Horsepower doesn't matter

Discussion in 'Lounge' started by gun, Oct 26, 2011.

  1. gun

    gun Powered by air Member

    I've been looking at various cars and trying to research online. In doing so I endlessly encountered one of my pet peeves. I'm amazed at how many people, including engineers, say completely silly and ignorant things when it comes to the performance of cars. Accordingly, I thought it might make for an interesting post here.

    HORSEPOWER DOESN'T MATTER. There I said it. Shocking I know. Before you circle the wagons and send a messenger for the cavalry, please let me explain. Horsepower is a wonderful metric for relative measurement. And by relative, I mean, in comparison to the same engine and powertrain. Note I said, "engine", and, "powertrain", and not vehicle. Again, I've made a provocative statement and have yet to back it up. But hang in there.

    The first thing you need to know about horsepower (HP) is that it is a derived unit of measure of work. Horsepower is always derived from torque and RPM. In fact, a simple explanation of HP is HP = TxRPM; where T = torque and RPM = RPM of the motor. Its actually slightly more involved than that but that simple formula expresses everything that is important for this discussion. Real formula is TxRPM/5252; but as 5252 is a constant, for ease of comparison we can ignore it.

    The problem with HP is people all too frequently compare HP between vehicles; all too often vehicles with completely different powertrains and even weights. Its at this jumping off point alone which invalids the irrational obsession people have with HP. But we'll dig deeper yet. You see what really matters is weight to torque ratios and even that is frequently not an apples to apples comparison. I'm sure many are still wondering why and how I came to these conclusions based an incomplete HP formula of HP=TxRPM.

    In far too many conversations people simply compare HP between two cars and assume the one with the highest rating is the fastest. The truth is, such comparisons are almost always completely meaningless; and therefore useless. Most engines create their HP using one of two philosophies. One, low torque and high RPMs. Two, high torque and low RPMs. This distinction, if we're to generalize, wonderfully categorizes the differences between four cylinders and eight cylinder engines. This is, of course, why they tend to provide wonderfully different performance profiles.

    So here's an example. Please remember we're using a simplified equation so the numbers won't necessarily seem sane. Engine A makes 300HP with 100ft/lbs of torque and 3RPM. Engine B makes 300HP with 3ft/lbs of torque and 100RPM. So which engine is "better?" The simple answer is, it depends. Both have their uses. For a car, chances are the engine with the highest torque is the superior engine. Notice the measure which held sway was torque. The reasoning is simple. Torque is what pulls you out of the corners and off the line. Torque is what determines how quickly you can generate and build horsepower, and therefore, use that horsepower. The fact HP is directly derived from torque only serves to dilute torque's significance in its contribution to performance.

    Ultimately, usable HP is all too often a function of time. If it takes you 60-seconds to build to peak HP-RPM, pragmatically, its of no use to us as a daily driver; let alone racer. This is, of course, why hard core motor-heads love long strokes and big displacement; both of which are associated with "torqie" engines.

    You might think this is the end of the story but I think we can dig deeper yet. You see, that HP rating is typically the rating of the engine at the crank. And chances are, your crank doesn't directly attach to your drive wheels. Chances are, you have a transmission and one or more gear boxes (example, "rear end"") between your engine's output crank and the drive wheels. Moving power from the crank to the drive wheel(s) requires mechanical motion, which in turn requires friction. Not to mention the mere act of transferring power requires movement of other heavy components. This all means the act of transferring power from the crank to the drive wheels loads the engine before it ever turns your wheel. Classically, the amount of power lost between the crank and drive wheel is 10%-15%.

    What this ultimately means, in most cases, is that the engine which is promoted as, say, 200HP, only has between 170-180HP at the wheel. But frankly, they typically never tell us the efficiency of the transmission and gearbox(s). This difference is normally denoted by the use of WHP vs BHP (or just HP). The WHP means Wheel HorsePower.

    And yet, this gets even more convoluted. The reason being, when you transfer power through the powertrain, it goes through a number of gears. Each gear, in turn, changes the gear ratio between input and output. What this means is that while the crank may make one rotation, the rear wheels may only spin .5 rotations. This, in turn, directly affects HP and usable torque, not to mention the time required to use our torque to build RPMs which in turn drives usable HP. Made even more confusing, these ratios change with every gear. And while that may not sound like any big deal, the issue is, unless your ratio is 1:1 (one turn input to one turn output), even ignoring transfer losses, the input (from the crank) HP will never equal the output (to the wheels) HP. Which means, when you add it all together, comparing two entirely different vehicles, each having 200HP, its completely impossible to know which one has the advantage without lots of extra details.

    So when its all said and done, horsepower makes for a incredibly poor basis of non-relative comparison. Which is to say, unless its the same powertrain, comparison on the basis of horsepower alone is completely meaningless. One might even say, horsepower doesn't matter.
     
