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Discussion Starter #1
I am building a 331 and plan to use a pony down turbo set up and was wondering what is a good compression ratio to have in the 331 ..thanks for any help
 

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Read this, i quoted this from Induction Motorsports.

"What's better, low compression and more boost or high compression and less boost?"

"There are certainly reasons to try to raise compression ratio, namely when off-boost performance matters, like on a street car, or when using a very small displacement motor. but when talking purely about on-boost power potential, compression just doesn't make any sense.
People have tested the power effects of raising compression for decades, and the most optimistic results are about 3% more power with an additional point of compression (going from 9:1 to 10:1, for example). All combinations will be limited by detonation at some boost and timing threshold, regardless of the fuel used. The decrease in compression allows you to run more boost, which introduces more oxygen into the cylinder. Raising the boost from 14psi to 15psi (just a 1psi increase) adds an additional 3.4% of oxygen. So right there, you are already past the break even mark of losing a point of compression. And obviously, lowering the compression a full point allows you to run much more than 1 additional psi of boost. In other words, you always pick up more power by adding boost and lowering compression, because power potential is based primarily on your ability to burn fuel, and that is directly proportional to the amount of oxygen that you have in the cylinder. Raising compression doesn't change the amount of oxygen/fuel in the cylinder, it just squeezes it a bit more."
 

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Discussion Starter #6
compression ratio

thanks for the help..I will try for about 8.5 compression ..I always thought and read that with lower compression you could force in more air , and with more air you get more oxygen , so you can add more fuel and get more power out ...now I hope it will work
 

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There are a couple of things to consider here.
Keep in mind a static compression ratio like mentioned above will only have an effect if the engine is completely together, and then you change it by milling the heads or changing pistons. If it's a brand new motor, it really doesn't mean anything at all until you bring the camshaft specs into the picture.

Compression alone isn't the boost killer - cylinder pressure is - and that's determined by the cam. FWIW, my static CR is 9.25:1, running an X cam, and my dynamic compression is 8.3:1 - typically I like to see this number between 7.8:1 and 8.4:1 on a pump gas setup.
 

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FWIW, most OEM turbocharged engines are in the 8.0-8.5:1 range. There are some slightly lower and some slightly higher, but I think it would be safe to say 90% fall into that 8.0-8.5:1 range.

Having a higher compression ratio will make it a little more snappy off boost, but you'll limit maximum power.

1st gen DSM 7.8:1
2nd gen DSM 8.5:1
Ford 2.3 Turbo 8.0:1
WRX 8.0:1
WRX STI 8.2:1
Neon SRT-4 8.1:1
Dodge 2.2L turbo 8.5:1 then 8.0:1 for later years
GN/GNX 8.0:1
Lancer EVO 8.8:1
Syclone 8.35:1
Cosworth Sierra 8.0:1
 

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There are two basic rules:
1. You want to run the most compression you can get away with at the power level you want
2. The less compression you run the more boost you can use and power you can make for a given fuel

Like others have said many other factors effect dynamic compression ratio such as cam design but as an example here is come calculations on a pump gas 350cid engine.

10:1 432Hp N/A [email protected]
9:1 414Hp N/A [email protected]
8:1 396Hp N/A [email protected]
7:1 378Hp N/A [email protected]
 

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I would also think that which turbo you use would also effect what compression to run.One thing I never see talked about is turbo effiecency. Turbos will loose power when you boost beyound the effiency of the turbo. depending on the turbo you could make more power 20 pounds than 25 pounds of boost
 
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