I don't have my old calculus book handy, but I'm pretty sure Y=X^2 was
the very first example of an exponential function.
And, the losses between the flywheel and the road are probably mostly
proportional to power. So, for your example of 105hp at the flywheel
and 95 at the rear wheels, an added 5hp at the flywheel would add only
4.5 at the rear wheels. IOW the percentage increase is the same,
whether measured at the road or at the flywheel.
I believe the drivetrain losses are a lot closer to 10% than 50%.
Otherwise, the diff & tranny would get a lot hotter than they do, and
need more cooling. I've seen the measurements somewhere, just can't
remember where.
OTOH, it's worth noting that most car manufacturers now use ATF instead
of 90W, apparently to reduce friction to the oil. The racers seem fond
of 5W synthetic gear oil, but again I have no hard numbers on how much
difference it makes.
Randall
Trevor Jordan wrote:
>
> At 1:59 PM +1100 15/3/99, DANMAS@aol.com wrote:
>
> >Yes, but I hedged my bets! I didn't know if it was the square or the cube,
>so
> >I just said "exponentially," which covers them both. Slick, huh?
>
> Close, but not quite. Exponential is n^^x, not x^^n (where n is 2 or 3 in
> this instance). Actually I believe that your original suspicions were
> correct and the load will fall between the square and the cube of the
> speed. The work done will be a cube function, but the losses (drag, etc)
> will be a square of the speed. I have no idea of the relative proportions.
>
> >Yep, using the cube changes my statement below to "will draw much more,"
> >rather than "may draw more." I don't think there is any question but that an
> >electrical fan is the way to go for competition, but I wonder how many of the
> >folks on this list have EVER had their engine up to 5000RPM? Show of hands,
> >please! I wonder how many even routinely run the engines over 3500 RPM (app
> >75 mph in 4th)? 0.09 HP X (3500/1000)^^3 or 0.09 X 43 = 3.87 HP
>
> Well I will confess to this one. I do not use it for cruising but I do for
> acceleration. Up to about 4500 typically. My perception (no measurements)
> of electric fans is that they give improved acceleration at speed - pulling
> out to overtake. At low speeds it is hard to pick the difference, but
> accelerating from 50 to 60 mph I notice it.
>
> >I agree, but there is another advantage to using a mechanical fan - if the
> >engine is running, the fan is running. With an electrical fan, there are
> >several failure modes that can leave you with a functioning engine, but not
> >enough cooling to get you out of the traffic jam and back home. If you can
> >make to the open road, you'll be OK, but if you don't get moving soon, you'll
> >be in big trouble.
>
> Fit a manual bypass switch or know how to rearrange the wiring to have the
> fan on permanently in an emergency if the thermo sensor or relay fail.
> Then there are only the fan motor and the wiring to worry about.
>
> Conversely, an electric fan running at 1800 rpm may save you in heavy
> traffic on a hot day whereas a mechanical fan idling at 900 might not.
>
> An important apsect that has not been raised is how much power gets to the
> rear wheels in the first place. A carb TR6 develops about 105 hp at the
> flywheel. If 95 get to the wheels then another 5 from an electric fan
> represents a 5 per cent improvement. If only 50 get to the wheels then an
> electric fan represents a 10 per cent improvement. Any suggestions?
>
> Trevor Jordan
> 74 TR6 CF29281U
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