Hey guys I have a question to the ones that have gone through this before. I have an ap1 with 115k miles, and i got a Garret GT35 journal bearing with 15psi. It will probably bring the car to around 350HP or somewhere around there. I am changing pistons and wanted to know what compression should I buy the new ones? As of right now I only want 15psi, but later I might want to get it higher without having to change the pistons again, what would you recommend?

 

there are a lot of people I've notice running stock pistons at this level of psi. so I would imagine stock compression ratio would be fine. but I think if you go more than 15 you might want to lower it a bit. stock is supposedly 11to1. at 115k on the clock, what are your compression numbers at for compression checks?

 

Definitely this. Why are you swapping out the pistons in the first place? People boost up to 600+WHP on stock engines relatively often on these boards, so unless you're going balls out with a drag build I don't really see the point. Lower compression will allow you to run more boost and make a bigger top end, but will also make for a later powerband.

Also, I'm pretty sure a GT35 pushing 15PSI on an F2*C engine should be producing quite a bit more power than 350WHP, someone feel free to correct me though if I'm wrong.

 

^^ He's right. At 15psi on a gt3582, people are seeing 480whp+ depending on fuel and build. If you only want 350hp, you can boost 15psi on a gt3071 and get the same power with a MUCH broader torque band.

 

Careful on piston swap. Stock bock has frm sleeves. Needs specific Pistons. And even then needs fresh hone or slight over bore and hone.

 

My compression was bad, that's why I took it apart. I was getting 140 all the way across (as I was told I was suppose to be at around 240, am I correct?) so since I'm fixing it, mind as well see what to look for when talking about compression. I already had the gt35 installed before, I was estimating around 350 but you are right, it could have been more. So I'm just going to keep this same turbo. My concern is also reducing the compression and getting the legging even bigger. Since its a journal bearing, the car before the boost kicks in is very slow, and weak:/

 

240 is ideal, perfect conditions, but normally people say 215+ is good, with minimal differences between the numbers (IE, consistency - 222, 223, 222, 224).

Are you sure the block was the cause for the compression loss? If that's the case you'll want to double check that your cylinder sleeves are still in appropriate condition as well. In all likelyhood you'll probably want to bore them out a bit and get them re-honed, at which point I'd personally drop in some Mahle (FRM compatible) 10.5:1 or 11:1 pistons and new rings.

Just my .02

 

Also It could be the gauge you used?? I did a compression test with a cheap harbor freight tester and got 150,130,150,150 but yet I put down almost 600whp and 420tq. So I'm not going to worry till I notice power loss or oil/coolant loss ,and my car only has 22k miles on it.

 

So I just brought my block to the shop, the guy says that these blocks are aluminum and they can only be bored out so much. Then a buddy of mine tells me that the only Pistons that would be ok without adding sleeves to it would be Mahle pistons. Does anyone know anything about that?

 

ya the guy in post #7 explained it