Should i get a higher or lower compresion ratio piston

[dajoker]

I've looked around and tried to reasearch this a little bit but i just dont understand which one to get. My goel is to run low 10's i was thinking about getting ross pistons (85.7/ at 8.3 C/R) and Crower rods but i dont understand what compression ratio to get whether it be a 9-1 or lower and i dont know what size pistons to get. Thanks alot

 

[DSMJim]

Only get a larger size because you need to in order to clean up your bore. Going slightly over sized say 20thou will not give you any perfoemance gain so ignore that.

The higher the compression ratio the more power you will make. 9:1 is always a favorite among 4G63 engine builders, higher then stock, but not 10:1. 10:1 has been used on many race engines, but you had better have a really good tuner or your in trouble.

 

[LousyDriver]

Compression ratio is the volume of the cylinder when the piston is at BDC (bottom dead center) divded by the volume of the cylinder at TDC.
Increasing the compression ratio on a Non-turbo motor typically increases the power output, but increases the minimum octane fuel that can be used.

However, when building a turbo motor, the air is already 'compressed' somewhat due to the turbo. Having a high compression ratio will cause the car to detonate (and destroy your engine)

So, if you plan on increasing the boost past the stock boost, you are better off lowering the compression ratio. Lower compression will greatly increase the level of boost you can run prior to getting detonation.

However, this is only the case for a highly modified engine (up to and over 20psi boost). If you are running close to stock configuration, you are best off keeping stock compression ratios. Running high compression with a turbo is a quick way to turn an engine into junk.

 

[Groomz]

A higher compression ratio means less room for error in tuning.

 

[jpolizo]

Originally posted by Groomz
A higher compression ratio means less room for error in tuning.

Yep -- don't go higher unless you've got the resources, skills, and patience to tune it.

 

[gpsturbo]

I agree, i would much rather have 7:8:1 cr then 9:1. Or you can be like the honda boys and run like 8psi

 

[turbohappy]

Remember that 15 psi @ 9:1 is ROUGHLY equal to 20 psi @ 7.8:1 in terms of detonation. With 9:1 you'll have more power off boost and spool faster, but with 7.8:1 you'll make more peak power.

 

[TurboTerror]

Originally posted by turbohappy
Remember that 15 psi @ 9:1 is ROUGHLY equal to 20 psi @ 7.8:1 in terms of detonation. With 9:1 you'll have more power off boost and spool faster, but with 7.8:1 you'll make more peak power.

you can still make lots of peak power with higher C/R. I have a 1.9L honda motor that runs 10.0:1 with a Garrett GT-4540. So far with only 6 hours on the dyno I have put down 587WHP on 20psi. I am shooting for 800WHP from 30psi. The possibility is there, but it does take A LOT LONGER amount of time to tune it properly. These is also A LOT smaller margin for error in the tuning phase.

If I was you, picking the 9:1 would be a safe bet. You can still make good power, faster spool, and better daily drivability w/out having to be into the boost a lot.

 

[TurboTerror]

Originally posted by turbohappy
Remember that 15 psi @ 9:1 is ROUGHLY equal to 20 psi @ 7.8:1 in terms of detonation. With 9:1 you'll have more power off boost and spool faster, but with 7.8:1 you'll make more peak power.

you can still make lots of peak power with higher C/R. I have a 1.9L honda motor that runs 10.0:1 with a Garrett GT-4540. So far with only 6 hours on the dyno I have put down 587WHP on 20psi. I am shooting for 800WHP from 30psi. The possibility is there, but it does take A LOT LONGER amount of time to tune it properly. These is also A LOT smaller margin for error in the tuning phase.

If I was you, picking the 9:1 would be a safe bet. You can still make good power, faster spool, and better daily drivability w/out having to be into the boost all the time.

 

[turbohappy]

Of course you're going to make more power with higher cylinder pressures. But keeping cylinder pressures constant (which keeps your detonation threshold constant), you'll make more peak power with low compression.