9.1 Compression with 12-15psi

[john_V63400]

Ok so I got my pistons back and they made my compresion 9.1, i was under the understanding they were 8.1
Should I be concerned or happy, I dont mind the compression change, but with a peak of 15 psi of boost be too much for my engine.
Its a 3400 forged pistons RSM all the goodies except forged Rods

[Joseph Upson]

Ok so I got my pistons back and they made my compresion 9.1, i was under the understanding they were 8.1
Should I be concerned or happy, I dont mind the compression change, but with a peak of 15 psi of boost be too much for my engine.
Its a 3400 forged pistons RSM all the goodies except forged Rods

Compression is not the problem, detonation is. The higher the compression you can run without detonation the better. The new supercharged 4.4L northstar runs 9:1 and the Cobalt SC motor if I recall correctly runs 9 or 9.5:1 compression. I would suggest an intercooler, water/meth injection and a gradual climb to the desired boost level while tuning along with 92 octane and you should be very safe since the water meth injection offers very good anti-knock properties by itself. The aluminum heads will help also.

[AaronGTR]

I'd say you're better off with 9.1:1 compression. That's still a drop from the stock 9.5:1 and people have run that much boost on the stock bottom end. I think 8.1:1 is lower than you would have needed to go and you would have lost a lot of low end torque and hp because of it. As Joseph said keeping it from detonating is the key and if you have an intercooler and enough fuel you should be fine. You can always swap to 3500 heads and drop the compression a little more if you want, and gain some flow while you're at it. And that should let you run either more boost or more ignition timing.

[bjornmk1]

why would he loose low end with 8:1 ?
he is using a sc instead of a turbocharger so that shouldn't be
noticable right ??

B

[AaronGTR]

why would he loose low end with 8:1 ?
he is using a sc instead of a turbocharger so that shouldn't be
noticable right ??

B

He's using a centrifugal SC which builds boost slower than a roots blower. Plus the SC's only build boost above a certain throttle position so if he's just cruising around town he won't be under boost and the lower compression will make acceleration more sluggish. The SC is extra load on the engine too even at low rpm, it just builds proportionate to boost and rpm.

My SC starts making boost as low as 35% throttle. I've noticed though with the cam, headers, and heads etc all increasing the top end flow and shifting the powerband up, that when I'm cruising at 1200rpm and only step on the gas a little bit it will bog sometimes and not accelerate much. Tuning will help that somewhat but it's always going to be there to some degree. Lower compression would contribute even more to that.