Yup, horsepower is a derived parameter. I think generally hp means the rate
at which torque is doing work, like ft-lb per second or minute. Or is that
lb-feet / sec ? I don't know..it is so hard to be politically correct these
days...
mayf, out in pahrumo
----- Original Message -----
From: "Steve Laifman" <SLaifman@socal.rr.com>
To: <tigers@autox.team.net>
Sent: Sunday, February 13, 2005 4:10 PM
Subject: Torque and Horsepower
> Just to throw water on the simmering missives:
>
> Talk about speaking at cross purposes. Break out the Mechanical
> Engineering textbooks, and lets all talk the same issue.
>
> We have owners, who are knowledgeable about practical mechanical
> relationships, talking about "torque" and "horsepower" as if they were
> independent quantities. They are not, but rather very dependent
> quantities, the ONLY variable being the RPM that's being investigated,
> and perhaps the throttle position if talking about "potential
> increase". They are being answered by responses that rely upon
> engineering definitions. Having differing views on the relationship of
> the variables, there are bound to be misunderstandings.
>
> On "Horsepower", "Torque", and "RPM":
>
> The only time they are "independent" is when you are at one set RPM, in
> a cruise mode, and decide to change the rpm. Then you will absolutely
> follow the engine design potential ("torque curve)" as the RPM changes.
>
> This is sometimes confusing to those not familiar with the origin and
> definition of these terms.
>
> If one were more familiar with electricity, it would be like arguing
> whether amps or volts has the most influence over power (watts). They
> are only dependent variables in the defining equation E = I x R (or
> Volts = Current x Resistance) and P = I x V (Power = Current x Volts).
> Arguing whether it is current (I) or volts (V) that produces power (P)
> obviously does not make sense here. They are independent variable that
> DEFINE power. Note that Power is also = I^2 x R by simple math
> substitution of the defining terms. No slight of hand.
>
> Horsepower = (N x T) / 33,000 (unit adjusting constant).
>
> The N is the rpm at which you want the information on the horsepower
> being generated, the T is the dyno measured torque the engine produces
> at full throttle when the external load drags the RPM down to the level
> being considered. If an engine is dyno tested 500 rpm increments, and
> the load increased to maximum the engine can stay at each rpm level at
> full throttle, you can get those pretty curves you see on the Edelbrock
> site.
>
> Many dyno houses do NOT do this, but get the maximum horsepower of the
> design. Less work and strain.
>
> It is the engine design that dictates the torque curve, hence the
> maximum torque/rpm relationship that DEFINES the horsepower/rpm values.
>
> It is just confusing to hop back and forth between different engine
> designs (torque curves) and try to compare one to another at a given
> rpm. They are not the same engine, and the results are only meaningful
> when like is compared to like. It is not intuitively obvious to one
> not familiar with the interrelationships between these parameters, and
> how they are determined, that get confused when one independent variable
> is compared to one from a different design. You need ALL the design
> parameters to make a comparison.
>
> Although these are very simple mathematical terms, those not familiar
> with their derivation and interrelationship, or the mathematics
> involved, can get misled by making comparisons between apples and
> oranges by comparing fruit diameter, or acidity.
>
> There are plenty of technical discussion that can be located by
> "Google", if not Marks Handbook of Engineering and Physics.
>
>
> --
> -----
>
> Steve Laifman
> Editor
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