Compression Ratios

The compression ratio of RM Rileys is quite low even by the standards of the day and raising it can be beneficial. The compression ratio can be raised either by fitting high compression pistons or by shaving the cylinder head face to reduce the depth of the combustion chamber. However it is done care must be taken to ensure that the piston cannot contact the valve heads.

Compression ratio is define as (V1 + V2)/V1 where V1 is the volume of the combustion chamber with the piston at its highest point and V2 is the swept volume of the cylinder.

The table below shows the compression ratio achieved by removing metal from the cylinder head face. Note that these are theoretical numbers and do not imply that it will be safe to remove that much metal from the cylinder head face. The numbers shown in red are probably unsafe.  In particular the water passages meet the head face at an oblique angle on 2½ litre engines and Riley recommended that only 32 thou. be removed from these cylinder heads. The numbers are based on cylinders with standard diameter bores. An engine which has been rebored and uses over size pistons will have a slightly larger swept volume and hence a slightly higher compression ratio.


Depth Removed (thou) RMA RME RMB RMF
0 6.8 6.6 6.8 6.6
20 6.997  6.764 6.946 6.736
40 7.164 6.938 7.100 6.879
60 7.363 7.123 7.261 7.029
80 7.576 7.320 7.432 7.187
100 7.804 7.530 7.612 7.353
120 8.048 7.754 7.802 7.529
140 8.310 7.994 8.004 7.714
160 8.592 8.253 8.218 7.911

Noting the limited amount that can safely be removed from a 2½ litre cylinder head there is little gain in compression ratio to be achieved by this method  High compression pistons can be used although there were reports when these first became available in UK that they hit the valves in some engines. There do not appear to be any recent reports of this problem.  The Pathfinder engine (RMH) with its normal higher compression pistons has a compression ratio of 7.25 but used a thinner head gasket.

The author has removed 100 thou. from an RMA cylinder head successfully and has heard of 125 thou. being removed without any problems.  With 100 thou. removed (CR = 7.8) the engine was found to be more responsive with improved low end torque although this engine was fitted with twin carburettors.  An engine with a single carburettor had 80 thou. removed from the cylinder head and that too showed a noticeable improvement in performance. Both engines were fitted with normal pistons. High compression pistons can be obtained for these engines too.

If anything is shaved off the cylinder head it will be necessary to make spacers to lift the rocker pedestals to their former height.

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