I think you may be misled by the 20/25hp nomenclature and I've been here before elsewhere in the forum.

However, these figures, which are quoted for ALL pre-war UK car engines, and indeed sometimes the name like Austin 7 and Morris 8, they are just a mathematical figure to give a definition for annual road tax. They have virtually nothing whatsoever to do with the actual power of the engine.

This engine was an "upgrade" from the previous 20hp engine/chassis from RR hence the 20/25hp designation. It is an in-line OHV 6cyl of just under 3.7 litres having a bore of 3¼" and stroke of 4½".

Using the RAC rating formula:

D²N/2.5 where D is the bore and N the number of cylinders you will get 25.35hp.

Pre-war this would have cost you GBP25pa in tax therefore; a huge sum at the time.

I can only hazard a guess at the actual power realised by this engine, probably around 80 BHP from the performance of the large and heavy cars it was fitted into. Bear in mind that your Roller in pre-war times came from the factory as a chassis and you then took it to a coach builder like Park-Ward, Hooper or Mulliner for a bespoke body. . . . . . . . . . Hand well in pocket twice then, and a rare sight on the roads obviously.

R.