool time is slight. I feel 9:1 or 9.5:1 can take the same amount of boost with better off boost response and slightly better spool. The ecu must be tuned properly of course but I personally prefer the higher compression myself.
2011-10-26 17:16:59
Thanx guys for the replies.
I'm going for some CP pistons and Manley rods so c/r is all open, but 9:1 it is then.
I'm not planning on pushing more boost than 20-22 psi or so since 400whp seems enough for me now.

Any thoughts on Wiseco pistons compared to CP or is it more or less the same?
2011-10-26 17:26:26
- Instead of 86mm at 8.5:1, I decided to get 87mm at 9.0:1 pistons for better spool responds.
2011-10-26 17:38:14
You will notice a slightly better response with the 9:1 compression. You should still be able to run the same amount of boost as long as the tuning is good.
2011-10-26 17:43:45
Originally Posted by ashtonsser
You will notice a slightly better response with the 9:1 compression. You should still be able to run the same amount of boost as long as the tuning is good.

Seems like we agree on that one Ashton Now if it is on a non-tuneable ecu I would maybe want the lower compression for a safety margin. All in all that .5 compression won't make much of a difference for tuning anyway but I guess at 20psi on the turbo you are running the tuning will need to be very good either way.
2011-10-26 17:55:26
I have Wiseco 9:1 forged pistons, and BC Sportsman rods in my DE+T. I really couldn't tell much difference from the stock 9.5:1 CR. It's still responsive, and has decent torque off-boost. I have my wastegate set for 21 psi, and have never gone beyond that. I've never had any problems out of my setup though. The timing did take a little while to get just right though. The higher CR is much nicer for a daily, and is all-around more fun to drive IMO.

Can't advise you on the CP pistons personally, but I haven't heard anything bad about them so far.
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