I run the 63mm cylinder as does my son and both of us run about 280 psi on the compression either hot or cold. Maybe 5 lbs is lost when engine is hot . The important thing with a thin cylinder is to install one with a steel sleeve and not a cast sleeve. The problem with a 63mm cast sleeve is after time the bottom of the sleeve will start to crack due to the side pressures by the piston especially a short skirt piston. The cast has a larger expansion rate then the steel has if a good quality steel. Also the cast is more prone to crack from its brittleness as compared to a steel sleeve.
The squish area is also called the quench area, or quench band, as it increases the surface area of the combustion area and actually cools or quenches the burning mixture around the edges of the combustion chamber. This assists in maintaining a steady (not violent) burn rate, and offsets any tendency for high speed detonation or pre-ignition to occur. With a low quench area the combustion chamber and cylinder will over heat causing the pre-ignition (violent burn rate in cylinder prior to TDC and plug firing). So the larger the quench the cooler the engine will run. Now if the compression is high this sometimes will also cause pre-ignition due to the heat caused by compression igniting the mixture prior to TDC or plug fire. In this case a higher octane fuel will be needed due to its higher flash point.
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Originally Posted by toomanytoys2
As Johnny 5 mentioned, if you go with a big bore kit, you want to go with one with a thick cylinder liner. I am an old school VW guy and have been working on air cooled VWs for about 35 years. In an air cooled engine, most of the cooling actually takes place at the head, with a much smaller percentage of cooling occurring at the cylinder. The problem with running a thin cylinder is when they get hot, the cylinder itself will expand. when this occurs the rings can no longer form a good sealing contact with the cylinder and you end up with blow by and a lost of compression. This doesn't do any damage to the engine, but you actually lose power when the engine is hot. Also the combustion squish is more critical in an air cooled engine. Since the head is where most of the cooling is done, keeping you squish numbers low is a big determining factor in engine temperature. If you have a high squish number, more of the combustion will take place in the cylinder and will raise your engine temperature. Also, an oil cooler is always a good idea for a hopped up air cooled engine. Since air is not a good medium for the transfer of heat compared to water, adding an oil cooler will cool the oil and the cooler oil will help to cool the head. I can't tell you what direction will give you the most bang for the buck since I installed my A12 cam and the big bore kit at the same time. At the same time I P&P the head, did the redneck intake and opened up the exhaust. However, I can tell you that unless you make the intake and exhaust more free flowing, you will not see huge results in what ever mod you end up going with.
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