Re: M3 shift points, Mr. Collie's flywheel, etc.

[Luis Marques <marques@xxxxxx>]

Bruce Augenstein <Bruce.Augenstein@domain.elided> wrote:
>
> Awhile back, a gentleman named Luis Marques went to considerable time
> and trouble to post figures and graphics that demonstrate optimal
> shift points on a 3.2 liter M3, stock, and after being sharked. His
> page is still up and available at:
> http://www.geocities.com/MotorCity/Speedway/6172/m3shift.html
> 
> Not wanting to peak too early on this :-), I haven't gotten around to
> responding until now, but the thing is, those shift points are
> incorrect. I intend no flame here, and the data is accurate as far as
> it goes, but it completely ignores rotational inertia in the engine
> and drivetrain.

That was me :)  As you noticed, it was just a quick stab at it and
completely neglected any rotational or acceleration factors.  It was
intended to correct some mis-information about shifting to the peak of
the torque curve of the next gear regardless of the shape of the power
curve, etc.  I guess that since it ignored the dynamics involved or the
actual gear shifts, it could be called the "optimal static shift
points".  They are still the optimal points if you are accelerating
slowly (as in higher gears), but I can see how neglecting the moment of
inertia and all that may not yield the right answer on the lower gears. 
The only really accurate way of determining the shift points is testing
in your own car, since there are so many variables.  Maybe another
option is to dyno a car on rollers in each gear and use those curves,
instead of just taking the high gear curve and compress it to account
for the lower gearing on the lower gears.

I'll have to give this some more thought, but keep in mind that all the
energy that is fed into the flywheel while you rev the engine in the
lower gear (ok, _most_ of the energy - some is lost in friction as the
clutch disengages and reengages) is not wasted, but is fed back to the
drivetrain when you complete the shift and the engine slows down.

Luis Marques
'97 328i

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