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Post by Eric R on Aug 29, 2014 11:19:51 GMT
Out of interest i would like to calculate the horsepower of my Mk1 3-litre but all the websites display calculators requiring information that i don't have and there doesn't seem to be much point in calculating it (muck in = much out!) Engine is 3L7 77.8mm bore x 105mm stroke but i cant find any info such as rev counter. Is this information lying somewhere or can anyone kindly compute for me?
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Post by Phil Nottingham on Aug 29, 2014 17:30:16 GMT
You mean the RAC Horsepower which is an antiquated theoretical figure. Rover used the industry standard brake horsepower as measured but was not the actual road use NET DIN figure
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Post by Eric R on Aug 30, 2014 6:56:05 GMT
Phil i dont know what i mean but i am often asked the horsepower at events and feel stupid that i have no idea! Im thinking that in days of yore, cars were 8,10,12,14 HP etc which seems simple!
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tonyl
Rover Rookie
Posts: 55
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Post by tonyl on Aug 30, 2014 8:00:00 GMT
The old RAC horsepower figure was D2*n/2.5, where D is the bore in inches and n the number of cylinders, which comes out at about 22.5 for your engine. That stacks up with other cars of the era. The Alvis TA21...TF21 were all 3 litre cars, for instance, rated at 21hp. It can't be read across to brake horsepower in later terms, which was based on measurement rather than a calculation. According to Wikipedia, Rover claimed 115 bhp for the Mk1. Even that was measured in a different way from the current (DIN) way of measuring it.
Tony
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Post by Phil Nottingham on Aug 30, 2014 9:39:52 GMT
The IOE 3 Litre engine is a very long stroke engine as it was designed for the P3 2.2 Litres approx 16HP but the RAC HP tax basis was changed to the flat rate system that lasted into the 2000's before it was launched in 1948 so it was called the Rover 75 not to be confused with the 1950 P4 75 Cyclops.
The bore based RAC system was only ever for road tax assessment (£1 per HP) and even when new in the 1920's it did not equate anywhere near to real HP in the most up to date cars of the 1920's
Our 2.1 Litre P2 Rover 16 is 55 BHP and was also a long stroke 6 (all capacities, 4's & 6's were 100mm stroke) but not IOE - no one could have afforded the road tax on the over-square V8 at 39HP and £1-5s per HP at post war rate!
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Post by Eric R on Sept 2, 2014 11:02:35 GMT
Have found this general comment: Horsepower (common) - A unit of power. Equal to 33000 foot-pound-force per minute. Very much in use today. Horsepower (RAC) - A strange unit, used only to tax cars in the first decades to the 20th century. It was based on the cylinder diameter, not the swept volume or power, which seems to have inspired W O Bentley at least to design long-stroke engines to get them into a lower taxation class. I have a replica tax disc dated 1959 with the weight shown in imperial (of course!) but the HP panel to the right is blank so perhaps the calculation problem existed way back then, although I guess the old 8-10-12-14 etc was used. Thanks for your interest.
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Post by Warwick on Sept 2, 2014 14:25:23 GMT
The terms used here are/were rated horsepower, and developed horsepower.
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kaiser
Rover Fanatic
worth his V8 in gold!
Posts: 136
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Post by kaiser on Sept 22, 2014 18:00:02 GMT
The old RAC horsepower rating has nothing to do with horsepower as such. It is a formula used purely for taxation, and it is, almost singlehandedly responsible for the dead-end the British motor industry went into. In the early days, the fromula, developed by RAC was fairly close to the actual power, and easy to calulate diameter of the piston squared, times number of cylinders, divided by 2.5. This figure was then used to determine the tax the car was liable for. In order to minimize taxation British manufacturers developed long stoke engines with small piston diameters, which resulted in a stream of engines which went directly against the modern trend of oversquare (bigger piston diameters than stroke) fast reving engines seen at the British rivals, mainly in Germany. The result are typical in the Rover engines, which were difficult to get to breathe and to get to produce horsepower, because at revs, they would tear themselves apart, and the narrow bores made it difficult to use large valves. Further results of that were typical low gearing of the cars in the UK, which was also a result of the cramped conditions, and even the development of the electric overdrive, which was used to a large extent on British products, but only on a few other products. Add to this poor fuel, and the scene is set for a long term problem.
That is mainly why we sit with heavy long stroke side valved engines like the three liter. A charming engine in many ways, but not fast, not light and not very frugal. The revelation dawned on Rover as they got the chance of using the Buick V8, which was more economical, lighter, more powerful and had far more torque. Once they saw it and tried it, out went the 3 litre with very little regrets, I think.
It is a classic example of rules which far transcends and exceeds what was grasped when introduced.
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Post by Warwick on Sept 23, 2014 2:56:51 GMT
It certainly wasn't restricted to Britain. The method was still used to determine annual registration fees here in the 1960s (perhaps beyond - can't remember), and it was used in the early days of Grand Prix racing. The cars were classified by the size of the bore. There was, among others, a 4-cylinder Fiat racing car with a massive 28-litre engine. The stroke was so long that the driver couldn't see over the engine. Peugeot had a similar car, although not quite that capacity if I recall correctly. David (Admin) has a photo of one he took at a hill climb several years ago.
In Japan, where the streets were narrow, they taxed cars on width. That resulted in the early exported Japanese cars of the '60s being quite narrow.
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