Does higher compression ratio raise TQ off boost?

turbotalon1g · 11-16-2012

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

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

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

Quote:

Originally Posted by JET

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

Tachyon might be a good resource on this

**Murlo26** · 11-16-2012

Quote:

Originally Posted by goodhart

Tachyon might be a good resource on this

Indeed, he runs 11:1 I think, maybe even 11.5:1.

**JET** · 11-16-2012

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

Quote:

Originally Posted by Murlo26

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

^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

Exactly right. If the turbo is maxed but the fuel has room left then you can raise CR to take advantage of that.

**Halon** · 11-16-2012

I don't have a ton of first hand experience, but the theory I've basically accepted is:
- It does increase off-boost torque
- It does not affect spool (or not enough to really notice)
- It will make the car feel as if it's spooling quicker because of the increased off-boost torque/response, but if full spool was at 4k RPM at the lower CR, it will likely still be 4k. You'll just get to 4k quicker because of the increased off-boost torque.
- More power at the same boost level, if done properly.

Also the other thing that I'm learning a lot about on the 2JZ platform that I rarely ever heard with the 4G63 platform, is to consider quench area is as ideal as possible. I'm not an expert what so ever, but I've heard some individuals whom I'd consider to be experts, say that quench pad area/design should be as important and thought out as CR is when building a motor.

**Shane@DBPerformance** · 11-16-2012

Magnus was always big on designing his custom Ross pistons to try to take full advantage of the quench area.

Higher compression will add power/torque when not making boost or on an NA car, so it will make a car feel stronger during normal driving. Higher compression doesn't create more exhaust though, so it doesn't help spool a turbo faster, but it will make more power with less boost, so it can feel like it's spooling faster. As you go up in compression ratio, the gains become smaller and smaller though, so going from 8.0:1 to 9.0:1 is a much larger power gain than 12.0:1 to 13.0:1.

**Halon** · 11-16-2012

Yep, Magnus is the only one I ever heard pay any attention to quench on 4g63's.

**Goat Blower** · 11-16-2012

It's not apples to apples, but I was VERY happy with the 10.5 to 1's I put in my Evo, that thing spooled at idle, or at least felt like it. When I rebuild my current DSM motor someday, I'll definitely be going higher compression. For a street car, I think it makes a noticeable difference, if it's a drag car, lower compression and higher boost might be better.

**Murlo26** · 11-16-2012

For most of us locals who run strictly e85, I don't see a downside to running higher compression.

We all run our turbos at MAX boost anyway since we are on E85, so if you are building the motor anyway, mind as well do higher compression.

If you are in fact doing a straight race/drag motor then yea you might be better off with lower comp more boost, but us street guys it seems like a no brainer.

turbotalon1g · 11-16-2012

I'm planning on 10.1:1 instead of 10.5 on wiseco HDs, but this was for a guy with a 2.3 9:1 motor.

I'm pondering AL rods vs. howard HDs now

**Halon** · 11-16-2012

Have you considered talking with Magnus about their pistons with (marketed) improved quench design? No idea where those land price-wise though...

10:1 (stock CR) is what I'm running on the SC300.