Horsepower Claims: The Good, The Bad and the Confusing By Shiv S. Pathak You hear it everywhere. Horsepower claims. Along with their sheer ubiquity comes a whole lot of confusion. You see, horsepower claims, by judged by themselves, are nearly impossible to quantify. Let alone compare amongst themselves. Here's whyÂ Virtually all dyno testing done by aftermarket tuning houses are done on something known as a "chassis dynamometer." As the name implies, the entire car is parked on the dyno apparatus where its wheels sit upon big rollers. When the wheels spin, so do these big rollers. The dyno computer monitors the force at which these rollers spin to determine horsepower. Sounds simple, doesn't it? Problem is that these numbers represent wheel horsepower, not engine horsepower which is what most people assume when playing the horsepower game. After all, we all know how much engine horsepower our stock car makes by just looking at the manufacturer's claims. But wheel horsepower is something all together very different. It does not take into account frictional losses caused by the drivetrain. That's right. The transmission, differentials, axle joints, etc., suck up their fair share of horsepower. As you can imagine, a car's wheel horsepower is always lower than its engine horsepower. So how do aftermarket tuners make engine horsepower claims if they only have wheel horsepower dyno results to go off of? Simple. They take an educated guess. Now here's where things get tricky. Not everyone guesses the same way. Don't ask us why. If it were up to us, there would be standard guessing procedures. But until that happens (don't hold your breathe), customers need to know what they are getting. But first, let's look at a totally hypothetical example. In our make-believe world, a stock Pontiac Firechicken makes 100 horsepower on the Supa Fresh Chassis Dyno. According to Pontiac, the engine is rated at 135 horsepower. Assuming this to be true, we can safely say that 35 horsepower is used to spin the drivetrain. But now let's say we put on our tuning hats and bolt on a big fat turbocharger. While we're at it, let's stick on an intercooler, bigger injectors, a stand-alone engine management system, a full exhaust and a pair of fuzzy dice. All of a sudden, the car is spinning the dyno rollers with 200 wheel horsepower. That's one fast Firechicken! But the question remains: How much power is the Firechicken's engine making? Now comes the time when we place our bets and make our guesses. One of the more popular guesses is to assume that the Firechicken, which doubled its wheel horsepower, has also doubled its engine horsepower. Sounds reasonable, doesn't it? Ladies and Gentlemen, we now have a 270 horsepower Firechicken. Not so fast buck-o, Take a look at the new driveline loss figure. Yep, it went from just 35 horsepower to a whopping 70 horsepower. Does this sound reasonable to you? It may to some people. Others, however, may balk at this is say that doubling the wheel output shouldn't double the driveline loss figure. After all, why should the driveline suddenly need twice as much power to spin it? Sure, the extra power will generate more heat. But an extra 35 horsepower of heat is enough to warm a small Eskimo Village. Where did all that heat go? Good question. No one knows. This brings us to the other, more conservative, guessing technique which assumes that the driveline loss figure of 35 wheel horsepower is going to remain constant regardless of horsepower output. Assuming this is the case, we simply add the 100 wheel hp gains on top of the 135 engine horsepower and come up with 235 engine horsepower. In summary, the optimistic guess yields 270 engine horsepower. While the more conservative guess yields 235 engine horsepower. Which is right? Neither, of course! If either guessing technique were 100% correct, the other wouldn't be used by anyone, right? The truth of the matter probably lies somewhere between the two methods, only quantifiable with big equations with multiple variables. We'll leave that for the mathematicians and physicists who want to figure out who has the fastest Firechicken. For consumers, it's far easier to ignore the whole engine horsepower thing and look only at wheel horsepower results. After all, isn't that what we're actually measuring?