So if you are going to install a SC on your LS2 and they perform better at a lower compression ratio. Than what after market heads would work the best? What is the best CP on a centri super charged engine? If you are going to the expense of the complete kit than might as well get the ideal compression while your at it. Your thoughts?

Thanks

 

No such thing as a "best" compression ratio. It akin to asking what cam is best; it does not exist. Almost every single aspect of engine performance is a trade off between compromises in low end torque vs peak hp vs reliability vs driveability, etc etc...

Reducing the compression ratio will have the following effect on the engine:
Pros: You will be able to run more boost at any given octane ratio and make more power.
Cons: You will lose efficiency: low end torque will be reduced, gas mileage will drop and the responsiveness of the engine will suffer some. You will also need to run more boost to make the same kind of power as a high compression low boost motor. This will create added parasitic drag from the supercharger, which will increase the likelyhood of belt slip, will increase the intake air temperatures and will put a larger heat load on the system overall.

It all depends on what you are trying to achieve... I believe that for 99% of users the stock compression ratio will give you the best trade off between power potential at any given octane rating and efficiency. I think dropping the compression ratio on a stock bottom end is a bit of a moot point because you can already exceed the stock bottom end's safe horsepower limits with stock heads and stock compression.
That said if I was building an all out ultra high HP racing engine with a massive supercharger or twin turbos I would probably want to drop compression down to maybe 9.5:1 or 10:1 and run all forged internals. It would make the kind of power that is unuseable in the street though...

 

I've made 750 RWHP with mid 10s compression with no issues.
I definately wouldn't go below 10 on a stock bottom end or 9.5 on a forged bottom end.
These are generalities but are pretty good rules of thumb.
I prefer a little higher compression and a little less boost as opposed to the other way around.
I am presently at 10.3 on my LS3 and am right at 700 RWHP.
I feel it's a great combination. Low end is good and power under boost is amazing.
Just my $.02

 

Not to take this thread off topic but can you describe what driving your setup is like? I am trying to get my head wrapped around what 700 rwhp would be like?!

 

Let me expand the question, If I want to add a set of heads ( TF or AFR ) what flow characteristic's will work best with my set up.

Note:
Not sure how big of a head unit yet
Ported FAST, ported TB, Kooks 17/8 headers w cats.

Thanks

 

Could not have explained it better myself, high comp. and lower boost is the best IMHO for all around performance. The myth of "you must have low comp for boosted engines" is just that a myth.


As far as heads don't bother, its not going to make any difference in your overall performance if they are lower comp heads they will drop you hp numbers. Good flowing heads are very important on N/A engines, boosted engines are forcing the air in and the new LS2 and specially the LS3 heads flow very well. There is just no need to dump 2k for very little performance, keep the heads and go with a good blower cam for a fraction of the cost and make 50 to 100 more hp. I have friends here in Houston that made 740rwhp on stock heads. Anything over that has to be forged.

 

The cost of aftermarket heads is a waste in my opinion on a boosted car where you are not trying to set any records. Its almost pointless. So is the FAST. First, where are you trying to be with your car? I believe in keeping the compression up on a street car and running less boost. I don't like losing the charcateristics of the higher compression car. If I wanted to make 800rwhp on 17psi, then yes I would run 8.5~9.5 to 1.

My car is at 10.8:1. Cam eats a bit of compression. I run 10psi. With a safe 18 degrees of timing I make 761/670. I sold my FAST and used the money toward the blower. My heads are very good, but were purchased because the car was intended to be N/A. A change in geography wrecked those plans.

I see, including myself early on, people not concentrating on what they really want. They shoot for pie in the sky numbers, spend thousands of dollars and are never happy. So, my advice is first ask your self where you want ot be. Is 600rwhp enough? I think it is but maybe you want sick numbers regardless of whether you can get it to hook? Its tough to say. I have seen stock bottom end stock head cars get close to 600rwhp with just a blower, cam and headers at the stock compression ratio. I would try that, with the stock heads and see what you think. Keep the $2500 in your pocket and run her with just the blower. Do it in stages. You may surprise yourself and be happy without spending the dough on heads. You may also get the power you want with a boost level that is ok with stock heads. If so you're good and can split the $2500 with ole AintQik. If not, change heads, swap pulleys and have at it. No real lose except perhaps the additional cost of another tune. Hope some of what I said makes sense! Good luck!

Oh, also, if you decide to go ahead and lower compression, don't spend tons of money on an aftermarket head. Its not going to be cost effective. Truck head or something would work fine.

 

Sooo true... I went through this with all my cars... First bolt ons, then more bolt ons, then more expensive bolt ons, and finally big changes (larger turbos, bigger intercoolers, engine swaps, etc)...
I think the reason that happens is because we normally don't have the benefit of driving the car we are looking for before we mod... I was **this close** to dropping 5K on heads and cam when I finally decided -without ever having driven or riden in one- to supercharge. Glad I did! As soon as I find more traction chances are I will want more power too It never ends

 

Thanks guys. I am looking for 500/550 with good torque. It is a street car and don't need those big numbers. When I step on the throttle I want it to go not spin!

 

In that case you don't even need a cam, just order Andy's S trim kit and thats all you'll need for 550rwhp.

 

I made 518FT-Lbs torque and 597HP at the wheels with Andy's kit and a pair of Kooks 1 3/4" headers... Stock heads, stock cam, stock everything else... And nothing but pump gas

 

Thanks guys that sounds just about right. If I get to the the point were I want more power that I can add cam or change pulley. For those numbers (mid 500) what are the boost numbers?

Thanks

 

 

It depends on your setup; if you have headers they will be a bit lower and they can also be lower with a larger blower.
I have Andy's Vortech V2 S-Trim kit with headers and a catted X-pipe and I am seeing 10.5PSI at redline, making 597rwhp/518rwtq.

 

8psi will make 550 or so with headers, 525 without.

 

I run WCCH stage 3, 317's... They are the VERY BEST, bang for the buck, they flow great, and they drop compression by .5
Next in line would be L92/LS3 heads, but they are double the cost and if you have an LS2 you must change your intake manifold... WCCH 317's for the win!!!