Here's my question im gonna build a 400sbc with a ati charger on it...about what compression would i need to run say about 12 lbs of boost? i was thinkin about 8:1 or like 8.5:1? or could i run maybe 9:1 or higher? just wonderin and still in the plannin stage atm ....thanks

 

Depends. If you have aluminum heads, then you can run more compression (about +1 more).

The lower the compression ratio, the more boost you can run.

If you cam is real "raddie", you can get away with more compression.

Altitude is also a factor.

I have a Crower Blower cam in my 383/ATI and I'm happy with the results. Good idle, well manors for drivability and great power when the boost kicks in. But this is in a boat.


If your thinking of putting the 305 TPI on the 400 and ATI, I believe this setup complements each other. The TPI is better suited for the low end to mid range, then the ATI takes over after 4000 RPMs and huffs away.

 

i would go for at least 10.5-1.you dont want a low compression slug.

dave

 

You shouldn't need to go lower than 8.5:1, and while you can definitely go over 9.5:1 you will likely have to give up some boost and/or timing, and perhaps still need better octane than premium pump.
From what I've seen, the average compression ratio of supercharged EFI street cars is pretty close to 9.0:1.

As a gereral rule, you will make more power in a supercharged engine (all other things equall) running less compression and more boost, instead of more compression with less boost.
At the other end of the spectrum, off idle transitions, and nonboost drivability are impaired somewhat with lower compression. That's why a range of 8.5:1 to 9.5:1 is used so often on true street cars.

 

THanks alot for the info guys:hail: :hail: :hail:

 

Im running 10.1:1 compression, with a 218/224, 0.495/0.502 cam, how low would i need to make my compression to run supercharger off 94 octane.Aluminum Edelbrock performer heads, and forged pistons, just for info.

 

retarding cam timing effectively lowers compression and also helps the engine breathe at upper RPMs

 

I'm running stock style Forged pistons with the 4 valve reliefs which probably gives me about 9.5:1 since I have a 350 bored .030 over and the stock compression is 9.3:1. (larger bores increase compression given all else the same)

I am running ATI's 12-14 psi pully and getting no detonation on pump gas. (I need to get a boost gauge so I know exactly how much boost I'm running. )

BTW, my cam is pretty mild. Comp Cams grind number 226H I think. The specs are .500/.510 210/220 114 LSA. I think its well matched to my combo.