Automotive Engine Horsepower: How It's Made and Where It's LostSAE gross crank HP (before 1972) This was measured at the rear of the crankshaft with no belt driven accessories attached, no air filter assembly, and no exhaust (only stock exhaust manifolds or sometimes long tube headers). Essentially this was indicated HP (generated by the combustion process pushing the pistons down the cylinders and rotating the crankshaft) minus internal frictional, inertial, and crankcase windage losses. Nowadays this is only a theoretical number as it's no longer used. Factors that can increase SAE gross HP include: -Increased displacement. -Improved combustion efficiency (higher compression ratio, optimized combustion chamber shape, optimized air/fuel ratio, optimized ignition timing). -Improved volumetric efficiency (optimized cylinder heads, cams, intake manifold, throttle body or carburettor, and forced induction). -Higher operating rpm. -Lower internal friction (synthetic engine oil, coated pistons, low friction piston rings, low friction bearings, roller cam followers, roller rocker arms). -Lower internal inertial losses (lighter crankshaft, connecting rods, pistons, lighter valvetrain). -Lower crankcase windage losses (good oil pan design, crank scraper, windage tray, lower viscosity engine oil, dry sump lubrication system, flat plane crankshaft). These factors will also increase SAE net crank HP and Wheel HP. SAE net crank HP (1972+) This is measured at the rear of the crankshaft with the engine in the same state as it would be if it was installed in the vehicle i.e. equipped with all standard belt-driven accessories, an air filter assembly, and a full exhaust system. This was substantially lower than SAE gross crank HP but it was a better reflection of a real world situation as opposed to the previously overinflated HP numbers. Factors that can increase SAE net crank HP and Wheel HP without affecting SAE gross crank HP include: -Reduced parasitic drag at the front of the crankshaft (deleting the smog pump, substituting the mechanical cooling fan for an electric unit, substituting the hydraulic PAS with EPAS or manual steering, deleting the AC compressor, substituting the belt-driven water pump with an EWP, installing underdrive pulleys). -Reduced pumping losses (higher-flowing air filter assembly, headers, free-flowing exhaust). Q. What is BHP (brake horsepower)? A. It's the same as SAE net crank HP. The term brake horsepower is often used interchangeably because engine dynos have a water brake to hold the engine at a desired steady rpm when measuring crank HP. Q. What is flywheel HP? A. SAE net crank HP is measured with an engine dyno connected via a driveshaft and driveplate directly to the crankshaft. Flywheel HP is essentially the same but with the driveplate of the engine dyno connected to the flywheel instead of the crankshaft, thus yielding slightly lower numbers. The flywheel is required on externally balanced engines to minimize vibrations. Q. What is PS? A. It's a unit of metric horsepower as measured in Germany, is used in Europe, and is often used interchangeably with DIN horsepower. It's equivalent to 0.9863 SAE net HP so the quoted PS number is slightly higher e.g. 304 PS or 304 DIN HP = 300 SAE net HP = 224 KW. One Imperial HP = 0.7457 KW, One Metric HP (PS) = 0.7355 KW Wheel HP This is measured by a chassis dyno at the driven wheels and yields a lower HP number than the SAE net crank HP due to frictional, inertial, and windage losses in the drivetrain (flywheel, clutch plate, torque converter, transmission, driveshaft, differential, axle shafts, driven wheels/tires). Factors that can increase Wheel HP without affecting SAE net crank HP (i.e. reduce drivetrain HP loss) are a lighter flywheel, lighter clutch plate, lighter torque converter, more efficient transmission, lighter driveshaft, more efficient differential, lighter axle shafts, lighter wheel/tire combination, and the driven wheels being on the same end of the vehicle as the engine (i.e. front engine & FWD, rear engine & RWD) as this minimizes the number of drivetrain components.
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