High altitude compression
 

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

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

You could retard the timing a couple of degrees too!

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