Thread: forced induction question

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Ok, I understand the compression limits of certain octane leveled fuels. If I put a blower on a motor:
Standard Atm. Pressure is about 14 psi we'll say
If I set up a blower pushing 7psi of boost, this brings us up to 21 psi. Everyone says if you run forced induction you have to lower compression ratio, which makes perfect sense. But it seems they say to lower it to around 7 or 8.
Wouldn't the compression have to be lowered by the same proportion that boost is added?
What I mean is, if I can run 11:1 maximum on 92 octane, and I go from natural asp. (14psi) up to 7 psi boost, then that's an increase in the amount of air inducted of 1.5 times. So shouldn't I adjust my compression by the inverse; 11/1.5 = 7.3333
That way if I start with 7.33:1 static, then the compression should come out to 11:1 if I push 50% more air in the cylinder.
How the heck do guys run 14psi boost, wouldn't you have to cut static comp. down to 5:1. How do you find heads with CC's that big?

How the heck do guys run 14psi boost, wouldn't you have to cut static comp. down to 5:1.

Well a lot has to do with the heads used, and what fuel you use(type of engine in some cases). The ones running 14 PSI are usually not running pump gas. The ones that do generally lower their compression lower than 7:1

How do you find heads with CC's that big?

Simple answer... They don't. In the same way you can use a 400SB crank and rods to make a 350 into a higher compression 383 stroker, you can take a 350SB crank and rods to make a 400 into a lower compression stroker(sorry I haven't done this yet so I couldn't tell you what it would be).

nice, I just wanted to make sure I had the theory down on this whole blower thing.
Someday I'll cut a hole in my hood too.

Another thing people do is to run different cams which lessen the amount of air that gets into the cylinder. Remember the compression can be lowered by the cam you use.

Originally posted by tcodi
nice, I just wanted to make sure I had the theory down on this whole blower thing.
Someday I'll cut a hole in my hood too.

tcody, youer on the right track but i dont think its that open and closed. the thing you need to keep in mind is with a aspriated motor you spend the big bucks on trying to get the piston to pull as much air into the cylinders that it can, with the blower you wont to get the valves open and closed as soon as possible, because when the valve open the blower is going to push the air into the cylinder, so you wont the valve to close as soon as possible and hold it in. on a reg motor from the factory it only fill the cylinder about 80%, so you can bolt a blower on and it will fill the cylinder up to 100% , so just by bolting the blower on you just picked up 20% more power. with the extra power comes the responsibility of extra cooling and more and better gas. before you cut the hood you should do some more reading about these blowers.

yeah, this blower project is years down the road, I'm still building up my hot rod now, it will have a 402 with normal induction. I'll drive that for a while, some winter when I don't have a good project I'll put a blower on it.