I got into an animated discussion the other day with some friends—how the hell is it that Ford Mustangs with a ton of power on paper always make so little power at the wheels? It’s always something dismal, like 600 wheel horsepower for an engine that makes 850. How does that happen?
Well, we always hear about the common 15 percent drivetrain loss explanation, but nobody ever really explains it. I wanted to see if that 15 percent guiding principle is right, though expressing drivetrain loss as a percentage is fundamentally wrong.
Allow me to explain in this latest installment of Motor101.
Where Does Horsepower Go?
Photo by: Audi
So what is drivetrain loss? Basically, it’s the horsepower it takes to spin parts of the drivetrain. That includes the transmission, driveshaft, differentials, transfer cases, half-shafts, wheels, tires, hubs, and brake rotors. Each one of those items takes a fixed amount of horsepower to overcome its natural friction, and each part carries its own inertia.
So what is drivetrain loss? Basically, it’s the horsepower it takes to spin parts of the drivetrain.
Some of the parts, like the differential or transmission, may have friction that increases with speed, which is where the percentage comes from. But the truest expression of drivetrain loss can be expressed more in the amount of horsepower rather than the percentage of horsepower—just because you increase the amount of horsepower your car makes, doesn’t mean you increase the drivetrain loss.
How Much Horsepower Does Each Part Require?
Photo by: Ford
The losses are much more of a function of the strength of the drivetrain. Generally speaking, stronger drivetrain parts, like transmissions, diffs, and axles, are heavier. Heavier parts take more energy to spin, thus requiring more horsepower.
Taking our Mustang example. A 5.0-liter Mustang GT or Dark Horse has a pretty stout drivetrain. The Getrag MT-82 has a reputation, but it still takes 480 hp’s worth of abuse. The Tremec in the Dark Horse is good for the 500 hp it makes, and it was also used in stuff like Vipers and GT500s, both of which had power deep into the 600s.
The transmission is overbuilt to hell. That amount of strength takes a bit of power to turn, simply because the gears are wider, and the gearbox shafts are heavier. That gearbox weighs about 125 pounds, and a good portion of that is the internals. Then, there’s usually a decently heavy flywheel and clutch, likely weighing around 20 pounds.
That cascades into the rest of the drivetrain. The driveshaft that sends power to the rear differential has to handle the horsepower, so it has to be big. Then, the Mustang 8.8-inch rear end is famously strong, but also lossy because of its size. The ring and pinion gears within the diff can weigh as much as 50 pounds.
Heavier parts take more energy to spin, thus requiring more horsepower.
Then, the axles from the diff to the wheels take horsepower to turn. But the biggest contributors are the wheels, tires, and brake discs. A single wheel and tire can weigh 50 pounds, plus a 10-pound rotor.
Roughly, the engine has to turn a good 200 pounds of drivetrain inertia before the horsepower even makes it to the ground. In the case of the Mustang, likely more. The average Dark Horse, which is advertised at 500 hp, will make around 380-420 horsepower to the wheels. So we can say a Mustang’s drivetrain takes 120 hp to spin, around a 20 percent loss. The discrepancy in numbers simply comes from the dyno and the conditions; temperature and humidity effect real world horsepower. The dyno itself can be hugely variable, too.
In contrast, let’s think about a lightweight car with modest power, like a Honda S2000. That car is advertised at 240 hp, and will make 200 hp at the wheels. Everything in the drivetrain is engineered to deliver 240 hp, which is substantially less. An S2000 gearbox weighs around 100 pounds, the gears within the diff weigh around 40 pounds, and the wheels and tires are around 30 pounds a piece, plus a 5-pound rotor.
All of that apparently takes 40 hp, or around 17 percent of the drivetrain.
So, 15 Percent Is Kinda Right?
As it turns out, 15 percent is a decent guiding principle. For a drivetrain that scales with horsepower, it does make sense. Theoretically, though, if you were to supercharge your Mustang or S2000, you wouldn’t lose more power. The drivetrain loss is fixed, not a percentage of overall output.
It’s interesting to think about how much horsepower cascades into other systems. So yes, 15 percent is actually kind of right. But we can express it more accurately—and that’s always better.
