Actual power for 398 cc

Hi

I search information about ACTUAL power of Sopwth 398cc / ABC 398 cc

I found controversial data.

From 3 hp to 9 hp .

The engine has a 3 h.p. Treasury rating for taxation purposes and this is based on capacity as a 500cc engine would be 3 1/2 h.p.
The 9 b.h.p. is the power it would produce on a dynamometer.

Roger wrote: The engine has a 3 h.p. Treasury rating for taxation purposes and this is based on capacity as a 500cc engine would be 3 1/2 h.p.
The 9 b.h.p. is the power it would produce on a dynamometer.

3 h.p. - after motocycle gearbox ?
9 b.h.p. - before motocycle gearbox ?

Are there any historical sources?

A kind of "The priciples of description motorcycles and automobiles and it's engines" or "How to read motocycles catalog"

Manual, books or magazines articles ?

Roger wrote: The engine has a 3 h.p. Treasury rating for taxation purposes and this is based on capacity as a 500cc engine would be 3 1/2 h.p.

Ops.

You mean RAC-rating

The RAC rating was devised by Col. Holden of the RAC and was based solely on the bore of the engine rather than the capacity and a modified version was adopted by the Treasury for car taxation. Holden's formula was an attempt to provide a method of handicapping in races based on engine performance and as it was published before it received royal approval it would have been ACGBI.
Capel Holden designed and built the first British motor bicycle in 1895 which had a four cylinder overhead camshaft engine as well as designing the Brooklands Motor Circuit.

I seem to recall reading about quoted power output, and it gets quite complex. It all depends on where the power output is taken from. So you have a variety from the theoretical power, via the flywheel, to the actual useful power, at the rear wheel. In between, you have the power output at the engine shaft, then the gearbox. So depending on how you want to sell something, you pick and choose.

I guess it also shows the efficiency of the various parts?

The Rating for an engine, seems to be based on pioneer engines, and has little reality after 1914ish. In the same frustrating way, they use capacity now....a 2019 1000cc, is going to be just a tad more powerful than a 1919 1000cc....Sorry, one of my irritations, that is killing off the vintage scene, is the need to learn to ride a 200mph bike to ride a 40mph bike....

Nicholas