| The other school of thought (which seems to come from people that actually went to college and took physics) asserts that a drivetrain has a fixed known mass, and the power needed to overcome the inertia is the same no matter how much power is put into the system. So, a drivetrain would take 15hp to turn. That 100hp engine from earlier is putting 85 to the ground, but the 200hp version would put down 185 I sure hope your friend isn't one of those who went to college and took physics, because he's way off :-) First of all, you don't "overcome" an inertia. Any amount of force that acts against the inertia helps move it. It doesn't take a set amount to "overcome" it. Accelerating a mass cannot account for the majority of the loss, because according to that theory a car cruising on the highway shouldn't have much of any drivetrain loss since you aren't accelerating the drivetrain mass, you're just keeping it going. Secondly, his theory doesn't take into account the *rate* of acceleration and the difference in force needed to accelerate the given mass at a different rate. So even within in his own example, he is wrong. A 100 hp engine will not accelerate the mass at the same rate as the 200 hp engine, therefore it wouldn't expend as much energy accelerating that mass. The way a dynojet works is that it measures the rate at which you accelerated a given amount of mass, therefore it can calculate how much horsepower was spent accelerating it. And by taking rpm into account, it can back-calculate the torque. Thirdly, (and supporting my last point), if the drivetrain took a constant amount of power to turn it, then you couldn't possibly turn it by hand, and cars wouldn't move when the starter motor engaged when you mistakenly left the car in gear. Starter motors don't make anywhere near 15 hp. The truth is that parasitic losses due to friction account for most of loss in the transmission, and that friction increases with speed- it's a percentage, not a set number. That's why using thinner transmission fluid helps reduce losses. It's also why automatic transmissions have higher losses, because of the friction within the torque converter (that's why the fluid in the torque converter gets hot). And it's also why cars with lockup torque converters get better highway mileage than those without lockup torque converters. Of course there is a very small hit due to inertia, but it is minimal, and nothing compared to the inertia of the car itself.
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