Stock, single carb engines: Not more than 6.6 for 87 octane to 6.9:1 for 91 octane. These recommendations are for the conventional head chamber. When the heads are properly semi-hemi cut you can increase these numbers by 0.3 points. Stock VW's originally had about 7.3:1 and this should be reduced to 6.6:1 when you intend to use 87 to 91 octane gas with the stock combustion chamber or 6.9:1 with the semi-hemi cut head.
Unleaded gas is no problem with any head combination that has the original factory stock heads of 1967 or later as they have the hard chrome stem valves required for unleaded fuel.
Dual carbs such as our GB 476-477A Kadrons with or without head work use 6.6:1 to 6.9:1 compression in correlation to the octane rating and chamber design. Dual Weber carbs such as our GB 479 and ported heads, use 6.9 to 7.1:1 compression.
Dual Weber carbs, welded big valve heads and large tube exhaust use 7.1 or 7.3:1 compression with the Semi-Hemi cut heads. All of the above recommendations are for sea level operation. Vehicles that ALWAYS run at high altitudes may run higher CR. See GB 801-HEADS, GB 801-CR or send a self-stamped envelope for high altitude formulas.
The duration of the cam has nothing to do with octane requirements. Just because you install a longer duration cam it will not allow you to run extra compression. Remember, conservative compression ratios 6.6:1 with normal combustion chamber and 6.9:1 (plus 0.3) if using semi-hemi heads, be sure to add 4 degrees more timing with the semi-Hemi cut heads to make the most power.
This will live longer and run cooler with better economy when 87 to 91 octane fuel is used.
Extreme high performance street cars driven regularly on the street with some race use, can also provide superior-to-stock VW life when the correct components are used. The gasoline octane rating must match the demands of the engine to achieve long life. For technical literature on CR vs. Octane, and formulas you can use to calculate your needs, order GB 801-CR.
Again, many people find our engine kits provide a package that works best for them or will use these as guide lines to create their own engine needs.