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Horespower - automotive?

  1. Apr 14, 2009 #1
    This is more of an automotive question I guess but anyways:

    I'm pretty familar with how torque and horsepower are related, the common equation most people think of is hp = ((tq X rpm)/ 5252).

    If you look at a lot of dyno graphs, the hp and torque curves always crossover around 5252. Obviously this is because they are equal around 5,252 rpm's. What i dont get is that they do not cross exactly at this point, and another thing ive heard is that some people will tune engines so that they do cross at this point. What is the advantage of this?

    here are some example dyno sheets/graphs:

    http://www.electricsupercharger.com/images/dyno/DYNO-PORSCHE_84_911_STOCK.jpg [Broken]

    http://dynos.evans-tuning.com/750whpsc300featurecar.jpg [Broken]
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Apr 15, 2009 #2
    Well, since the horsepower is a calculation based on the torque output, the lines always cross at 5252 rpm. Any graph that shows otherwise is incorrect. You cannot "tune the engine" to change the location of the crossing point.
  4. Apr 15, 2009 #3


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    Homework Helper

    Although the lines cross, the units are not the same. Also the lines only cross at 33000/(2 pi) = 5252.113... when using English units. Using kilowatts and newton meters, the lines cross at 60,000 / (2 pi) = 9549.296...

    Wiki link:

  5. Apr 15, 2009 #4
    Here are the equations:
    Torque measured in Newton meters. (USE MKS units)

    Power (watts) = torque x 2 pi (RPM/60) = torque x RPM/9.549

    Power (HP) = watts/746 = torque x RPM/(746 x 9.549) = torque x RPM/7124
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