building a 355 ci shortblock, with ported 52cc heads, cam and a twin intercooler P1-sc. (8# pully and a 12# pully)

What CR what be optimal for me?

31cc dish pistons would make the 8.6:1 CR
And
24cc dished pistons make 9.2:1 CR

what are the differences?
What rod size? i have head NA go 6'', Boost, go 5.7'' or maayybe 5.825''

And if you got time, what kinda power do you think? (considering good tuning great heads and moderate cam)

 

i dont know a lot about the P1sc, but assuming its in the same league as a S-trim, you should be able to make around 600rwhp pretty easily.


I would go with the 9.2:1 setup. if 12psi is your goal, this will be a great compromise for pump fuel.

with a stock stroke, you could prolly run 6" rods with out problems. i would say no to 5.850s cause its expensive. if JE has a nice off the shelf piston for 6s, go that route, but you will also be fine with 5.7s

Good luck, FI rocks.

 

I'm running 8.3/1 at ~13 psi intercooled. I still can only run about 28 degrees timing without detonation. However, I gain about 40 rwhp by upping the timing to 34 with some 102 unleaded mixed in the tank.

I've heard theories that low compression on a S/C engine increases power by more than just running more timing. The theory is that the increased volume of the cylinder (plus the chamber, plus the large piston dish) gives more total volume for the air to be pumped into. So, you get more air mass in the cylinder, which means more power.

I'm not sure if it holds water, but I can't think of a reason why it wouldn't be true.

Mike

 

There never is an easy answer.

The 8.6 Compression ratio will allow you to run more boost and more timing and you will ultimately make more peak horsepower.

The 9.2 Compression ratio will give you a bit more power in the lower rpm ranges for better driveability.

I opted for 9.0 Compression ratio and ... what engineermike said ... even though I'm at 3,500 feet, and see only 8psi of boost, I was getting spark retard on 91 octane pump gas. After putting in 2 gallons of Xylene so as to jump the octane rating, we saw no spark retard and were able to add 6 degrees of timing and thus make an additional 35 horsepower.

I think I'd opt for the higher compression and be prepared to add some race gas or what ever for the times you really want the maximum from the car.

I also agree with the 5.7" rods for forced induction, you get thicker ring lands with the 5.7 vs a longer rod.

As for power, anything over 500 HP is easily attainable. I got 525 at the wheels through a partially clogged cat ... that has been taken care of ...

Hope this helps

LWM

 

Cool. thank all you guys so far with the help. The Ati p1-sc is a lil more capable then a s-trim,
825 Hp
1200 cfm
30psi
62,000 rpm

You guys have any sugestion on what brand?
Cp( copston only tho??)
JE
SRP
Wiseco(only 20cc )
other?

 

stick with JE, they have some nice off the shelf pistons that sound like they will fit your combo well.

 

Speed Pro's aren't bad. The JE's are really nice too, but twice as much.

 

How are you getting such low compression #s with 52cc heads??? I ran your numbers through my compression prog and I am coming up with like 9.7:1 with 31cc dish pistons, zero deck and .029 gasket.

Are your #s are wrong or am I doing something wrong? Sorry for the off topic post

 

I agree with you. Don't know 'bout the .029" gasket issue, but even using a .039" gasket, would take an honest 57 cc chamber for a 9.0-1 ratio.

 

TRW makes a nice FI forged piston as well.

 

I have had problems getting mine to idle with the A/C on and had to have it programmed to run rich at idle to accomplish this simple task with 8.5 to 1. I now need rings due to ring wash from running rich at idle. I would , and am going to run 8.7-9.2 to 1 compression.

 

Again, a bit off the subject, but how rich were you running to cause this wall wash? You obviously were running O.L.

 

i was using
52cc
0.020 deck
0.039 felpro

 

I really don't know. I had it tuned by fastchips and Ed couldn't get it to idle with the a/c on without adding fuel at idle and keeping it in open loop. He seemed to think the compression was causing it. I would have just left it in closed loop and not ran the a/c in retrospect. I really need to learn how to tune these things.

 

Cringe.....It's fair to state his guesswork didn't cost him anything.