Saturday, November 25, 2006

AV - VW Reliability - Compression leaks

A key factor in the long term reliability of the Volkswagen engine, stock or modified, is maintaining a perfect seal between the combustion chamber and the barrel of the cylinder. Although compression ratio and cylinder head temperature can effect this seal the most critical aspect the amount of sealing surface relative to the pressure it must contain.

Here are some typical cylinder wall widths:

Stock 85.5 = 0.165"
90.5 = 0.150" (requires machining the case & heads)
92 = 0.121" ditto
94 = 0.139" ditto

The so-called 'slip-in big-bore' cylinders are merely over-bored stock cylinders. 87mm slip-in's have a wall thickness of about 0.105"

In the Photo archive I've created an album titled JUGS in which you'll find a few photos illustrating the relative diameters & wall thickness of the cylinders above, with the exception of the 'slip-in' type. (*)

(Ed. Note: The photos were posted to the CX4 Group. If I can find the originals, I'll attach them to this file.)

One reason for posting this information was to back-up my comments in recent posts regarding the suitability of various parts. You'll notice that the 94mm jugs actually have a thicker wall than the more common 92's.

As a general rule, compression leaks (ie, failure of the seal between the combustion chamber and the cylinder wall) are not a problem when the engine is properly assembled & maintained, the compression ratio is 7.5:1 or less and the cylinder wall is at least 0.120" in width.

You'll note that 92mm jugs are right on the line. When 92's are used on engines running higher CR's the joint between the barrel and the cylinder heads is usually fitted with a copper gasket, carefully annealed at the time of assembly.


(*) The jugs in the photos were what happened to be in the shop at the moment. The stock jug and the 92 came out of the junk box, the 90.5 and the 94 from engines under construction. I've no examples of the slip-in types because I don't use them.

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