drive train losses

[Dave Glubrecht]

One very interesting thing to know about the dyno results are that they are
affected by things like wheel/tire combinations.  Because of the way they
measure HP (how fast you can accelerate the rotation of a drum is one type)
everything that rotates will affect the numbers.
You can loose 5 HP with that big brake upgrade because the larger rotor has
more angular momentum.  Since the point of HP is to accelerate the car and
the wheels/tires/brakes that are on it, wheel HP on the dyno should not need
ANY correction number unless you are bench racing and then you don't want
real numbers as made up numbers sound better.

   Dave G






From: "Huw Powell" <audi at humanspeakers.com>
To: "Mike Arman" <armanmik at earthlink.net>
Cc: <quattro at audifans.com>
Sent: Thursday, August 26, 2004 6:40 PM
Subject: Re: drive train losses


>
> > 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
> car?).
>
> 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
> crank.
>
> 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.
>
> -- 
> Huw Powell
>
> http://www.humanspeakers.com/audi
>
> http://www.humanthoughts.org/
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