# Question about car engine horsepower rating

Hi all,
This group often sees questions about ratings (can my 2 hp electric motor develop 3 hp? can I draw 150 A from my 100 A supply? etc.) so I'd
like to ask a rating question. I think this one might spark some debate. If you buy a good electric motor, the power rating is a continuous one (or it says otherwise). Same with most pieces of industrial equipment. But what about a car? Is the 130 hp or whatever maximum engine power the manufacturer quotes a continuous or an intermittent rating? I was always under the impression that it was an intermittent rating, though quite where I got that idea I can't remember. I'm not asking if the engine will wear out quickly developing its maximum power rating continuously - I'm sure it will - I'm just wondering if it will overheat?
Any thoughts? Just a matter of curiousity...
Best wishes,
Chris
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I'd
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HP is not directly measured in either case. It is based on an equation using the torque and RPM. In a normal auto engine the torque and HP both have a peak number. That is the number most often quoted. If the cooling system is properly designed for the engine it will not overheat BUT it will run warmer than it normally would. However most AUTO engines built cannot handle running at the max numbers for long because they are made so light to save fuel.
-
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The rule of thumb we used for sizing brakes for overhead crane motors was:
5250 X RPM / HP = torque
Should be able to get the HP as the unknown with a little algebra.

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wrote:

Hi John
The equation in your post has been published with a significant error. I am sure it is a "typo". HP equals Torque times RPM. Yeah, the automotive design guys *do* measure the HP of the engines they develop.
It is kinda interesting that at about 5,250 RPM the Torque *and* the HP are the same number when measured with the commonly used USA dimensions. So, an engine producing 200 HP at 5,250 RPM will be producing 200 ft lbs of Torque.
Jerry
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wrote:

equation
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HP = Torque (lbft) X RPM / 5252
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The fundamental formula for horsepower sez:
HP = [ (WRN x 2 pi) / 33000 ] Visualize a rope on a windlass arrangement wound around a shaft of radius "R"; the rope is supporting a load of "W", and the shaft is turned at a number of revolutions, "RPM"
Where: R = Radius or length of a lever arm about the center of a shaft, in feet W = force, weight, in pounds N = Number of revolutions, RPM
Supposedly, this is the original formula, traceable to James Watt. Simplifying the formula by dividing out the 2 pi term yields: HP = [ WRN / 5252 ] WR is torque, thus the familiar form becomes: HP = [ (Torque x RPM) / 5252 ] or Torque = [ (HP x 5252) / RPM ]
Bob Swinney
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Boat engines are rated at a continuous rating, car engines are not. High performance boat engines can be limited to 15 minutes max power in some cases.
Lots of automotive engines would probably self destruct if held continuous.
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Besides that, the HP "rating" of an automotive engine is developed at a certain RPM at full throttle. Unless the car is on a dynamometer, it would be almost impossible to operate it for any time at that point.
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Leo Lichtman wrote:

Aircraft engines are operated at near peak torque for long times. Experimenters are having good luck with Subaru engines, my favorite (the Pietenpol), used an inline 4 cyl. Ford. Key items are cooling and lubrication, one needs to start with a good design, as most modifications are patches at best.
wws
Leo Lichtman wrote:

Aircraft engines are operated at near peak torque for long times. Experimenters are having good luck with Subaru engines, my favorite (the Pietenpol), used an inline 4 cyl. Ford. Key items are cooling and lubrication, one needs to start with a good design, as most modifications are patches at best.
wws
Leo Lichtman wrote:

Aircraft engines are operated at near peak torque for long times. Experimenters are having good luck with Subaru engines, my favorite (the Pietenpol), used an inline 4 cyl. Ford. Key items are cooling and lubrication, one needs to start with a good design, as most modifications are patches at best.
wws
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It's been a while so someone correct me if I'm wrong but IIRC peak torque occurs at a much lower RPM than peak HP. For a good size Ford V8 it was something like 3000-4000 RPM for peak torque and 5000-6500 for peak HP.
Steve.
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SteveF wrote:

Horsepower is an extrapolation to describe output condition at a point in time. A marketing number, hence: "horse." Torque is a constant over time, above time. The usable number. HP curves describe volumetric effenciency, tunable over a wide range. A 1 cu. in. model engine can produce 5 HP. At 25,000 rpm. But not enough torque to power a go cart.
That's all screwed up, HP is 33,000 pounds 1 foot in 1 minute, right? Thats one healthy horse. So torque, a static measurement, doing work over time, is "horsepower." I'm going for more coffee.
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.

