[Vwdiesel] torque vs. horsepower (was: something worth trading the A4 in for)

Terry Briggs vbriggs at stny.rr.com
Mon Jul 7 21:55:13 PDT 2008

Roger, you are the man !
On Jul 7, 2008, at 12:13 AM, Roger Brown wrote:

> David Schwarze wrote:
>> What you say below is simply not true.  Horsepower DOES move things.
>> More HP will get you down the road more quickly, regardless of torque.
>> The units of HP and Torque tell the story.  Power (hp) is a quantity
>> that you can use along with the weight of the vehicle, friction,
>> aerodynamic drag, etc. to predict how quickly a car will accelerate.  
>> It
>> is a measure of how much work is being done.  Accelerating a car is 
>> work.
>> Torque is a measurement of rotational force which CANNOT be used to
>> predict how quickly a car will accelerate (why not, you say?  Because
>> "it depends" on the RPM).
>> For a real world example, take a sport bike.  That fits your 
>> definition
>> of "massive amounts of hp and little to no torque".  Ever race one of
>> those at a stoplight in your diesel VW?  I'm sure with the massive
>> torque of the diesel engine that poor sport bike didn't stand a 
>> chance,
>> right?
> Torque, via the gearing in the transaxle, it what turns the 
> wheels/tires to push the car
> forward.  More torque, whether in raw form out of the engine or via 
> reduction gearing in
> the transmission equals more force transferred by the tires to the 
> road and more force
> acting on a given vehicle mass (or weight) results in an equal and 
> opposite force (per
> Isaac Newton) called acceleration.  The units of torque are the same 
> as the units of work,
> force over a distance.  And if you take things to finer and finer 
> increments of time (in
> mathematical terms), the RPM factor goes away, acceleration is defined 
> as the derivative
> of velocity with respect to time (dv/dt) and is simply reduces to the 
> mass and the force
> acting upon it.
> Power (or horsepower) is in units of work per unit of time.  That will 
> tell your how much
> work an engine can produce over a given period time.  Since engine HP 
> is a function of
> torque and RPM, you can vary both of those factors via gearing, tire 
> size, etc.  And since
> HP is a combination of these two values (torque and RPM) and since in 
> typical internal
> combustion engines torque varies with RPM, it is hard to separate the 
> two factors.
> Imagine one engine with a very peaky torque curve with tuned intakes 
> and exhaust made for
> maximum power/torque output at some high RPM.  And then an engine with 
> the same overall HP
> but one with a fairly flat torque vs. RPM curve.  If you use both 
> engines to accelerate a
> vehicle, (assuming fixed gearing) you need to integrate the torque 
> over the range of RPMs
> from beginning to end to get the overall time to speed.  Think of 
> taking the area under
> the HP vs. RPM curve.  A broad flatter HP curve has more area under it 
> than one that is
> shaped like a triangle.  A flatter curve is the result of a torque 
> that builds up at a low
> RPM and holds fairly constant at the RPM rises.  A steeper curve is 
> the result of an
> engine where the torque keeps rising with RPM.
> And yes, a 50 HP gas engine in a motorcycle will out accelerate a 50HP 
> diesel engine in an
> automobile.  Why?  Weight.  Put 50 HP behind a 300 lb. bike and put in 
> gearing to drop the
> high revving, low torque engine output down to a usable value for the 
> load, that low
> torque times the gear reduction results in a large torque at the wheel 
> and that torque
> times the lower mass equals higher acceleration.  Put 50 HP behind a 
> 2000 lb. Rabbit and
> guess what, it is slower to accelerate than a 6 times lighter 
> motorcycle.
> So it all really boils down to the fact that torque is the fundamental 
> unit and is what an
> engine produces.  RPMs are just the result of what the engine is 
> connected to and what
> that load does in response to the applied torque.  HP is just the 
> derived product of the
> supplied torque and the resultant RPM.
> -- 
>    Roger
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