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Is all HP created equal
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slowzuki
Posted 11/29/2012 08:53 (#2723089 - in reply to #2722096)
Subject: RE: Is all HP created equal


New Brunswick, Canada
HP is HP is HP, its 746 watts of power, or 550 ft*lbs/second of power, change it to any unit you want. The standards on whats hooked to the engine while its being dyno'd varies, some are a bare block which is higher than at the pto shaft turning all the pumps and fan etc.

Just because it has a rating doesn't tell you how long it can operate at that power either. Automotive diesels can't maintain high power levels, they just aren't made to handle the heat and loads because most of the time they are operating at low hp needs crusing along. Stationary power units and tractors need to operate continuously at rated power levels.

People get all excited about the peak torque vs peak hp but the basics are, the engine produces a torque on the crankshaft. HP is based on the speed you can apply that torque at, simple multiplication math. If you know the rpm and the torque, you can calculate the hp. If you know the hp and torque you can calculate the rpm those values are obtained at.

Torque is multiplied or divided by gearing, 500 ft lb at engine, 2-1 reduction gives 1000 ft lb at output. HP is not affected beyond losses in the gearbox to friction.

You can't flip power plants between applications looking only at hp because the gearing is set for a specific rpm and different engines have different torque vs rpm curves. A 50 hp piston steam engine produces massive amounts of torque right to stalled, a 30,000 hp gas turbine can't run under thousands of rpm without stalling. Even fixing the gear ratio, a turbine can't be hooked up to most loads without a clutch or torque convertor to allow slip and prevent stall. A large diesel operates over a fairly narrow speed range with a fairly flat torque curve requiring lots of gears to allow use of the engine.

Short answer hp is hp but don't pick a motor for an application based only on hp.
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