hi guys, im picking up a vortech t-trim with aftercooler as well as having a motor built for my 94 trans am. Im just confused as whether to go with a higher or lower compression ratio and how much boost to run to make the most power, Im also undecided as to what size motor to go with 355, 383, or 396. I know motor size will have alot to do with efficency as well. I see alot of guys running 9:1 CR's with about 12-14 lbs bt was wondering if like an 8.5 or 8.7:1 with more like 16-20 lbs might be the way to go. Also, what rings does everyone recommend? Im looking for a reliable driver probably a lil too overbuilt with about 600 rwhp on a conservative tune, ill probably also keep myself pullied down to around 6-8 psi for daily driver duty and toss on the big pullies for track days and special occassions.

 

I'm of the opinion that your CR can't be too low for boost. Lower CR and more boost will make more power than higher CR and less boost. The hellfire rings are a popular choice for boosted applications. The lower your compression ratio is the easier it will be to avoid detonation.

 

Thats how Ive always done it but im new to superchargers and lt1's...previously ive just turbo talons where 25psi was normal. I just wonder if going too low of compression would take more hp to spin up to a high amount of boost. This car needs to stay streetable. how low is too low

 

Lowering your compression will not change your spool up time. Ok, it will but only slightly. The turbo spins depending on the volume of air that you are pushing through the exhaust. Which is totally dependent on your engine displacement( ie 355 vs 383). The lower compression will not make as much hp off boost so you won't be able to increase the rpm's as fast to get the extra volume of air out which ends up being fractions of a second of difference. The big thing is that you will probably be running a higher boost level so it will take longer to reach that.

 

I should probably be more clear with my post, this will be a vortech t-trim supercharger with aftercooler, approx 15-19 lbs i assume

 

It sounds like your goals are similar to what mine are. Currently I have a 383, and am having different pistons put in for my D1SC. I plan on starting with 12#'s but if that doesn't make at least 600 RWHP, then I have also got at 15# pulley I will be putting on. The shop that is doing my motor work is reccomending I go with some 8.5-1 pistons, just to play it safe. Yes its true that lower compression pistons will give you some lag, but the way I see it is, I would have to go with at least 9.5-1's anyway, probably even 9-1's to play it safe, so whats a step to a 8.5-1 for detonation assurance really going to cost me lag wise from the step above I would pretty much have to go with anyway? Not much. As for the 355, 383, or 396 question, honestly, I would go with the 355 for a driver car. Anything more and your going to be putting added torque, on added torque from the supercharger. Mine was a NA 383 before I decided to supercharge it, otherwise, thats the size I would have picked. All above is just IMO.

 

The lower compression ratio really doesn't make as much difference on NA power as most people think it does. To go from 9.5 to 8.5 might cost you a 2 to 3 % power loss which is very minimal and is more than worth it IMO to help reduce detonation and allow for more boost/timing. The power difference from compression becomes exponentially greater as the CR increases. To drop from 13.5 to 12.5 would cause a larger power loss than going from 9.5 to 8.5

 

BreakmyFootoff is right, unless you are dipping down into the low 7's compression ratio isn't going to affect you that much. Just play with the HP calculator on here and adjust the Compression ratio, there is minimal difference. I think higher compression will give you better throttle response on the whole but what is really important is airflow. If you get 14.7 pounds of boost into the cylinder you will double your N/A hp (assuming the air is the same temperature as it would be N/A). As for 355/383/396, if your heads can support the airflow needs of more displacement then go with more. I went with a 355 because that's all my ported LT1 castings can handle.

My very opinionated $.02 (feel free to tell me I'm wrong)

Jeremy

 

I'm also building a similar setup. I'll be at 8.7:1 CR with a 383 and a D1SC ProCharger. Shooting for about 620+ rwhp. I went with the Hellfire Rings. I would think that its better to be a little lower rather than a little higher on the CR. Many people with 12-13 lbs. of boost shoot for 9:1 CR so the engine is responsive on the street w/o boost. With higher boost levels, the CR is usually a bit lower. You'll lose some initial throttle response at lower CR levels but that's just the tradeoff you have to accept with high boost applications. You can always pulley up to get the boost in sooner. Also, don't overlook the importance of dynamic CR. But you'll need a custom cam to get that figured right for your car. Do the 383. Doing the rebuild go for a stroker motor but a minimal bore for block strength.

My car should be running in a month. I'll post numbers when its done.