Real HP loss numbers

Daniel Ciobota dciobota at hiwaay.net
Fri Mar 12 19:24:47 GMT 1999


 Let me jump in on this discussion with an observation.  Rear wheel hp numbers
are measured by observing the acceleration of two 1000lb drums by the driving
wheels of the car.  That's how hp is calculated, rotational speed vs. time,
known weight of 2000lbs and known drum diameter.  From those numbers, and final
gear ratios (rpm/drum rotation), the chassis dyno calculates instantaneous
torque required to accelerate the drum at that rate.
 The key measurement here is rotational mass.  That parameter affects how fast
those drums will accelerate, and since it's a known quantity for the chassis
dyno, accurate hp numbers at the driving wheels can be calculated.
 To backtrack flywheel hp from driving wheel hp, a couple of parameters are most
important: frictional loss, as discussed here already, and _rotational mass_,
more accurately, momentum.  Take for example an aluminum flywheel and the
equivalent steel version.  In my stang, the aluminum piece weighed 10lbs, while
the steel piece was 23lbs.  In neutral, the aluminum flywheel accelerated
significantly faster than the steel piece; no surprises there.  The same concept
applies to the rotating pieces inside transmissions, driveshafts and rear ends;
the total momentum affects how fast the drive wheels can accelerate, thus
affecting rear wheel hp numbers.
 From observation of different dynoed cars, I think that gm transmissions have
slightly less momentum and frictional loss than their ford counterparts.  And,
of course, because of the fluid and rather massive torque converters, automatics
are significantly more sluggish and offer more loss.
 So, when someone is talking about a 75hp loss, not all that loss is friction,
otherwise our drivetrains would glow in the dark!  However, much of the loss due
to momentum is taken up by the engine (more strain), which shows up as heat in
the coolant and oil.

 If my physics are wrong, please correct me.

 Just my $0.02.

Daniel



Roger Heflin wrote:

> On Fri, 12 Mar 1999, Clive Apps   Techno-Logicals   416 510 0020 wrote:
>
> > >
> > > I have wondered about this myself.  I have heard anywhere from 15%
> > > to 25% in drivetrain losses.  What I can't figure out is where the
> > > energy is going.  If you have a 300HP engine with 25% drivetrain
> > > loss, then you are losing 75HP somewhere.  Since it doesn't just
> > > disappear, something has to be soaking up 75HP of energy.  My
> > > guess is that the loss would be in the form of heat which would
> > > mean of lot of drivetrain parts would have to be getting really hot
> > > (assuming the 300HP load on the engine).  Since I have only seen
> > > my transmission get warm, it doesn't really make much sense unless
> > > I just haven't had the load on for a long enough period.
> >
> >
> > just estimate how much torque it takes to compress a valve spring
> > multiply x 1/2 revs x # springs X length of valve travel x
> > x losses in the vlave gear and add in the amount that it takes to
> > move the valve train components around
> > 15% sounds reasonable
> > on 300 HP that would meav that another 52 HP was being eaten by the engine
> > to move the valves
> >
> > Clive
> >
>
> For the 15% number everyone has generally ment the amount of hp lost
> after the flywheel.  The wheel dynos hp nubmer vs.  the engine only
> dyno numbers.  So that claim is rear wheel hp * 1.15 (manual tranny)
> is roughly equal to the engine hp at the flywheel.   The valves are
> the same and already accounted for in both cases.
>
>                         Roger




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