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
>
> >
|