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

[Brian M Kennedy <kennedy@xxxxxx>]

Wow, helluva post, Bruce.  Thanks.

Bruce Augenstein wrote:

> 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.

Does anyone know if CarTest compensates for these effects properly?
CarTest does compute lower shift points than those on the web page.


> The shortfall of about 17.5% out on the road (.644 observed over .781
> theoretical) is due almost entirely to rotational inertia - "flywheel
> effect", which only comes about when you are accelerating (or, in fact,
> decelerating). In first gear, that M3 gains about 189 engine rpm for each
> mile per hour gained, while in second, it's down to 112 rpm. It takes energy
> to accelerate these rotating parts, and this energy is then unavailable to
> accelerate the car.

Nice explanation.  

>  The parts affected are basically every rotating thingy
> forward of the transmission tailshaft. This obviously includes the engine,
> flywheel and all engine driven accessories, as well as the clutch, pressure
> plate, transmission input shaft, various gear clusters, etc.

Any formulas for how to relate the mass of those "rotating thingy"s to the
power drain??  If I reduce that mass by 5%, can I assume a 5% lower drain?

Any estimates on the total rotational mass we are talking about here?
(You need to know that to know how much of a reduction you are making,
percentage-wise.)

After a lightweight flywheel, the next targets might be those accessories.
One option is to put in pulleys that gear down the things they are pulling,
thereby making their effective rotational drag on the engine much less.
I know one person who claims such "underdrive pulleys" were the biggest
performance gain he had done -- on the order of 17hp.  Comments?


Thanks again for the well-written and informative post,

Brian

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