best compression for a rebuild?

[soulcontroller]

that you want to slap a turbo kit on? im going to get the howell bottom end, but im wondering what compression i should set up with for a stage II bolt on...

 

[ninja0331]

I'm kinda new to these engines but I have rebuilt other motors. Based on my experience, compression ratio should be kept under 10:1 with aluminum heads for a naturally aspirated street car running 93 octane.

For turbo applications, it should be no higher than 8.5:1. If you plan on boosting alot, I suggest going lower than 8:1.

However, compression ratio doesn't mean much with turbocharged applications. It's compression pressure what you really want to look at. You can increase boost as long as you increase intercooler efficiency. Also another thing that will allow you to run good boost without detonation is polishing the chambers and freeing up the exhaust runners.

Another thing, getting a camshaft with a longer intake/exhaust duration and some overlap will enable you to run much more boost and will improve efficiency tremendously because the pressure from the incoming charge will push the burnt gasses out of the chamber and it will cool it down at the same time.

Hope it helps.

 

[L2RTSiAWD]

Depending on all the mods you want to do 9:1 is a good CR for a built street turbo motor on pump gas. check out the pistons thread in 2.0 performance for some very good piston info.

Later,

 

[xhypno]

The 8cyl belief on our engines. Aluminum head can take more preassure then cast (older engines). I have seen alot of NA cars run 12:1 and 13:1 ratios and heard of even higher. But turbo, you always hear that the lower the better (that is an old 8cyl belief). Our engines were by DSM (a conglomerate between Crysler motor(in the 60's was know as the Hemi motor corp) and Mitsu motor corp), Our heads are designed to be strong (except for the 2gnt). A 4G63 can take very high compression, even for turbo, with the right fuel system. The average ratio used for pump gas is 8.5:1 (same as 2g turbo), but one of my friends is running je custom 10:1, with a 20g, 1000cc injectors, SAFC, and a custom 400lph fuel pump. His car dynos at 700hp (also built with cams new crank and alot of other stuff). The most important thing about turbos, compression and our engines is to us a VERY STRONG HEAD GASKET and VERY GOOD FUEL SYSTEM. I am right now building a ESI race car, with a NT block, JE 9.5:1 forged pistions, and a HKS 1.25mm head gasket ($200). That should give me a ratio of about 9.3:1, on 93 octane, that should produce alot of hp with barily any detonation.