Aircooled Volkswagen Type 1 engine building video: Top End Install Part 1, Type 1, 15001600cc.
In this segment, VW engine building guru Mike Smith shows how to measure the combustion chamber volume inside the head, in order to calculate the compression ratio of the engine. Other subjects covered here include proper positioning of the piston rings inside the jug, and assembling the piston/piston pin/jug on the rods. The deck height of the VW engine is also measured here, and that volume is included in the compression calculations.

Formulas for setting compression ratio:

Swept volume of one cylinder = Engine size divided by four

Bore (in mm’s) x Bore (in mm’s) x Deck Height (inches) x .01996 = Deck Volume (in cc’s)

Head cc’s + Deck cc’s + 1 cylinder swept volume
= CR (:1)
Head cc’s + Deck cc’s

Depending on the octane you are running, your target compression ratio should be set at factory level, or preferably lower, for street gas. Use machined, not stamped, spacers under the jugs to lower the C.R. Note that the compression ratio on an engine assembled with stock, brand new parts is almost 7.7:1. The exception to the newparts rule might be if you were using the very latest Mexican factory dualport heads, because some of them have 5758cc combustion chambers, which allows you to run the smallest possible deck height.

These C.R. recommendations revolve around running a stock cam. If you are building a performance motor that uses a more radical camshaft, there is some room to run more compression. The reason for that is because cams with more duration are less efficient at lower rpm's, so the dynamic C.R.(which is the C.R. actually seen by the motor) is less at lower rpms. The tendency to detonate is also most prevalent at lower rpm's. However, as the octane level of pump gas continues to plunge, there still isn't much room for high C.R. on the street. In most of California, for instance, the best pump gas you can get is only 91 octane.

The compression ratio used on the 1970’s Beetles was 7.3:1, but the chemical composition and octane level of the gas was different back then. That octane rating system(RON) involved a different testing procedure than the rating system used today, which is (RON+MON)/2. It is now called the CLC rating system.

In 1978 or so the VW factory tried to end the confusion over what gas to run by claiming that “91 RON equals 87 CLC”. This statement was made in the 1978 bus owners manual, but it is conspicuously absent from the 1979 Bentley manual, which recommends only RONrated fuel.

If your compression is set too high for the octane you are running, there is a risk of engine damage. The best way to maximize your engine's efficiency is to run the tightest deck and the biggest possible squish area. You will need the largest possible head cc's to do this.

There is further relevant information on octane ratings and compression in the excellent internet gasoline faq. Here is an excerpt regarding octane requirements for carbureted engines with nonmanagement systems (ratings are CLC):

Compression Ratio: Octane Number Requirement: Brake Thermal Efficiency( Full Throttle )
6:1 81 25%
7:1 87 28%
8:1 92 30%
9:1 96 32%

It appears that, at 7.0:1 compression, a minimum octane level of 87 is required. Since the octane test engine used here is water cooled, you must increase the octane requirements slightly to compensate for your hotter aircooled engine. For instance, that 7.0:1 compression setting could require an octane level of 88 or more on the VW engine.

For more specific information on the problems caused by running high compression with street gas take a look at the writings of Gene Berg. He goes into detail discussing the damaging effects of things like engine pinging and knocking that isn’t loud enough to hear.