drive train losses
audi at humanspeakers.com
Thu Aug 26 21:40:28 EDT 2004
> My point is this - the dyno measures what comes out at the drive wheels
> - and then adding an arbitrary 20 or 25 or 30% "for losses" is simply
> incorrect, and does not do justice to the effort, expense and skill of
> the person who modified the engine.
What would be more useful, besides the good old seat of the pants test,
would simply be fairly standard, duplicable tests, especially if run at
various stages of the project - especially before starting (since that
gives a baseline of what was getting to the wheels "stock," rather then
the factory figure).
It strikes me that there are three, of varying usefulness:
1. Dynamometer. advantage: consistent measurement, factors out air
resistance and vehicle weight by measuring car standing still.
disadvantage: expensive to run, hard to find awd setups, and heat soak
factors will be different when sitting still (or do they run fans on the
2. G-tech type measurement. advantage: all the tuner-types probably
have ready access to one to play with, so lots of measurements can be
taken. Hopefully, if you get the vehicle weight and air resistance
figures right (and gear ratios, and tire size... right?), the numbers
will be consistent and repeatable. disadvantage: with poor weight and
Cd numbers, the calculations won't give accurate results.
3. Drag strip time slips. advantage: it's the raw unvarnished truth -
if you get a 13.0, there is no debate about it, the car did it.
disadvantage: driver technique and clutch/tire slippage issues make it
harder to compare apples to apples. Although, they do give you your
reaction time, don't they?
Anyway, I guess my point is that playing the numbers backwards using
"rule of thumb" percentages is just cheesy and who cares? If you dyno
300 hp, or turn in that 13.0, you know you have something good. If you
improve the numbers, they're better, whatever might be happening at the
By the way, I suspect that the actual "drive line loss" formula, which
of course will vary from car to car, is a complex second or third degree
(or worse) formulation - ie, a constant, plus a factor linear to power,
and perhaps ones that are proportional to power squared (or cubed, or a
power below one, like a square root...). And it probably varies by rpm,
too, so whether you get max hp at 5500 or 6500 will change things.
More information about the quattro