Compression Ratios and Engine Shims

[Psucamper]

This note describes a simplified method of determining the thickness of a block to cylinder shim spacer needed to reduce the compression ratio of an air cooled VW engine. The necessary assumption is that THE SAME PISTON /CYLINDER set will be used and there have been no modifications to head or cylinder geometry. The only change will be in the effective "deck height". There is considerable mathematical manipulation involved which is tedious but not difficult. The analysis is not presented here but is available on request. I believe this is original work but don't rally know (or care). Anyway here are the defimitions you need to make the calculation, C1=initial CR, C2= desired (lower) CR, s= piston stroke, h=the (calculated) shim/spacer thickness.
C1 is determined from the mfgrs data sheets or from previously measured data.

The equation that determines the needed shim thickness is: h=s(C1-c2)/(C1-1)(C2-1). Note that the shim thickness is independent of things like cylinder head volume etc. Any consistent set of units can be used.

An example is instructive: A type 4 GD engine is a nominal 2000cc with a bore of 94mm and a stroke of 71mm. The VW shop manual (Bentley) says the stock engines have a CR of 7.3:1 and the recommended fuel is 91RON. The most popular current fuel is not 91RON but is (R+M)/2 . Research octane +Motor octane. This is better called an Anti Knock octane index or AKI. RON 91 is about 87AKI (pump gas). The chemistry of octane ratings is a whole nother subject and to be avoided if possible. To permit this engine to run safely/well under load some "experts" (I'm not one) recommend the Cr be reduced to 7.0:1 or less.

Barring reliable engineering data on measured knock performance under varying loads. I will not comment on th efficacy or desirability of such CR reductions.
Finally, some numbers. If one chooses a new (lower) CR of say 7.0:1 the previous equation yields a shim thickness of 0.563mm/0.22". Not a readily available thickness but close. 0.020" shims are commonly available. Using the differential calculus I have developed an expression that shows the "sensitivity' to small changes in shim thickness and its effect on changes in the compression ratio (if anybody really cares). RDL 6/16/12

[Gypsie]

This is interesting and I'm sure it was a pate scratching challenge, but the take away I have is that it seems like we are going through Maine to get to Texas (ie the long way around).

Why even use the numbers. If you know you want to reduce compression ratio and don't want to do the calcs, why not just put on a thicker spacer (the next thickness up or further...).

If one is inclined to calculate CR, one is likely able to measure and calc some of the assumptions (piston/cyl deck, piston dish cc's, Head CCs, etc.) and plug data into the ready made calculators out there.


I think when one is elbow deep into a rebuild they will want to have accurate measurements to avoid a re-rebuild.

[Psucamper]

As I said....you HAVE to know the initial CR to make it work. Yes, there is no substitute for determining the existing parameters (cc's.deck, etc.) when either acquiring an unknown engine or doing a buildup. But if you can even guess at an existing range of CR's you can test for that complementary range of shim sizes very quickly. Puts you in the "ballpark" fast. RDL

[Amskeptic]

RON 91 is about 87AKI (pump gas).
To permit this engine to run safely/well under load some "experts" (I'm not one) recommend the Cr be reduced to 7.0:1 or less.

I do not recommend reducing the compression ratio on a normally-aspirated VW engine below 7.3:1.
This IS the reduced compression ratio across the model range in 1973.
VWs were famous for handling most any fuel quality in any country across the planet.
Colin