-----Original Message-----
From: jim gilmore <jgilmore@oeonline.com>
To: mil-veh@skylee.com <mil-veh@skylee.com>
Date: 14 December 1999 12:37
Subject: Re: [MV] Indian built WWII MVs
>Lars wrote:
>>The description of the (bad) picture stated:
>>Indian pattern Mk. 2 A Anit-Aircraft carrier, powered
>>by Ford V-8 engine with 32 hp....32 ????
>
> I have a copy of "Army Service Training Course 1944" published by Ford
>Motor Company of India Ltd. On the cover of this book is a drawing of one
>of these type armored cars. In the listing of specifications the 32 hp V-8
>is listed as a "Mercury "8" " I believe this is the 100 hp flathead V-8 .
> It appears that the horsepower rating used by Ford of India is different
>than the US rating. The standard (1941 to 1944) 95 hp V-8 is rated at 30 hp
>and the earlier (1937 to 1940) 85 hp V-8 is listed as 22 hp. The 4 cyl
>motors are rated at 8 and 10 hp. Interestingly, the " 1/4 ton 4 x 4 Jeep"
>is listed as 15.6 hp.
>
I think I can sort this out.
You are looking at the official RAC (Royal Automobile Club) horsepower rating
that was used for taxation purposes until the end of W.W.II leaving the UK with
a legacy of small car and truck engines owing to the swingeing annual taxation
on anything over about 10 (RAC Rating) HP. Post WWII the continuing fuel
rationing and then huge taxes on fuel carried on the tradition of small engines.
This peculiarly British rating system would also be applied to the Dominions
therefore so Canadian and Indian built (did India make any at all ?) MVs against
UK orders would be RAC rated.
The RAC HP rating has virtually no connection at all with the actual power
generated by the engine and it favours smaller 6 or 8 cyl engines with a long
stroke, pre-war up-market cars were to be found with 1.5L sixes to take
advantage of this. Now you may understand why such things were called Morris
8's, Austin 10's and Standard 12's to pick just a few.
The RAC rating is founded on an assumed piston speed of 1,000 ft/min and a mean
effective pressure of 90psi with a mechanical efficiency of 75%. These
assumptions are largely false for even W.W.II engines let alone now, typically
the W.W.II engine developed three times the RAC rated power.
The formula for obtaining the RAC rating is:-
HP = (D²n) ÷ 2.5
Where D is the bore in inches and n the number of cylinders. If the bore is
measured in millimetres the denominator should be 1613. Note the stroke is not
taken into account at all.
Having the Ford CMP Manual MB-F1 actually in front of me, page K-38 gives the
stroke at 3.1875" and this calculates to 32.5125 HP (RAC), line 5 on this engine
spec page actually states "Taxable HP. . . . . . . . . . . 32.5" (Just for fun
a RR Merlin is 203 HP)
Incidentally James Watt coined the horsepower initially needing a yardstick to
sell his steam engines against, he found that a large cart-horse could work
continuously at 22,000 ft lb/min, wishing to massage the power of his engines
upwards he increased this by 50% thus giving 33,000 ft lb/min to the HP, which
is what we have to this day.
Richard
(Southampton UK)
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