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Horsepower

From: "Angela & Darren Fritzsch"

Quick questions for anybody with opinions (Guess that's everybody) :) My current understanding is that metric hp, DIN and ps are the same thing; if they aren't can someone detail the conversion factors? The theoretical difference between SAE and DIN isn't very large, but the way they were originally measured made a big difference. Did the SAE people ever change their measurement criteria ? Or did everyone just stop using SAE? For the sake of comparing old SAE hp estimates with new(er) DIN ones, is there a kludge conversion factor commonly accepted? What is it? As a side issue, I don't suppose the US people are quoting SAE in their dyno's per chance. Finally, bhp. (No not the big Australian). I read somewhere recently that it was a measure of rear wheel hp. I thought bhp was still commonly flywheel figures and simply referred to the measurement technique of applying a brake. Comments? ta, Darren


Dear A & D,

I'm a mechanical engineer who grew up long enough ago that I was raised with Imperial units ("wotsa Futt Mista?" ), went through my training with SI (Metric) units, then worked 10 years for a Yank company who supplied us with equipment from the US, UK, Italy and Japan. 

I have endured much sucking of teeth and scratching of foreheads over exactly these sorts of questions. The basic concept of power is that it is "the rate of performing work". The confusion arises out of how and where the power is measured, and who is specifying the procedures used.

A traditional "horsepower" is specified as a work-rate of (I think) 33000 foot-pounds per minute, and is actually a good deal more than the average nag can deliver for any length of time.

Similarly a Watt is specified as 1 Newton-Metre per Second. (Approximately one gerbil-power) The forces and distances can be measured linearly or rotationally, with identical power results, (i.e. Force multiplied by linear distance divided by time gives exactly the same results as Torque multiplied by angular movement divided by time). In automotive practice we are usually interested in rotational power, i.e. the combination of torque and rpm.

When you crunch the conversion numbers a traditional horsepower works out to be 746 Watts (0.746kW) Those people working in metric countries, i.e. Europe and Japan, do all their work in metric, but are trying to flog their gear to us, the UK, and more importantly the Yanks, who refuse to understand metric. They have to quote a horsepower figure because your potential customer no-comprendez this kilowatt thing. To "simplify" the conversion a metric horsepower was defined by (I think) the French as 0.75kW. I think the Japs use the same and call it a ps, but don't know about the Germans.

SAE is an American industry body (The Society of Automotive Engineers) that sets standards for all kinds of technical issues in the US auto industry, although their specs. are regularly "borrowed" by other industries.

DIN is a German technical body (The Deutsche Institut fur Normung; - equivalent to the Australian Standards) that sets standards for all kinds of industries over there. JIS is the Japanese equivalent of DIN, AS, BSI etc. etc. and does the same job over there, and I believe sets the spec for ps.

The term "bhp" stands for "brake horse power" and derives from the days of steam and even current marine practise, when engines were running very slowly. (e.g. the main engine on the ship I currently work on has a max speed of 111rpm; mind you it has a 60cm bore and 2.3m stroke and delivers around 12000bhp) This was slow enough to measure the gas pressures inside the cylinder and plot them against the piston position over one revolution.

You could then measure the work done by the cylinder gases on the piston;- the "indicated horsepower" (ihp). By comparing this with the actual output of the engine you could see how much you were losing in making all those bits and pieces thrash up and down (i.e. work out the mechanical efficiency of the engine)

Brake horse power means you load the engine you are testing up with a brake (dynamometer), which you also use to measure the torque delivered, and by simultaneously measuring the rpm of the engine you can calculate the power delivery. Read this carefully;- brake horsepower specifies how you measured the power, not where you measured it. A manufacturer will commonly test an engine with the dynamometer attached to the flywheel, with the whole thing mounted on a test stand, i.e. with no power steering, no air conditioning, no alternator, and often no water pump. They do this for the simple reason that it gives them the biggest figure to print in the spec. sheet. You will never find any direct admission of this in any of their literature, they will just specify a particular testing protocol as laid out by one of the standards societies, i.e. a DIN, SAE, JIS spec. They are not actually lying, just trying to get you to buy their machine instead of somebody else's.

For us mug drivers the only really useful measurement of power is what you get at the back wheels, hence the usefulness of rolling dynamometers such as Bob Whyms'. The numbers measured here are always lower than the factory figures because they show what is left after taking account of the losses for all the accessories and also the drive train.

When trying to compare figures from different sources I would suggest that you convert everything to kW and compare those numbers. It is usually safe to assume that any figures you get will be engine output as described above because all the manufacturers are playing the same game.

Sorry if the above sounds like a mechanics lecture, but many skilled individuals have spent a great deal of effort over many years making a simple concept as complicated as possible.

Jim Snee, '83 928 5-speed

Please note: these comments are the writer's personal opinions. I take no responsibility nor give any kind of warranty as to the accuracy of the statements included on these pages. The information in Tech Tips is based on the extensive experience of Porsche specialists and owners who drive, service and even race their 928. Although this information is generally considered 'best practice' niether 928 Australia, Landsharkoz or the Webmaster will accept any liability for such information, or, for any use to which it is put. If you have doubts about a specific point, or if you have any questions, you should consult your Porsche specialist or post a question on the Landshark mailing list. - Leonard Zech, Webmaster.

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