After doing some valve work on my 71 B20E (replaced a couple of valves, had hardened seats put in on the exhausts and a 3 angle valve job), I decided to cc the combustion chambers. I measured volumes of 46.7, 47.1, 46.9 and 46.3 cc which translates to compression ratios that vary from a low of 10.54:1 to a high of 10.73:1. The burette I used to check the volumes had a measuring error of about +/- .1 cc, which in the best circumstances would reduce the maximum volume difference to 0.6 cc (or 1 cc in the worst circumstances). Does anybody have any suggestions as to what is an acceptable variation between cylinders in terms of cc volume / compression ratio?

If you measure the depth of the valve heads into the combustion chambers I expect it will account for the variation in chamber volume. Those chambers with thicker new valves or seats that are not as deeply cut into the heads will have the smallest volume.

As to how much variation is acceptable, that depends on how close you are running to "too much" compression as determined by the octane of your fuel, or in some case the racing rule book. At 10.5+:1 your have a lot of compression for pump gas. In this case equalizing the chamber volume and possibly increasing it may be desirable. Some racers will sink the valves a bit on the small chambers to do this, although sinking valves very much is generally undesirable.

Unless restricted by a rulebook some chamber modification my better serve to equalize and increase the volume. The ridge on either side of the spark plug and the chamber wall around the intake valve in order to "unshroud" it are suggested. Don't screw up the head gasket sealing surface. drop in an old valve to protect the valve seat while grinding. Try to have a minimum reduction of the area that provides "squish". Use eye protection when grinding.

Good luck

Charlie

The ridge on either side of the spark plug and the chamber wall around the intake valve in order to "unshroud" it are suggested.

I won't put a head back on without removing the intake side ridge adjacent to the plug. It helps a lot with swirl, much more than tight squish.

Did you measure them once or more than once? The burette error is definitely NOT the biggest error
in your measurement! In fact if you are using grease and glass on the face of the head and filling
through the spark plug hole the thickness of the grease film could give you a bigger variation than
what you got if you were not VERY careful. Half a cc is not very much! ~1% of your measurement.
--
George Downs, Bartlesville, Heart of the USA!

In theory of course they all should be the same but I suspect you won't notice anything with the engine running. Finding out why they are different might allow you to balance them up a bit. I'd suspect that the seat inserts are in slightly different positions and you can check the head face to valve centre measurements to find out but a straight edge across the the valve stems would give you a quick check. It's hard to make 4 similar combustion chamber shapes that give equal firing efficiency and flow, while keeping the cc of the chambers the same.

The original question related to the difference unequal chamber volumes have on static compression ratio.

Might I suggest that since the volume used for the computation of static compression ratio includes the volume of the deck height - the height of the piston above or below the deck of the block at top dead center - unless you add that volume to the actual combustion chamber volume, you don't know whether there is any actual difference in static compression ratio between cylinders or what that difference is.

Might I also suggest that engines never actually see static compression ratios.
What they do see is actual or operating compression pressures or ratios. The biggest difference in actual, operating compression between cylinders is the flow of the ports. Unless port flow is equal, the compression pressure based on the actual volume compressed in each cylinder will not be.

John
V-performance.com