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Technical What's up with horsepower

Discussion in 'Traditional Hot Rods' started by Pothole, Oct 12, 2019.

  1. Pothole
    Joined: Dec 14, 2017
    Posts: 60


    Can someone please educate me on horsepower and torque? I recently took my A-V8 to a dyno and was surprised at how low the hp (at the rear wheels) read. It was just over 72hp. My engine is a 59ab with dual carbs, headers, aluminum Sharp heads, and Clay Smith cam. The transmission is a three speed and I'm not sure about the rear except I was told it was from a 42.
    I hear terms like hp at the flywheel and the rear but don't understand the difference. The reason for the dyno was to try and determine the best rpm to shift from 2nd to third. I just returned from racing at TROG where I did good but could have done better if I had known more about my power curves in second and third. At TROG because of the sand I started in second and got the jump on all but 1 other car. In some runs I stayed in 2nd the whole way and in others I shifted to 3rd about 2/3 the way with mixed results. In my runs I tried to go by feel rather then the tach reading. The dyno indicates I should shift at 3000 rpm, the curve drops off after that. Any advice would be greatly appreciated.
    TROG was a wonderful experience- the cars, the drivers and the TROG organization.
    Thanks, Pothole Bob
    lothiandon1940 likes this.
  2. The drivetrain itself along with other variables saps some HP so "at the wheels" HP is going to be lower than at the flywheel.
    Montana1, upspirate, Deuces and 3 others like this.
  3. Someone (Squirrel where are you?) with greater engineering knowledge than myself will be able to explain this all better than me. I'm sure it involves laws of physics like inertia, etc. and the amount of energy lost while spinning transmissions, drive-shafts, rear gears, axles, wheels, etc. and there may actually be formulas to calculate HP loss through these challenges. There may actually be formulas that can predict the amount of loss relative to the aforementioned flywheel HP rating. It's all way over my head, but rest assured there is a loss to be considered. Your 72 HP at the wheels could translate to maybe 100 HP at the flywheel. This is just an example, your results may vary.:).....Don.
    Montana1, mgtstumpy, weps and 5 others like this.
  4. bill gruendeman
    Joined: Jun 18, 2019
    Posts: 119

    bill gruendeman

    I don't remember the the numbers but a man trans eats like 10%, auto trans is more. And rear end also eats like 10% of your horsepower
    Montana1, Deuces and lothiandon1940 like this.
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  5. Tickety Boo
    Joined: Feb 2, 2015
    Posts: 970

    Tickety Boo
    from Wisconsin

    Drive train uses up to about about 20%, depending on the transmission used, some automatics like the turbo 400 use up more to run than a powerglide. Manual transmissions don't have a pump to run
  6. dumprat
    Joined: Dec 27, 2006
    Posts: 2,280

    from b.c.

    Friction and gear drag from components is what causes it.
    There was an article published years ago where they ran the same car with several rear axles on a chassis dyno. Different rear axles used up different amounts of horse power. The 9" ford was one of the worst (least efficient)
  7. squirrel
    Joined: Sep 23, 2004
    Posts: 43,429


    Losses in the drivetrain are pretty hefty, no matter what type of drivetrain it is. Also, horsepower numbers bandied about by guys who've never put their engine or car on a dyno, are generally out to lunch. Even so, what matters is winning races.

    As for shift points...look at "area under the curve" more than just at the RPM where HP starts to fall off. This means, you want to shift at a point where you are not losing HP by going into the next gear too soon, and dropping RPM to where the HP is lower than what it was before you shifted. The only way to figure out the best shift points, is to make timed runs with slight changes, and see what makes you go fastest.

    I hear a lot about how the TH400 takes more power to turn, than most engines make. And the 9" ford will slow you down because it has so much friction from it's large pinion center line offset. The funny thing is, the street/strip drag racing guys I know who go fast mostly use the TH400 and 9" ford, because they can survive the punishment. Unless you're racing in a very competitive super stock class, you probably won't notice the slightly higher percentage power loss from using either of these parts.
    1953naegle, weps, upspirate and 12 others like this.
  8. Thanks, Jim.
    Deuces and hrm2k like this.
  9. Squirrels explanation is spot on, As he said people throw out horsepower numbers, when I was young every small block Chevy with headers was 300horsepower, most of them are talking out of their ass without dyno sheets to back it up.

    Sent from my iPhone using H.A.M.B.
  10. town sedan
    Joined: Aug 18, 2011
    Posts: 1,072

    town sedan

    Just because no one mentioned it:

    torque x rpm / 5252 = hp
    Desoto291Hemi, hrm2k and Blues4U like this.
  11. TagMan
    Joined: Dec 12, 2002
    Posts: 5,864


    You're asking two different questions -
    - "What's the difference between torque & horsepower ?"
    - What is my optimal shift points while racing ?".

    If you google those questions or permutations of them (i.e. "Torque Vs horsepower" or "optimum drag race shift points"), you can find all kind of information.
    Flathead Dave likes this.
  12. RustyDogg
    Joined: Oct 8, 2014
    Posts: 106


    I maybe wrong here... but if you're getting 72 h.p. to the wheels that's a pretty strong flathead. IMO
    Last edited: Oct 12, 2019
  13. There's a couple good reasons to do Dyno time but for most Hot Rod owners the info learned is often disappointing. Knowing the sweet spot to shift is a good one.
    Frankie47 likes this.
  14. Pothole
    Joined: Dec 14, 2017
    Posts: 60


    I can't thank you guys enough, information presented in different ways really helps to drive the points in. Jim, excellent info, concise and understandable, thanks.
    KoolKat-57 and lothiandon1940 like this.
  15. Blues4U
    Joined: Oct 1, 2015
    Posts: 3,811

    from So Cal

    Yeah, I thought he was looking for more information on the terms, horsepower, & torque, not so much about drive train efficiency.

