How can I determine if I have a 3.4l?

[Porschefile]

The title says it. I've been searching hour after endless hour and it's driving me a little crazy. I recently bought an '87 911 with a Protomotive turbo kit. I was told it was a built 3.4l using the 3.2l as a base, and that it supposedly has 964 cams. Is there any way I can physically distinguish if this is a 3.4l by looking at the motor? I've looked at lots of pics and can't see any notable differences in the external cylinder designs that would easily distinguish between the 2. I'm afraid of doing anything with the car without knowing what displacement it is, and what compression ratio it has. The thing that scares me most is the compression ratio. It looks to me like the lowest CR 3.4l pistons for the 3.2l engine were Mahle 9.8:1. If the CR is really that high, then I probably shouldn't even be able to run the boost I'm running (1bar). Short of pulling the motor apart, I'm not sure if there is a way I can figure these things out with the motor in the car. Anyone have any ideas?

[Walko]

Pull it apart then you will know

and while your in there....

Michael

[Wayne 962]

You can get a ballpark idea by running a compression test and seeing what the numbers are...

-Wayne

[2.7RACER]

A 1 bar boost effectively doubles the compression ratio. Not likely anything close to 9.8:1 pistons.
You recently bought this, I would contact the seller for more details.

[Porschefile]

I have, the problem is he didn't have the car built (it was the guy he purchased it from), and the shop that he thought built it doesn't have any records except for some regular maintenance.

Wayne, I've been trying to figure this out. Is there some sort of math for using the compression numbers to determine an approximate compression ratio?

Thanks for the help guys.

[Henry Schmidt]

Call or email Protomotive and asked what they use in 3.4 liter turbos. Mahle made 98 mm pistons for both turbo and none turbo applications.
The Mahle turbo pistons (as used by Ruff were 7.5 to 1 ) had a small problem. Too short piston skirts. The pistons have a tendency to rattle.

[Porschefile]

Thanks for the help Henry.

[David]

I'm no expert, but I've noticed the compression readings to be a little higher than atmospheric pressure (14.7psi) times the compression ratio.