Miata Mailing List: October 1999, Message #54

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Brian makes some good points.


>>I also pose this: who think that this very green S2000 will dyno over 200
WHP after 5-10K miles?

Car mags (C/D in particular) do performance tests on their long term
vehicles sometimes.  Often enough you'll see a car that's quicker with
30,000 miles than it was new.


>>So, a 200 WHP car rated at say 240 loses about 18%. If you
>>boost that car to say 400 WHP, does that scale exactly to 480 crank
>>HP? 

Pretty close, yeah.


>>So instead of losing 40 HP in the drive line, you now lose
>>80? Why would you if the drive line losses objects have not changed:

The OBJECTS haven't changed, but the conditions they';re experiencing have
changed.  A drivetrain isn't perfectly efficient - you input work at one
end, and you get some heat output in the middle and some work output at the
other end.  The output and the heat add up to the input.  If you put twice
the torque through, or the same amount of torque through at twice the speed,
you make (not exactly, but close enough for our purposes) twice the heat.
{The "not exactly" part involves how efficiently the drivetrain passes heat
on to the air.  It's miniscule, really.  It changes depending on how fast
the u-joints turn.  But this "not exactly" aint the difference between
loosing 18% at the rear wheels or 19%, it's more like the difference between
18.7865% and 18.7871%.)

You could get a pretty good understanding of this from a good Thermodynamics
text, if you can find a clear one (the calculus is useful but not necessary)
but this ain't the Science list, it's the Miata list, so try it this way
instead:  Drive slowly and gently around a parking lot for ten minutes using
very little power, and then park and put your hand on the differential
casing - it'll be warm.  Do ten minutes of hard driving or highway driving
using a LOT of power, park, and - well, I can't tell anyone to touch the
casing, 'cause it'll be hotter than heck and they'll burn their fingers and
sue me - but IF they were to try this, they'd find that it's really hot.
The more power you put through the drivetrain, the more power willl be lost
as heat - that's why it burns your hand after it's been used to transmit a
lot of power.  And it's not speed dependent - if you drive for ten minutes
around a parking lot slowly but with the gas pedal to the floor, using your
brakes to keep your speed down, the diff will heat up a lot that way as
well.  This is why tow vehicles have tranny oil coolers.  It doesn't depend
JUST on the torque you're putting through or JUST on the speed - it's the
product of torque AND speed that's important.  (And coincidentally, "power"
is work done per unit time, which is the product of force and speed.)

{ding} Class dismissed.

>>Their base line is a good start, but just not yet. They 
>>need some more baselines. One at 3K, then 5K, then 10K.

I totally agree.  Ideally, their results would be repeatable if you switch
back to stock and then back to their modifications.  Otherwise, you're
right, you could be crediting your mods for an unrelated improvement.  ( The
day after I upgraded to radial tires on my motorcycle, Desert Storm ended.
Therefore, my tires gave better handling, longer life, and peace in the
Middle East.)

- Bill