    Last edited: Oct 26, 2011
  2. jim580

    jim580 Member

    HP is important because if your neighbor buys a 300hp sedan, you can go out and get a 301hp sedan. It's just human nature.

    Remember when a 48" riding mower had a 10hp engine? They worked just fine and didn't need more. Now you can hardly find one with less than 18, 20, 22hp. Take that neighbor Jones!

    It's all about marketing and sales. They know if they build it, we'll buy it to "one-up" each other.
     

  3. jim580

    jim580 Member

    ...and before I forget, did you ever think you would see 2000cc motorcycles?
     
  4. My old Allis B is rated at 16 HP I think.

    Thing is a beast of a tractor.

    Usable power is all in the gearing, as most off road guys already know. You could run your Jeep with a weed whacker engine if you could get the gears steep enough.

    Although it is fun to romp on an old GM 350 with a good sounding exhaust.

    Makes the girls all tingly in the pants region. (It's science, see #5 at the link)

    http://www.cracked.com/article_16780_6-sex-myths-as-explained-by-science.html
     
    Last edited: Oct 26, 2011
  5. MMCM(SS)_RET

    MMCM(SS)_RET New Member

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    Texas
    Someone always has to tell, there is no Santa Claus!!!!!!!
     
  6. gun

    gun Powered by air Member

    Certainly didn't expect that post to be this well received. I can't tell you how many times I've seen people pop a vein when I tell them that. Some even get angry and accusatory. Though I must admit, those who became visibly angry usually have a vested interest in their self-image via their high horsepower vehicle. Seriously, you should try it some time.

    There's not a reply I disagree with here. And MMCM(SS)_RET, ouch and ahhhhh... ;) You guys are an enlightened lot. Must be why I like it here.

    Cheers.

    P.S. None of this means HP can't be fun or that there is anything wrong with fun. :D
     
  7. DEDUBE

    DEDUBE Member

    Back in 1978 I bought a new Harley Davison Low rider 1200cc. It was one of the first Low riders made. I can't remember the torque but it only had 29 hp. I was amazed and thought then that HP doesn't mean power to pull the hills. I have been amused by all the car and truck adds that boast of their HP a 5800 rpm. Now how many drive around at 5800 rpm. Its a bunch of empty hipe to excite motor heads into purchasing the biggest engine in the vehicle.
     
  8. duster066

    duster066 Supporting Member

    Excellent subject, and I agree with the logic. From a racing perspective the only numbers that matter are found on the scales, a clock, and a bank statement. If the bank statement can be kept black the other two numbers go down.
     
  9. showpare

    showpare Expert in Superfluities Member

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    Great overview, gun. I feel better prepared to compare vehicles now. I've never been much of a gear head. We have been looking casually at the three retro muscle cars: Challenger, Camero, and Mustang. One of these may be my Mid Life Crisis Car (MLCC).
     
  10. KnurledNut

    KnurledNut 995 Range Hound Member

    Gotta love a girl who appreciates horsepower.

    [​IMG]
     
  11. gun

    gun Powered by air Member

    I don't know if you said that with tongue in cheek. I know that's a popular meme for guys once they hit 40-ish, but I personally disbelieve. Unless mommy and daddy are buying for you, or you've had an especially fortunate career, or no kids, etc., most people simply can't afford such toys until they hit 40-ish. Having fun or rewarding yourself for your own success is not a midlife crisis.

    Since you're not a "motor head", IMOHO, get the car that you find most attractive, most comfortable, and the best seat of the pants fun. Don't worry about 1/4" times; though 0-60 and rolling to 100 times frequently reflects more real world fun. Which is to say, passing, entering the highway, or just having some fun from the light up to the speed limit or a little beyond.

    I rebuilt my first motor at 16 and helped/watched my father do several others way before that. I still consider automatic transmissions to be something of a hydro-mechanical marvel. My cars have been T/A, Firebird, Mustang, T/A, Galant. I grew up somewhat of a motor head but never particularly hard core. I don't change vehicles like underwear and tend to keep them until they are long in the tooth. My favorite car was my 5.0 'stang, which had no problem whipping the 4.6's which came later in spite of the 4.6's having more HP on paper. My second favorite was my 5.7 '99 T/A. Both of these were manuals. To this day I still miss my 5.0 'stang.

    These days autos can provide both better economy and faster shifting than sticks. Also, mod chips (be careful here, there is lots of fraud here) can be had which can dramatically improve performance and even speed shifting for automatics. So if you're not a very hands-on/tactile driver or if you constantly drive in heavy traffic, don't be shy about looking at cars with automatics. In fact, with an automatic, the average driver can easily give the more experienced driver, with a stick, a really good run for their money; especially on the roll. Not to mention, more consistent times.

