Less static compression and more boost?
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Less static compression and more boost?
I just bought a new LS2 block from a forum member and will soon start building a stock displacement LS2 short block that will be supercharged. Sticking with off the shelf pistons, I have some choices to make regarding compression ratios. Using Wiseco's as an example; I can stick with a flat top that will give me 10.6:1, or I can select an 11cc dish that would put me at 9.7:1.
The 10.6:1 choice would likely be more responsive, with snappier throttle response, but with 91 pump gas, boost levels would be limited. I'd like to leave meth out of the equation for now. Alternatively, I could select the 9.7:1 dished piston, and likely lose some throttle response, but would be able to run more boost and/or timing. Again, leaving meth out of the equation.
If you were starting from scratch, which direction would you go - less static compression and more boost? Or, more static compression and less boost?
The 10.6:1 choice would likely be more responsive, with snappier throttle response, but with 91 pump gas, boost levels would be limited. I'd like to leave meth out of the equation for now. Alternatively, I could select the 9.7:1 dished piston, and likely lose some throttle response, but would be able to run more boost and/or timing. Again, leaving meth out of the equation.
If you were starting from scratch, which direction would you go - less static compression and more boost? Or, more static compression and less boost?
Instructor
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From: enid oklahoma
Le Mans Master
Joined: Dec 2005
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From: Southeast TX
Unless your build is a "***** out....scratch and claw for every last hp, competition motor", and you have the blower to support the extra air flow, I'd shoot for the higher compression ratio. There's a jillion stock short block LS2's, 3's, and even 7's living large in the 650 to 700 rwhp range. That's with crap pistons. Yours will be forged. I've had lower compression supercharged motors before. Never again.
Dynamic compression ratio (camshaft choice along with static ratio) has a say in how all this plays out too. That and timing dictate cylinder pressure. Available octane dictates how much cylinder pressure you can deal with. Lots of variables to consider. If you don't mind being a little more sluggish out of boost and the fuel eco penalties, the lower comp ratio might be for you. I won't do it. Been there, done that, have the t-shirt....won't wear it
Dynamic compression ratio (camshaft choice along with static ratio) has a say in how all this plays out too. That and timing dictate cylinder pressure. Available octane dictates how much cylinder pressure you can deal with. Lots of variables to consider. If you don't mind being a little more sluggish out of boost and the fuel eco penalties, the lower comp ratio might be for you. I won't do it. Been there, done that, have the t-shirt....won't wear it
Le Mans Master
Joined: Apr 2011
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From: Montreal
St. Jude Donor '13
If starting from scratch I would consider the total package. Type of supercharger, gears, and transmission will play a roll in off idle and low rpm throttle response also.
Thread Starter
Instructor
Joined: Jul 2004
Posts: 177
Likes: 0
Unless your build is a "***** out....scratch and claw for every last hp, competition motor", and you have the blower to support the extra air flow, I'd shoot for the higher compression ratio. There's a jillion stock short block LS2's, 3's, and even 7's living large in the 650 to 700 rwhp range. That's with crap pistons. Yours will be forged. I've had lower compression supercharged motors before. Never again.
Dynamic compression ratio (camshaft choice along with static ratio) has a say in how all this plays out too. That and timing dictate cylinder pressure. Available octane dictates how much cylinder pressure you can deal with. Lots of variables to consider. If you don't mind being a little more sluggish out of boost and the fuel eco penalties, the lower comp ratio might be for you. I won't do it. Been there, done that, have the t-shirt....won't wear it
Dynamic compression ratio (camshaft choice along with static ratio) has a say in how all this plays out too. That and timing dictate cylinder pressure. Available octane dictates how much cylinder pressure you can deal with. Lots of variables to consider. If you don't mind being a little more sluggish out of boost and the fuel eco penalties, the lower comp ratio might be for you. I won't do it. Been there, done that, have the t-shirt....won't wear it
Thanks for everyone's input.
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Le Mans Master
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From: Montreal
St. Jude Donor '13
Le Mans Master
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From: Southeast TX
I have a new set of TEA Stage II 243s with a 65cc chamber. If I use the 243s,the Wiseco flat top would put the CR at 10.7. I also have a set of unworked L92s that I could use. I *think* the L92s have a 70cc chamber. Using the same Wiseco flat tops, the L92s would put me at 10.1. Not too far off from your recommended 10.2-10.3.
I hear you. I was all set just to move forward with the stockish 10.7 CR. Plenty of people have made that work. But, just out of curiosity I took a look at the compression ratios that the OEMs are using in their forced induction applications. Ford's GT500 is making 662hp with a 9.0 CR and the ZR1 is making 638hp with a 9.1 CR. They are making those numbers with OEM reliability and no meth. That made me rethink how I might move forward. I at least wanted to get some additional input on the subject before I pulled the trigger.
Thanks for everyone's input.
I hear you. I was all set just to move forward with the stockish 10.7 CR. Plenty of people have made that work. But, just out of curiosity I took a look at the compression ratios that the OEMs are using in their forced induction applications. Ford's GT500 is making 662hp with a 9.0 CR and the ZR1 is making 638hp with a 9.1 CR. They are making those numbers with OEM reliability and no meth. That made me rethink how I might move forward. I at least wanted to get some additional input on the subject before I pulled the trigger.
Thanks for everyone's input.
Also consider that the numbers you're quoting for the OEM's are flywheel numbers. A ZR1 typically dynoes 540 rwhp on a Dynojet. Just about any F/I build on an LS3 with bolt ons is going to be over that at barely over half the boost.
Last edited by old motorhead; Jul 13, 2014 at 07:10 PM.