912 engine rebuild compression ratios

[mike farmer]

I am planning to rebuild a 1600 cc engine for a '67 912. Can anyone recommend a target compression ratio that will assure long engine life with readily available gas in the USA? I think the original C. R. for the 912 was 9.3:1. Any thoughts on cylinder shims as a means of reducing compression vs. buying lower compression piston/cylinder sets? I have read guidelines by Gene Berg Enterprises for air cooled VWs, which recommend a
C. R. of 7.0:1 or less for regular gas. Are the Berg recommendations appropriate for a 912 engine?

[Griznant]

My personal opinion is that Gene Berg did not know more than Porsche. He did have experience, and at some time, in some combination for a T1 VW, he may be right, but making a blanket statement like he did is erroneous.

While it is true that 9.3:1 is high, and today's gasoline does not have quite the octane rating of old-school gasoline, many people run this on the street with little trouble. It's all about the combination of parts you are going to run, and how you plan to use it. Even Gene Berg ran higher compression in his competition engines. If you plan to keep it all stock, then the stock compression is probably close to perfect (maybe a little less if you can't get good gas), but definitely do not run 7.0:1 in that engine. You'll end up with a slow smokin' pig.

My T4 engine is getting 9.2:1, and I know this is a viable compression for use with my combo. Why? It has been proven, and runs great for the people who have this setup. However, running this on a stock bus engine would result in a big fat mess. Just figure out what your performance parameters are for your car, reliability, available octane, and do some research. Do not rely soley on Gene's "Blue Bible". The guy wasn't stupid, but he didn't know everything either.

Later,

G

[snowman]

There is nothing wrong with 9.3:1 compression in a 912. You can probably use 89 octane gas with no trouble. If you do experience knocking, have the timing retraded a bit to prevent it.