    Let us know what you wind up getting.
     
  12. AGuyNamedMike

    AGuyNamedMike Lifetime Supporter

    I agree wholeheartedly.

    (Unless the car is red. Then it is absolutely a MLCC. :D )
     
  13. duster066

    duster066 Supporting Member

    I agree too. When we got married I was instantly introduced to her idea of spending money, and I didn't like it. We needed a car, and decided to buy new. I wanted a 5.0 Stang, and she absolutely wouldn't stand for the sticker price. We compromised and SHE (she always got the good car) got an 85 Shelby Charger.

    Four years ago I needed a car. Just for fun I stopped by the local Ford dealer and looked at Shelbys. Before it was all over she told me to go get what you want, "you've earned it." I ended up with the Mustang I've always wanted, a new GT. This time it really is mine! Oh and of course it's red.
     
  14. showpare

    showpare Expert in Superfluities Member

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    Busted! My MLCC will be a reward for being 50ish and having the house paid off and my daughter through college. We paid off the house about 5 years ago and our daughter is one semester from her degree. However, I'm a pretty conservative guy. It may be a while longer. We are looking at ~50 acres in some rough country. I may do neither or I may do both.

    Right now, I'm liking the Mustang. We have a two car garage, daughter moves, then the MLCC gets a bay.
     
  15. duster066

    duster066 Supporting Member

    I feel a little ripped off. If I had waited just three years I could have had another 110hp! Oops now I'm busted cause I agree with everything Gun said.
    However I have enough experience driving cars hard that I know if that hp figure goes up, all other things being equal, and the driver knows what he's doing, the car will go faster, and 110hp will make it a lot faster.
     
  16. AGuyNamedMike

    AGuyNamedMike Lifetime Supporter

    Depends on the race. When I was muuuch younger (ah, sweet misspent youth) I had a baby blue and primer gray '77 Pinto wagon with the 2800cc V6 mated to a slightly worked C4 tranny (I know, 2.8 liter, but back then this engine was referred to as the 2800cc). My circle of friends was a group of guys who mostly drove late '70s f-bodies (Camaros and Firebirds if you don't know.) Sometimes we would race odd routes very late at night through the back roads (think redneck rally racing), and just as often as not, I'd win. Maybe the fact that I wasn't afraid to cut through a field or scoot down a rutted gravel track at speed helped, but it still felt good.

    I'm 44 and expect to drive my dependable truck for the next dozen or so years, and I left the need for speed behind a long time ago. When that hankering for the MLCC hits me I'm more likely to find a restored antique convertible or classic motorcycle and sidecar for weekend cruising
     
  17. duster066

    duster066 Supporting Member

    I know the car well. My first hot rod was a 71 Pinto with the Capri 2600 V6 and a 4 speed. It could take mild v8 cars. My comparison is an apples to apples, cars built to a set of rules. HP is real hard to beat if ya ain't got any. And most guys smart enough to build good power also know chassis and tires. They are generally very hard to beat.
     
  18. gun... thanks for the post and education. My wife has a small diesel Jetta with a turbo on it and it will out run just about anything on the road, even out of the hole. It has so much torque even with the auto tranny that it can bark the small but sticky tires (FWD).
     
  19. gun

    gun Powered by air Member

    I had hoped it would be an interesting if not provocative discussion. If it helps you, I'm flattered. I'm sincerely happy you got something out of it. Though honestly, a little disappointed it turned out to be not so provocative here. ;)

    Diesels and gas direct injection engines, IMOHO, are the future. With gas direct injection engines, suddenly they are able to take on a lot of advantages diesels offer, such as much greater thermal efficiency and lots of low end torque. Unlike what I described above in the generalizations of low torque, high RPM vs high torque, low RPM, most diesels, regardless of displacement, subscribe to high torque, low RPM models for the sake of combustion efficiency. Diesel engines are famous for their low end torque capabilities. And while most people tend to not think of diesel engines as high performance engines, that's all started to change in the last decade, plus. In fact, those gas direct injection engines have a heritage from diesel technology; which originates from direct injection diesel. IMOHO, modern diesel engines are horribly underrated and under appreciated in the US. Not so in Europe.
     
  20. duster066

    duster066 Supporting Member

    I was prepared to have a spirited debate on the original point, but your position is to strong. I think you are referring to the way performance figures are used in advertising and a general lack of knowledge by those who buy cars based on those numbers. I can't argue with your logic. However hp, in general, does indeed matter a bunch when comparing apples to apples. My 07 Mustang has 40 more advertised hp than my 00 had. I don't know what the curves look like for the two cars, but although the 07 is several hundred pounds heavier it does go faster and accelerates quicker than the 00. So while advertised figures are often misleading, when considering similar cars the way that they make power does matter. I think you said that though.