This old and quaint rating system we had often throws our colonial and ex-colonial chums over the Atlantic into a complete tizzy. I have seen endless head scratching elsewhere especially over the Ford CMP manual that actually quotes 32 rated HP in the spec but everyone knows the old flathead V8 makes 90hp.

Its done for taxation and correctly called the RAC rating, works like this:

HP = D²n/2.5

where D is the bore in inches and n the number of cylinders, notice the stroke is not accounted for and the resultant figure has really absolutely nothing to do with the real power made.

We had to pay tax at the rate of 1 GBP/rated HP until the end of WWII and hence why England is blighted with a history of tiny engined cars, the common pre-war models like the Austin 7, 10, 12; the Morris 8, Standard 10 etc, use the HP rating number as the model designation. Even these tiny engines represented about a months salary per year in taxation, would you pay this now. . . . . . . .?

It is now obvious how wealthy people were to even run a car, especially those with say a Morris 25, 30HP Lanchester or a 40HP Rolls.

No one was exempt, even the army paid up to the start of WWII and you will note for, example, the Morris Commercial types had a tax disc holder fitted as standard to the left rear of the bonnet (hood). I expect when ordered the War Office needed the makers HP rating officially stated to register and pay taxes on the CMP imports impressed into the British Army.

Incidentally the accepted standard horsepower of 33,000 ft lbs/min is wrong; James Watt when selling his steam pumping engines needed a performance comparison. He found the best cart horses here could work continuously at 22,000 ft lbs/min and simply increased this by 50% and called it horsepower, thus making his engines very attractive indeed.

R.