What is the difference between the HP measured by the Gtech, and the HP measured at a dyno, and the HP given in publications?
First of all, there are variances even between dynos. That is to say, if you take your car to 5 different dynos, you will probably get 5 different results. None of them are wrong, its just that they all measure in a slightly different way, under slightly different conditions. As any experienced tuner will tell you, when you are tuning/modifying your vehicle, you should always go back to the same dyno for comparison. Most publications list the crank horsepower, which is the horsepower put out at the crankshaft, with no transmission, etc… connected to the crankshaft. Most dynos measure wheel horsepower, which is the power delivered through the wheels (front, rear or all wheels); this power is what accelerates the vehicle forward. Consider all the mechanical components between the engine crankshaft and the wheels, as well as the friction (rolling resistance) between the rubber tires and the surface of the dyno roller, and it is no surprise that dynos measure a lower HP figure (after all
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