What is the difference between horsepower and horsepower "on the crank"?

Baloo55th

Not heard the term 'on the crank', but there's a thing called brake horsepower I do know. Horsepower is calculated theoretically, and there are different systems for doing this. In the UK, the system before WWII for road vehicles was based on the bore and stroke of the engine, and as vehicles were taxed on their horsepower by this method, most manufacturers produced long stroks narrow bore engines which gave low ratings for tax. (When this was abolished, the oversquare engine (wide bore, short stroke) came in with a marked difference to performance and economy.) Brake horsepower is the power of a motor working against a force (the brake) that tries to stop it. The force applied can be measured, and this gives a more accurate indication of the power of the motor. A 8 hp engine could develop 40 bhp, for example. The hp of electrical motors was calculated another way, but the bhp method can be applied to any motor. However (isn't there always a however?), if wanting to know the real power of a motor properly, torque has to be taken into account as well depending on the intended use of the motor. A Ferrari engine develops very high bhp, but is of little use when towing a heavy trailer. Equally, an old Field Marshall tractor would pull the side off a house, but wouldn't be much good at Le Mans... Basically: Horsepower is theoretical; 'on the crank' is practical. Apr 12 06, 12:00 PM

deuceu72

I watched a tv show last night called wrecks to riches. In the show, the guy building the car said the car had 550 horsepower on the crank. Thats where I heard the term. I think it has to do with the crankshaft only and not the entire drivetrain but I am unsure. Apr 12 06, 12:36 PM

xfacilitatorx

I imagine this term is similar in definition to 'hosre power at the wheels'. When using a Dynomometer to determine the power of an Automobile , the horse power is measured 'at the wheels' by applying a load to the drivetrain. Horsepower can also be determined directly 'at' or 'on' the crank output shaft of the motor. --------------------------------------------------------- Measuring Horsepower If you want to know the horsepower of an engine, you hook the engine up to a dynamometer. A dynamometer places a load on the engine and measures the amount of power that the engine can produce against the load. You can get an idea of how a dynamometer works in the following way: Imagine that you turn on a car engine, put it in neutral and floor it. The engine would run so fast it would explode. That's no good, so on a dynamometer you apply a load to the floored engine and measure the load the engine can handle at different engine speeds. You might hook an engine to a dynamometer, floor it and use the dynamometer to apply enough of a load to the engine to keep it at, say, 7,000 rpm. You record how much load the engine can handle. Then you apply additional load to knock the engine speed down to 6,500 rpm and record the load there. Then you apply additional load to get it down to 6,000 rpm, and so on. You can do the same thing starting down at 500 or 1,000 rpm and working your way up. What dynamometers actually measure is torque (in pound-feet), and to convert torque to horsepower you simply multiply torque by rpm/5,252. http://auto.howstuffworks.com/horsepower2.htm http://auto.howstuffworks.com/horsepower2.htm ------------------------------------------------------- Factory ratings are all well and good, but many enthusiasts modify their cars and then want to see how much of an improvement they got from their labors. The problem is that most of the time people are not interested in ripping the engine out of their car to have it tested on an engine dyno; no, they're going to be testing on a chassis dyno. The most common chassis dyno, the inertial dynamometer (popularized by DynoJet), measures the horsepower as delivered at the power wheels -- whether front or rear. But testing rear-wheel horsepower (rwhp -- obviously, front drivers would be measuring fwhp) makes it difficult to convert from what the dyno says to what the manufacturer says. The manufacturer, remember, measures horsepower at the flywheel. All that equipment between the engine and the wheels -- the transmission, driveshaft, differential, and axles -- introduce friction and inertial losses summarized as "powertrain loss" or "parasitic losses". The efficiency of the driveline can greatly affect the amount of the powertrain loss: Ford's AOD transmission, for example, is notoriously inefficient. As a very general rule, rear-wheel horsepower on a manual-transmission car is about 15% less than SAE net, and rear-wheel horsepower on an automatic-transmission car is about 20% less than SAE net. http://neptune.spacebears.com/opine/horsepwr.html --------------------------------------------------------- 416.74 Horsepower at the crank (345.90 Horsepower at the rear wheels by adding 17% of 5-speed automatic transmission power-loss.) 434.93 ft lb of Torque at the crank (360.99 ft lb of Torque at the rear wheels by adding 17% of 5-speed automatic transmission power-loss. According to Mercedes-Benz factory data, a stock CLK55 is rated 342HP & 372ft lb of Torque. V8 / 5.5 Liter / SOHC) http://www.jimmy540i.com/clk55sprewell.htm Apr 12 06, 12:37 PM

Baloo55th

Yeh. The drive-train takes power. If you just hook up a motor to the measuring gadget, you'll get a max reading. But on the road, it'll be different. On the road, you've got gearbox (automatics use more power than manuals), differentials, bearings at the wheels, tyres, plus the power taken by the alternator - all taking power out. He's calling it 'on the crank' but it actually means what comes out at the flywheel before anything else is counted. The flywheel is mounted on the end of the crankshaft, anyway. But that 550 horsepower he refers to is really brake horsepower as I described it. No-one usually uses horsepower as such for internal combustion engines. Of course, where the term horsepower came from was the number of horses it would take to pull a load that an engine could pull. In the days of steam, they could do a direct comparison, as the engines were fairly low horsepower. You try getting 550 horses to pull something... Apr 12 06, 1:13 PM

BMCBRID5

Horsepower refers to power at the rear wheels ( or front ). Crank Horsepower is total engine output less trans and differential Apr 12 06, 1:19 PM

deuceu72

Thanks All!!!! Apr 12 06, 1:20 PM