Neat article...

Discussion in 'Technical' started by spytfyre, Nov 27, 2004.

  1. 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 why…

    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?
  2. To get an accurate number on drivetrain losses, take the engine and put it on an engine dyno, to get crank horsepower. Then put it in the car, put it on a chassis dyno, and get wheel horsepower.
  3. It's sometimes inconvenient to remove your entire engine, parasitic accessories and all, and stick it on an engine dyno. Do you not agree?
  4. Who cares what the engine makes? What matters is what gets down to the ground. Whp is a much more important figure.
  5. Relax: I thought it was a neat article. You don't have to make it your Bible. It even friggin' concludes by saying something similar to what you said, you stupid tool.
  6. It seems to me that if you are in the business of engineering products for people to modify their engines with, that an engine dyno would be the obvious tool for testing product development, and therefore part of the total process.
    If you are just someone who is trying to tune their enigne, then you would have no need for a flywheel output figure, unless of course you dont know what youre doing.
  7. Indeed...
  8. Wasn't that a Sport Compact Car article from some years ago exposing the shady nature of the import aftermarket business? One in which cars weren't even dynoed in the same gear from run to run.
  9. Absolutely. It is inconvenient. But then in certain cases, for instance, to find what kind of power an engine REALLY makes, or to find actual drivetrain losses, it could be done.
  10. um for this reason i belive BHP was created to measure the power at the crank!! and we use HP to measure the overall horsepower at the wheels.. Duh..

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