Can't seem to decide on what compression ratio to use

[Chadillac2000]

Been doing tons of research the past couple days before I continue on to ordering my piston and rod combo. Been trying to decide between the Wiseco 9:1, Eagle 8.5:1, or Wiseco 8.3:1 pistons. Going to be running a large turbo such as a gt35r or similar and it probably won't be really efficient until the upper 20 to lower 30 psi levels and will probably be running up to 35psi at times. The car will be propane or alcohol injected and would love to run pump gas except when at the track. I know that the 9:1 compression will see more power under the curve which can seem to spool the turbo up faster, but I know i'll be able to run more boost with the 8.5:1 and 8.3:1 compressions. You guys have any suggestions?

BTW will be using a cometic HG and i've heard that lowers the compression a bit.

 

[Chadillac2000]

Anyone?

 

[jpbrazill]

As far as the gasket goes, you can opt to choose thickness. As far as an opinion on the compression ratio for you pistons, given the choice in turbo, I would advise the 8.5:1. This will be just fine for you street/strip car.

 

[TSIfreek]

You will be better off running the 8.5 or even 8.3 compression. I would not be concerned about off boost performance. You can have a much more agressive tune early in the rpm range to spool the turbo faster and run a leaner AFR as well. On pump gas it get tough to make decent HP expessially on the smaller turbos.
Going the 9.0 comp your going to need to run higher octane fuel when you start to want to make power on your tune with out knocking.
Like I said you can work around the low compession with a more agressive timing and leaner AFR on spool up. All power in our motors is from the turbo itself not what pistons or any thing of that matter. I have been having good results on a daily driver with ross pistons.

 

[fourreGsixty3]

I would suggest running the 8.3:1 or the 8.5:1. You can advance timing in the low end to give you decent power when off boost, and where the turbo has yet to kick in, then retard it when boost kick's in hard, and dump fuel in for an 11.0-11.5 AFR...give or take some.

 

[T9S1i]

What is the highest octane pump gas you can get? Thats the question i had to answer to myself. Here in cali, i can only get 91 octane, so i decided to go with the weisco 8.3's... my car sees a lot of daily driving so its going to have 91pump in it all the time so i didnt want to go too aggressive with the compression.

If you can get 93 or 94 octane pump gas for daily driving, then i wouldnt be too concerned with 8.5-9.0

 

[Artago]

Here in cali, i can only get 91 octane,

You can only get 91 in Cali? Weak. We get 94 in Vancouver Canada and in Europe 95 is the MINIMUM and you get get 100 almost everywhere. Damn lucky Europeans .
Tom.

 

[Chadillac2000]

Yeah I bounce back and forth between NC, SC, and GA so 93 octane is pretty readily available.

 

[xveganxcowboyx]

You can only get 91 in Cali? Weak. We get 94 in Vancouver Canada and in Europe 95 is the MINIMUM and you get get 100 almost everywhere. Damn lucky Europeans .
Tom.

Different rating system. Subtract roughly 5 points to get our equivalent.

 

[Maglin]

Just a FYI. Just finished building my buddies 97 TSI AWD. Used Wiseco's 8.3:1 pistons. Had the head freshened up and decked. Block milled and used HKS 280 cams. Running a HX35 (large frame turbo) and a Megasquirt for fuel and ignition duties. Car runs 91 pump gas and since it's being broken in only 26 PSI. On it's 200 mile drive home the car managed to get 28.5 mpg. And is crazy fast @ 25.5 PSI which I just round up to 26 when talking about it.

 

[Rick@AP]

I seriously doubt you will be able to tell the difference between 8.3 and 8.5 compression.
I've built motors with 10:1 compression pistons that have worked just fine on 93 pump.
Focus more on total compression after the rebuild (piston compression, headgasket thickness, bowl shape (if porting) , etc.)

 

[silver bullit]

I would do a 9.1 but 8.5 would not be bad anything less than 8.5 really is like running your stock compression cause there is only a small gain.

You run 9.1 you can make more hp at less boost, or 8.5 and make more boost. But if i was to build something i would do 9.5.

 

[dsm-onster]

The question is what is the knock threshold with respect to compression for your setup. I don't know the answer to that . But, I do know the simple thing that lowering comprssion raises that threshold.

How much do you want to run pump gas? Is it worth the loss in power from lower compression pistons? What is the loss in power? I SERIOUSLY doubt there is that much loss in power. But HIGHLY suspect at 8.5:1, you're running close to the knock threshold for compression.

It's that simple to me.

I agree w/ Rick. If you get higher compression pistons, then you can get a way with more valve unshrowding. I think any change for the sake of flow w/ no loss in actual results is good