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Australian Ford Member
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In actuality, you can't. It is impossible, due to the fact power is lost between the engine and the wheels; lost as friction, noise, heat as all the bits in the gearbox and diff soak up thier little share of the power the engine spits out to the clutch. You can't measure power loss if you don't know how much there was in the first place! Car manufacturers advertise actual engine output eg 220kW which is measured with the engien removed from the car. Most half-smart car modifyers quote rear wheel kW or horsepower (rwkw or rwhp) as that is alot easier to measure - you back your car onto a dyno and 'pull the string' (do a test) However, there is a rough rule of thumb - flywheel kW equates to rear-wheel horspower. eg a 200kW Falcon should punch out 'around' 200hp atthe treads; a 165kW Commy around 165hp etc. Each car is different; each dyno is different (although in theory, they should all be calibrated the same!); autos soak up more power than manuals. Wide tyres ands sticky tread compounds 'rob' more power, too. Very rubbery figures... but a good lie detector if someone says they have 300kW and only develop 200hp atthe wheels. Dynos are good at comparitive measurements (ie before and after - eg, extra power gained after an exhaust or other change to the engien)
 
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