Pat Kennedy

My technician mentioned that we could raise the compression ratio of my engine from 8.0:1 to 8.5:1. How is this done, and what are the ramifications?

Thanks.

Danno

Shave off 0.050" from the cylinder head. Milledge and Powerhaus does it all the time. Only a caveat is that you want to make sure your valve-springs and lifters be in good shape.

Andy

According to Smokemup.com's calculator of <a href="http://www.smokemup.com/auto_math/eff_cr.php" target="_blank">effective compression ratio</a> an 8.0:1 motor with 18psi boost has an eff CR of 17.8:1, if you bump static compression to 8.5:1 you will need to limit boost to 16psi to have the same eff CR. That will aid your off-boost performance somewhat but I've been told that less CR and more boost will make more HP than more CR and less boost. Say that real fast.

Danno

"That will aid your off-boost performance somewhat but I've been told that less CR and more boost will make more HP than more CR and less boost. Say that real fast. "

Brfvfrhhvfsb.... too fast, my tongue's all tied up..

There's a difference between power potential and power achieved and the resultant torque curve. Unless you're using the absolute maximum boost a certain compression-ratio will support, you'd be better off raising the compression to get better off-boost torque in the low-end and avoid Dr.Jekyll/Mr.Hyde all on/off powerband.

For example the power-potential on 92-oct. street-gas:

8.0:1 @ 22psi boost = 380-390rwhp
8.5:1 @ 18psi boost = 350-360rwhp
9.0:1 @ 15psi boost = 320-330rwhp
9.5:1 @ 12psi boost = 300-310rwhp
10.0:1 @ 8psi boost = 280-290rwhp

So if your goal is a measly 300rwhp, you might as well go with 9.5:1 compression @ 12psi to have the best partial-throttle/mid-range instant response with a wide & broad torque curve (and faster car too).

It would be a waste to use 8.0:1 and 17psi because you'll have a lot more turbo lag. If you're going to put up with that, might as well run 22psi and get a whole lot more power.

Pat Kennedy

Thanks guys. Indeed, my tech indicated the slightly higher compression ratio would result in less turbo lag, albeit at a lower boost. That is our objective.

TurboTim

We have ran compresion ratios as high as 9 to 1 on the 951. Personally, I did not like it, nor would I do it on my personal car. We were way limited on the amount of boost we could run on pump gas (like 4-6 psi less boost then 8 to 1 compression). There was not a big difference on spool-up or low end torque either. I did have total control of timing and fuel with an Electromotive unit but we could not avoid the pinging above 12 psi. 12 psi with 9 to 1 compression is not going to make as much power as 18 psi with 8 to 1 compression. At least this is what my experience dictates. Take care.