High altitude compression

[BenI]

I am building an engine for high altitude (5000 ft) operation. It’s a 2056cc carbed engine for my 914. From the factory this engine ran about 8.5:1 CR which yielded a sea-level compression reading of about 125 psi per cylinder. When this same engine operates at 5000 ft it should only reach about 107.5 psi/cylinder. If I try to reach 125 psi/cyl at 5000 ft, I will be running a CR of about 9.65:1 and will have a cylinder pressure at sea level equal to 141 psi. Should I take the safe route and run about 8.5:1 or should I try for the 9.65:1? What will happen if this car finds itself at sea level then? Thanks for any help…

Ben

[Ron Meier]

At sea level, buy better gas! You'll probably get preignition.

You could retard the timing a couple of degrees too!

[Dave at Pelican Parts]

If you run higher-octane fuel, you can use a higher compression ratio. If you run a lumpier cam with more overlap, you can run a higher compression ratio. And there are other factors which I don't have time or energy to go into right now.

I feel that calculating compression pressures from compression ratios will give you incorrect results, and it's a bit short-sighted to back-calculate compression ratios that way.

I would guess (and it's just a guess) that you can go with the higher number, as long as you are careful about going down to low altitude--always run top-grade fuel when you head down, and back off on your timing.

--DD