Whats a good compression ratio?
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Teching In
Joined: Aug 2016
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Whats a good compression ratio? I'm building up a lq4 with built-up internals to deal with it turbo charged set up. I'm interested to see what compression ratios people have on their builds. Soon the engine will be going to the machine shop and I wanted to make sure that I have my pistons purchased to send with the block.
#317 heads
going to 6.125 rods
standard stroke
#317 heads
going to 6.125 rods
standard stroke
TECH Fanatic
Joined: Oct 2014
Posts: 1,220
Likes: 179
From: Virginia
I'm building up a lq4 with built-up internals to deal with it turbo charged set up. I'm interested to see what compression ratios people have on their builds. Soon the engine will be going to the machine shop and I wanted to make sure that I have my pistons purchased to send with the block.
#317 heads
going to 6.125 rods
standard stroke
#317 heads
going to 6.125 rods
standard stroke
Keeping static and dynamic compression low allows you to maximize use of boost.
Here's a simple formula to calculate compression under boost:
14.7 / SCR (or DCR) + 1 * boost PSI
It gives you an idea on how to set up the motor.
I'm N/A at 11.8:1, DCR is 8.2:1, still on 93.
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Teching In
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The biggest consideration is application. I don't have anything running boost, currently, but knowing a lot about tuning and MAP values, I would say a static compression in the 8-9:1 range is good for boost. If you are staying on pump gas, keep your dynamic compression under 8.6:1. This is decided by the cam and valve timing. 93 will detonate above that DCR.
Keeping static and dynamic compression low allows you to maximize use of boost.
Here's a simple formula to calculate compression under boost:
14.7 / SCR (or DCR) + 1 * boost PSI
It gives you an idea on how to set up the motor.
I'm N/A at 11.8:1, DCR is 8.2:1, still on 93.
Keeping static and dynamic compression low allows you to maximize use of boost.
Here's a simple formula to calculate compression under boost:
14.7 / SCR (or DCR) + 1 * boost PSI
It gives you an idea on how to set up the motor.
I'm N/A at 11.8:1, DCR is 8.2:1, still on 93.
TECH Fanatic
Joined: Oct 2014
Posts: 1,220
Likes: 179
From: Virginia
It really all depends on the boost you'll be running. You can safely put about 10 PSI on a stock LS1 and get away with it on a proper tune. Seen it, done it, tuned it. Still working just fine.
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I run just under 10:1 (stock rotating assembly) in my street car with boost, but summer here is usually mid 80s, I have a large front mount intercooler, and I run 93 with water/meth. My cam is actually fairly small and I have strong cranking compression numbers. Zero complaints. Rarely ever any detonation.
Without water/meth, I would try to stick to around 9.5:1. With water/meth I try to stay up towards 10:1.
Without water/meth, I would try to stick to around 9.5:1. With water/meth I try to stay up towards 10:1.
If 93 is all you have available, then that's very important unless you're planning on running a solid Methanol setup.
Like others said, Keep the compression down down to 9.5 - 9.8 for decent boost pressure.
I'm stuck with 91 octane here in Northern NV. I have to run no more than 12 psi on an A2A intercooler without Meth. 408, Martin turbo cam and TSP 2.5 317's with 9.6:1 CR.
Once I get a GOOD reliable Meth system, I'll be turning it up to the 18-20 psi range to see how things are looking.
Just keep an eye on your IAT, knock etc... and inch it up little by little
Like others said, Keep the compression down down to 9.5 - 9.8 for decent boost pressure.
I'm stuck with 91 octane here in Northern NV. I have to run no more than 12 psi on an A2A intercooler without Meth. 408, Martin turbo cam and TSP 2.5 317's with 9.6:1 CR.
Once I get a GOOD reliable Meth system, I'll be turning it up to the 18-20 psi range to see how things are looking.
Just keep an eye on your IAT, knock etc... and inch it up little by little
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From: Virginia
My formula for boost compression is wrong. Should be:
SCR (or DCR) / 14.7 + 1 * boost PSI
Sorry about that. Had a brain fart lol.
Example:
9.5 (SCR) / 14.7 = .646
.646 + 1 = 1.646 * 15psi = 24.7:1 SCR.
SCR (or DCR) / 14.7 + 1 * boost PSI
Sorry about that. Had a brain fart lol.
Example:
9.5 (SCR) / 14.7 = .646
.646 + 1 = 1.646 * 15psi = 24.7:1 SCR.
TECH Fanatic
Joined: Oct 2014
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From: Virginia
It's actually better to use dynamic compression ratio, since the real limit is 93 itself, which will detonate above 8.6:1, according to everything I can find on it. Obviously heat is not going to help, and something like a meth injection can improve this number.