[rjsmd]

When considering a built engine, there is the option to increase the CR. What are the potential benefits and at what costs do they come?

I realize that it offers the potential to improve off boost power, but will it be noticable? How much more sensitive to knock would a higher compression turbocharged engine be?

Is there a conventional wisdom on this issue? I believe that turbocharged engines typicaly run relatively low compression ratios, but I was wondering what is currently considered to be the ideal?

 

[SUPRA180]

lower compresion will alow you to run more boost , and im shure that higher comp motor under boost will be more sensitive to knock ....what are your goals ? track car ? street car ? track/street ? race gas all the time ? once in a while ?

 

[coreapoc]

Ideal?

Thats more of a sit down and discuss what youre going to be doing with the car.

If you are going from the ground up building an engine and plan to add boost into the mix at any point, and since its a built engine Im ASSUMING you were aiming for high boost psi and a larger single turbo. I would leave it to stock compression.

 

[Hefti2]

...or even reduce it

 

[Espeefan]

At what point does a lower compression motor with higher boost pressures become less efficient at making power, when compared to stock compression at a lower specific boost level?

 

[Hefti2]

As a rule of the thumb you can say: The lower the CR the higher the possible boost and the less the power under vacuum condition. That is no black and white thema but rather a fluent transition.
For e.g. a dynoqueen a very low CR combined with boost of 30+lbs is cool but on the street that´s "not so nice"...

 

[rjsmd]

I asked the question because I was thinking of purchasing a friend's built motor. He is going 3.4L, and was willing to part with his built 3.0 for a reasonable price. Only thing is that the CR is 9:1 on the pistons, and the block was decked so the overall CR may be higher. Although he is willing to sell at a reasonable price, given the current cost of increased displacement options, it would only cost me a few thousand more to go with a package that included the SP3.2L stroker or a few above that, the BC3.4L. My goal is a street beast. Currently running a GTR70 turbo, with which I am quite happy. Ideally, I would like to get the turbo to spool earlier, and shift the powerband to the left. I would more than likely swap out my turbo for the SP71DBB then: go out, insert key, turn key, hit the gas, and repeat.

 

[SwiftDarkness]

9.1 are fine. ive ran that compression for months. higher compression will give you more hp and spool out of the amount of boost you can run (which is a little bit lower then 8.5)

 

[mitulparmar787]

General rule of thumb is higher compression is more sensitive to knock (as said above) but will also give you much faster spoolup and more power "under the curve". The downside is you may end up having to run meth injection along with pump gas or race gas to run the motor. I'd look at the goals of your setup (street, track, dyno queen, etc) and speak to a knowledgable tuner/engine builder about what would suit you best .