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View Full Version : Does higher compression ratio raise TQ off boost?

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turbotalon1g
11-16-2012, 10:43 AM
Does a higher compression engine make more torque while off boost?

Just looking for a friend, he wants maybe rebuild his motor with higher comp.

Thanks
JET
11-16-2012, 11:35 AM
A bit, not a huge difference though. It is a trade off, higher comp gives a little more torque and better spool, but you can't run as high of boost (compression ratio doesn't get to run through the intercooler).
turbotalon1g
11-16-2012, 11:37 AM
So it does raise off boost TQ? Have you seen this on the dyno?
I know it feels a ton better, he has an auto car and wondering if moving up from 8.5 is worth it or not.
Murlo26
11-16-2012, 11:44 AM
A bit, not a huge difference though. It is a trade off, higher comp gives a little more torque and better spool, but you can't run as high of boost (compression ratio doesn't get to run through the intercooler).

Depending on the fuel, higher compression doesn't matter if you have the fuel to back it up...e85/e98.
goodhart
11-16-2012, 11:44 AM
Tachyon might be a good resource on this
Murlo26
11-16-2012, 11:45 AM
Tachyon might be a good resource on this

Indeed, he runs 11:1 I think, maybe even 11.5:1.
JET
11-16-2012, 11:46 AM
I haven't personally seen an apples to apples comparrison, but it is a commonly accepted theory and there are many sources on the net about it. Higher compression produces more power at the same boost level across the board, but in a turbo car you actually lose power up top if the rest of your system is capable of running with more boost and is intercooled.

8.5 to 9.5 seems to be the 4g63 sweet spot. If he is running E85 only I could see going higher than that.
JET
11-16-2012, 11:51 AM
Depending on the fuel, higher compression doesn't matter if you have the fuel to back it up...e85/e98.

That is not really correct, only if you have reached the limits of your turbo system. Here is a very simple static boost calculator to see the correlation between CR and boost. Just remember, boost gets to run through the intercooler after compression and CR does not.

http://www.rbracing-rsr.com/compression.htm
Murlo26
11-16-2012, 12:13 PM
^correct, I should've elaborated. I just meant psi for psi, higher comp is better assuming you aren't octane limited. Ethanols cooling capabilities helps too.

And I guess when comparing I'm assuming the turbo to be maxed.
JET
11-16-2012, 12:18 PM
Exactly right. If the turbo is maxed but the fuel has room left then you can raise CR to take advantage of that.