    Torque is a rating of how much work is performed.

    Horsepower is a rating of how fast that work is performed.

    @town sedan gave the formula for calculating HP. BTW, 5252 rpm is always the point where torque & HP always cross on a graph.

    Here's a video that explains it, though this guy usually bugs me, he does a good job with this one.
    pitman likes this.
  16. Stan Back
    Joined: Mar 9, 2007
    Posts: 476

    Stan Back
    from California

    . . . and for some older engines, you might not want to get to 5280 cuz some internals may cross.
  17. Blues4U
    Joined: Oct 1, 2015
    Posts: 3,811

    from So Cal

    lothiandon1940 likes this.
    Joined: Dec 14, 2009
    Posts: 1,898


    Not as technical as most of the other answers witch are all spot on

    How it was explained to me years ago it
    Horse power equals how fast you hit the wall.
    Torque equals how far through the wall you go.
  19. Nitro crew chief
    Joined: May 4, 2008
    Posts: 171

    Nitro crew chief
    from Illinois

    Horsepower is how fast you will hit a wall, torque is how far you will move the wall.
    akoutlaw, Blues4U and Hotrodmyk like this.
  20. The Shift Wizard
    Joined: Jan 10, 2017
    Posts: 1,422

    The Shift Wizard

    In a very general and unscientific answer, torque will get you acceleration and horsepower will get you top speed.
    Personally, I like plenty of each. :rolleyes:
    VANDENPLAS likes this.
  21. RichFox
    Joined: Dec 3, 2006
    Posts: 9,741


    When I had my 12 port Howard head GMC people would ask me how much horsepower it had. And I told them. I soon noticed people were disappointed in the number. Even though I had just put 44 mph on my record. So I raised the number. People liked that better. But still weren't real happy. So I raised it more. People happier. I raised it past any realistic number. Spectators satisfied. So I left it there for the rest of the week. I am not going to start again by telling the real number
    rpm56, KKrod, gas pumper and 8 others like this.
  22. Pothole
    Joined: Dec 14, 2017
    Posts: 60


    Thanks Blues4U, the video was excellent. I'll have to watch it several times to get it all but it was well done.
    Paul Madden likes this.
  23. Pothole
    Joined: Dec 14, 2017
    Posts: 60


    Thanks, now I know how the game is played.
  24. G-son
    Joined: Dec 19, 2012
    Posts: 557

    from Sweden

    You only need a couple words to tell people how much power your engine has: it's either "enough" or "not enough". ;) Numbers, taken out of context, are just useless numbers.
  25. studebakerjoe
    Joined: Jul 7, 2015
    Posts: 442


    Sounds like the old Rolls Royce thing. I believe the hp rating was listed as sufficient.
    lothiandon1940 likes this.
  26. jimmy six
    Joined: Mar 21, 2006
    Posts: 4,641

    jimmy six

    I like Rich have been asked about horsepower of my GMC's. I actually don't know as it has never been on a dyno. Others have told me how much you need to push a 32 grille shell 171mph, I don't put much belief in their numbers.
    If I think they are in my class I "just might" say "it looks like more than you are" but normally say "not nearly enough".;)
    I can say if I just lean on my roadster and it doesn't start moving, I must have left it in gear. :rolleyes:
    Texas Webb likes this.
  27. Desoto291Hemi
    Joined: Apr 21, 2009
    Posts: 738


    5252 is the constant used in the equation.
    They only intersect at 5252 rpm,,,because the two numbers equals the same.
    No big mystery,,,just normal logic.
    There are also formulas to calculate a lot of different results.
    You can calculate the horsepower of a car in the quarter mile,,,using the weight and mile per hour with the right constant,,,and it is very close to the dyno numbers as well .
    Back in school,,,,who would have thought that math would actually be useful someday,,,Lol.

    Texas Webb likes this.
  28. Ziggster
    Joined: Aug 27, 2018
    Posts: 331


    All good info. So is what your saying is that the most Hp you achieved was in 3rd at 3000 rpm? For others - Is this typical of a flathead? Seems low rpm to be maxing out Hp.
  29. Boneyard51
    Joined: Dec 10, 2017
    Posts: 2,696


    B7BC323C-C568-48A3-AD52-78682314AC21.jpeg 90095CAB-567C-44DD-BF4E-E0BB6D160666.jpeg I got to play on a dyno a couple of weeks ago.....for the first time! It was awesome!
    pitman and Bandit Billy like this.
  30. The Shift Wizard
    Joined: Jan 10, 2017
    Posts: 1,422

    The Shift Wizard

    One rather important thing about those nice printed out dyno graphs is not necessarily the peak number but rather how flat and fat the curve is in total. The sooner on the RPM scale that the torque (in particular) and the horsepower raise the better. Note that all the power is under the line(s) so if the curve is slow to ramp up and drops off quickly in a peak, that vehicle won't be nearly as quick and as much fun to drive on real streets and tracks as one with a curve that raises sooner and traces like a rounded mountain top but has a somewhat lower bragging number.

    The higher number doesn't always win the race but the vehicle with the highest total across the whole RPM band has a good chance of getting its fair share of wins. :cool:
    Elcohaulic and upspirate like this.

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