That's what gearboxes are for :-|
Mark Rand RTFM
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On Sat, 18 Mar 2006 01:20:59 +0000, Mark Rand

Conversely, you can hang a V8 block from the end of a lever and produce 1200 ft lbs of torque - it will move the go-cart a couple of feet when you let it drop. No speed - no power.
That 5 HP 25000 RPM engine is producing just a hair over 1 ft lb of torque at that RPM, and may produce 2 ft lbs at 12,500 for 4.6 HP. It MIGHT even produce 3.5 ft lbs at 6000 RPM for 3.9HP
It will still be rated as a 5HP engine, when in reality, it is a real nice 3.5 HP engine. The rating will be; A, accurate, and B, Misleading, at the same time. *** Free account sponsored by SecureIX.com *** *** Encrypt your Internet usage with a free VPN account from http://www.SecureIX.com ***
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250# x 132' in 1 minute. Sounds doable now, huh?

--
http://NewsGuy.com/overview.htm 30Gb \$9.95 Carry Forward and On Demand Bandwidth

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Correct. Torque always peaks below HP since torque is the only thing that matters. HP is just Torque x rpm x (constant). Torque goes up to it's peak, starts dropping off. RPM keeps rising past the torque peak so HP rises at a slower rate then drops off.
Google on "torque curve" lots of nice articles http://www.dur.ac.uk/r.g.bower/PoM/pom/node11.html http://www.auto-ware.com/combust_bytes/p_goal.htm
SteveF wrote:

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Aircraft engines typically redline at 2700 RPM or so; a few geared versions will redline at around 3300. The engines have somewhat longer strokes than auto engines, producing useable torque at lower RPMs. The propeller is the limiting factor with airplanes; it can't operate efficiently when the tips are above about 600 MPH and the centrifugal forces on the prop get too large. The Subaru is having some success, but it can't be operated at or near its redline for long periods, as most aircraft engines can be. The engine normally has a redrive to allow the engine to run up to 5600 or so while the prop turns at about 2500, the high engine RPM increases fuel burn enormously (loss of VE), and the light valves burn too easily if the engine is run lean. Consequently, the engine is cruised at about 4700, sacrificing HP and cruise speed. It's not an "equivalent" replacement for an aircraft engine. I put a Subaru in an airplane, had lots of fun doing it, and it flew OK except for the lower cruise. Very noisy, since there was no room in the cowl for mufflers of any significance, and I had used the full-size Subaru rad with some fancy ductwork to make it cool properly. Not easy. Lots of bugs to work out.
Dan
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I would think that hooking it up to a Nextel Cup radiator would keep it from overheating. Then again, racing engines have things like sodium filled valve stems and such to manage the localized heat around the combustion chamber so maybe not.
Steve.
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" Then again, racing engines have things like sodium filled valve stems and such to manage the localized heat around the combustion chamber "
Yes, it would be possible to have local regions overheat while the coolant was within OK limits.
I had a toyota celica where the limit was defined by the red-hot exhaust boiling the fluid in the brake lines, but that's a different story...
Dave
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I hope you moved the brake lines. My wife had the master brake cylinder split which covered the exhaust with brake fluid which ignited and turned the entire car from the windshield to the front bumper into a Crispy Critter. She was on a back road going very slowly in traffic so was able to roll off the road and get out without any problems. And since that Audi was the most unreliable piece of crap I ever owned I wasn't unhappy. Insurance adjuster took one look at the car, checked the VIN and was done.
Steve.
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You did well. I had a Ford that burned out die to blowing oil into the air filter box and then running the results onto the exhaust manifold. It took the fire brigade half an hour to get to it. Had it transported home where it stood in the rain for two weeks until the insurance adjustor came to see it. He tried to mark it down because the paintwork was in poor condition!
Mark Rand RTFM
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