Larry,
The engine and chassis dyno hp and torque peaked within 100 rpm. If my headers
were
bad or the mufflers restrictive I would expect the tailspin. But, the headers
are the
best that can fit into a Cobra, and the mufflers are 3.5 inch dia glass packs.
(Yes,
it's loud.) I have the data from the engine dyno on a spread sheet. I'll try
to find
the data from the chassis dyno and overlay it. I'll let you know in a day or
so. It
could be interesting. I wonder how to measure the rolling resistance of the
tire. It
could be one of the largest losses. I know that my car is not east to push, and
the
majority of that resistance is rolling resistance of the tires. This resistive
force
is constant and not proportional to speed.
Craig
Larry Gallo wrote:
> Craig,
>
> Did you note if the power curves on the engine versus
> chassis dyno:
> A) peak at the same RPM, or
> B) did they rise closely along the same line
> on the way up, but at some certain rpm
> diverge noticeably into a tailspin for
> the installed runs?
>
> I am guessing the B condition was encountered...
> Analysis of that nature could help answer the
> controversy of this 'thread'.
>
> Regards/Larry
>
> Craig Wright wrote:
>
> > I had my street 427 tested and broken in on an engine dyno. It put out 420
>hp.
> > Two months later I measured 310 hp at the rear wheels, a 26% change. I did
>gain
> > 10 hp back by jetting the carb for the in car conditions. The differences
>were,
> > in part caused by:
> >
> > Mufflers
> > Different headers
> > Transmission, drive shaft, differential, tire rolling resistance etc.
> > Engine inlet temperature.
> >
> > The last one has an effect on some cars, and can result in 2-4% lost hp.
>While
> > sitting stationary on the dyno, there is no air coming through the hood
>scoop.
> > Consequently the carb is sucking engine compartment air (at least, in my
>car).
> > This air has been heated by the radiator and is significantly hotter than
> > ambient.
> >
> > Next time I use an engine dyno, I'll be more careful to use the headers and
> > mufflers that will actually be on the car.
> >
> > Craig Wright
> >
> > Mark Palmer wrote:
> >
> > > Derek,
> > >
> > > Just sent another reply, but wanted to comment on one or two things here.
> > > Loss in gear mesh is due to friction, not "metal bending". Let's hope the
> > > teeth aren't bending much or we're in real trouble! Actually, there is
> > > always some deflection but bending does not dissipate energy -- bending is
> > > what springs do, which are energy STORAGE devices not energy ABSORBING
> > > devices. The energy put in to bending the gear tooth is returned to the
> > > system as the tooth unloads & returns to its "normal" position. Think of
>a
> > > sling shot, if that helps.
> > >
> > > Otherwise you're on the right track. Oil "churning" (usually called
> > > windage) losses are proportional to speed, oil shearing proportional to
> > > speed & load.
> > >
> > > Perhaps surprisingly, very little of the loss actually occurs at the gear
> > > mesh itself -- typically 1.5% of transmitted power per mesh. Most of the
> > > losses are due to the bearings, and windage. This is because, in involute
> > > profile gear teeth, the relative motion is actually a rolling motion, NOT
>a
> > > sliding motion (true for spur or "straight cut", & helical). The losses
> > > are typically 5% or so, if you include the radial & thrust bearings that
> > > support that gear mesh (1.5% mesh loss + 3.5% bearing loss). Bevel gears
> > > have a combination of rolling & sliding motion, hence are less efficient
> > > (typically 2.5 - 3% loss per mesh; 6 - 8% when you add in the supporting
> > > bearings) and worm gears are the worst, all sliding, that's why we don't
>see
> > > them much on cars.
> > >
> > > Regards,
> > > Mark
> > >
> > > >
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