Question about car engine horsepower rating

continuously -

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.

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.

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.

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

222 79236 body 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

222 79236 body 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

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.

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.

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

dimensions.

HP = Torque (lbft) X RPM / 5252

" 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

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

Back in the muschle car days, they would do everything they could to come up with a max Hp rating to advertise. IIRC, one major mfr took all the accessories off the engine (water pumps, generator, etc. and even used a vacuum system to extract waste gases to get a MAX number to put in the ads. I remember hearing that a mfr of industrial equipment ordered 60 hp engines from both Caterpillar and from Ford to test for some continuous duty application. The ford engine failed miserably when asked to deliver 60 hp continuosly, even though it was rated for it. I have a Ford engine in my backhoe. It has 3 different HP ratings, all the way from 62 HP down to 42 HP depending on whether its pumping its own water, etc., I consider it a 42 HP engine and it behaves that way.

Pete Stanaitis

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Christ> Hi all,

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.

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.

They ALL do this! Latest procedure is to run the camshaft with a big induction motor on a flux vector drive, so it is in sync with the crank, but not drawing any power from the crankshaft. This runs the oil pump off external power, too, on most engines.

Jon

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.

This is standard practice in the USA. SAE ratings exclude auxilliary drives, Euro ratings include them.

Just one reason why '70s US cars had huge engines with gazillion horsepower ratings, but European cars were often faster in reality.

Corners were the other reason.

Continuous, but at a constant rpm which is unrealistic for anything except motorway cruising. It's done by measuring on a dyno, either an engine dyno or a rolling road.

It's usually quoted at some ridiculous rpm limit too, fairly close to the redline. Long stroke engines with small valves will have failry flat curves (i.e. torque is approximately constant with speed), highly tuned engines (motorbikes) have "peaky" torque curves, so max power only happens when you rev hard.

.

That's what gearboxes are for :-|

Mark Rand RTFM

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