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Question on Compression Ration When Supercharging

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Old Dec 12, 2011 | 11:56 AM
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Question Question on Compression Ration When Supercharging

When engine builders including GM build engines for Supercharged cars its appears that the comp ratio is anywhere from 9.0 to 9.7 why not 10.0 to 11.0? Can somone clear this up for me?
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Old Dec 12, 2011 | 12:03 PM
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Lower comp allows more boost. Boost makes more hp than comp. Make sense?
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Old Dec 12, 2011 | 12:16 PM
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Question comp vs boost

Originally Posted by JRENIGAR
Lower comp allows more boost. Boost makes more hp than comp. Make sense?
So by filling up the chambers before the piston comes up to compression?
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Old Dec 12, 2011 | 12:22 PM
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There's a lot of variables. Fuel being used, Type of supercharger, Use (street/strip, full race) Intercooled or not. Most FI applications are usually set up to run 9.5:1 or below depending on type of induction...now remember thats on pump gas for the most part. If you run E-85 or something along those lines you can run a higher compression without worrying about detonation. It just really depends on what you want to do
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Old Dec 12, 2011 | 01:20 PM
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Question Comp vs SC

Originally Posted by blue00ZZleeper
There's a lot of variables. Fuel being used, Type of supercharger, Use (street/strip, full race) Intercooled or not. Most FI applications are usually set up to run 9.5:1 or below depending on type of induction...now remember thats on pump gas for the most part. If you run E-85 or something along those lines you can run a higher compression without worrying about detonation. It just really depends on what you want to do
ERL M1 LS3 427 Whipple SC Intercooled comp 9.5 pump gas street use
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Old Dec 12, 2011 | 01:44 PM
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Yeah on a forced induction setup, detonation is usually the limiting factor in the amount of boost you can run. There are many ways to combat detonation: Higher octane fuel, cooler incoming air (ie intercooler), lower compression ratio, meth kits, etc etc. For street use (where you wont normally be running race gas) lowering the compression and/or adding meth and/or adding an intercooler are usual methods.
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Old Dec 14, 2011 | 07:56 AM
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Keep in mind that unless an engine builder is putting together an engine for racing or a specific fuel, the builder is going to want to make sure the engine will survive with the average 91 to 94 octane fuel. 9.5/1 seems to be a good compromise. Bob
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Old Dec 14, 2011 | 11:18 AM
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Question Boost vs Comp

Originally Posted by Bob@BruteSpeed
Keep in mind that unless an engine builder is putting together an engine for racing or a specific fuel, the builder is going to want to make sure the engine will survive with the average 91 to 94 octane fuel. 9.5/1 seems to be a good compromise. Bob
Thank you Bob makes sense.
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