samuelblake

What is the best compression ratio for a turbo or supercharger?

I have a '97 Pontiac Trans Am and I am having the engine built up. The engine will be a 396 ci LT4 with all forged internals (Oliver "H" Rods, Callies Dragonslayer forged crank, JE Pistons, etc.) and will be converted to a 4 bolt design to handle the high boosts. I just need to know what compression ratio I need to have it stroked at?

I still have to decide on a twin turbo or a large supercharger. The rest of the drivetrain will be taken care of. I had the 6-speed manual transmission rebuilt for harsher HP/TQ. The driveshaft will be a Mark Williams Custom Carbon Fiber driveshaft. The rear end will be a Strange 12 Rear End with the 3.43 gears in it.

I want to keep this car as an everyday driver car, but I want to be able to crank up the boost to do at least 800 Flywheel HP and when I get done smokin' the competition at the track I want to be able to turn the boost down and drive the car home. This car will surely see alot of track time but more everyday driver time. I will drive this car probably 5 days a week of about 35-50 miles per day to and from work and just pimpin' it at redlights trying to get 4 bangers to race me <evil grin>

So what compression ratio would you guys recommend? Thanks.

Tech

Pick the blower/turbo first.

Bob 96STrimA4

Your plan seems very close to the way I've done my ride. Ive had various comp ratios and for my current set-up I like the 9.0 to 1. with 15 lbs of boost. I run this on pump gas and the car has more power on the street then I'll ever need, and I can baby it around at 1500RPM's all day if I can control myself.

I assume you are talking big boost. If you want to run pump gas I would keep the compression to 9.0-1 or even a bit lower depending on boost. You could go to 8.5 - 1 and 15lbs with a turbo set up and believe me, you won't be lacking any torque. Also, you don't need to have a turbo shut off. Just use your foot to moderate boost.

Bob 96STrimA4

Your plan seems very close to the way I've done my ride. Ive had various comp ratios and for my current set-up I like the 9.0 to 1. with 15 lbs of boost. I run this on pump gas and the car has more power on the street then I'll ever need, and I can baby it around at 1500RPM's all day if I can control myself.

I assume you are talking big boost. If you want to run pump gas I would keep the compression to 9.0-1 or even a bit lower depending on boost. You could go to 8.5 - 1 and 15lbs with a turbo set up and believe me, you won't be lacking any torque. Also, you don't need to have a turbo shut off. Just use your foot to moderate boost.

rskrause

You don't give enough info to really say, primarily how much boost and what fuel you will be using. But generally I say ~8.5:1 or even a little less unless you are using high octane leaded race gas.

BTW: I'd steer clear of that CF driveshaft. Gains a couple of rwhp at best and they are not known for their intestinal fortitude.

Rich Krause

samuelblake

Thanks Bob,

I was surely thinking of 8.5:1 but I didn't know if that was too low of a compression level.

Let me ask you this. I know quite a bit about Turbos but I don't know alot about Superchargers. I saw your website and I know you have FREAKING AWESOME numbers. Is yours street drivable mostly? If so...what is your MPG? Also....do superchargers have adjustable boost controls that you can change from the inside of the car like Turbos do? Just wondering. Also...why does everyone go with the Accel/DFI Aftermarket computer? What does it do that the stock computer cant? Thanks.

Bob 96STrimA4

My car is every bit as street drivable as a factory car. It idles rougher, is louder, and the clutch is heavier. Thats it. Streetablity is really a matter of cam selection. Forced induction cams by nature are more streetable.

My fuel mileage is about 11 MPG if I try and be good for a whole tank of gas. Forget abaout mileage if you're going big power. After market engine management advantages are tunability under boost. The factory PCM won't do this unless someone has a wasy to use a 2 bar Map sensor on one. Also you need one to run injectors over 50lbs.

But there is nothing like the factory computer for fuel efficiency.