Here's a "Butt Dyno":


Here's a Real Dyno:


Granted, my torque is lower since the "Butt Dyno" run was done in 2nd gear, while the real dyno was done in 3rd, so the turbo did not spool completely on the Butt Dyno. Check out the HP though. 174 vs 172. BTW, that's a 2000 GTI 1.8T with only GIAC chip.

Find a straight, flat road with no traffic. Log Group 005 as you do a full throttle run from idle to redline in THE HIGHEST GEAR THAT WILL STILL BE UNDER THE SPEED LIMIT AT REDLINE . At redline, push in the clutch and allow the car to coast as far as possible.

Crunch the data on your own, or send it to me.

* Requires VAG-COM

Group 005 gives you, among other things, time, speed, and RPM. Using those and vehicle mass, kinetic energy can be calculated at each time interval. The change in kinetic energy over time is power. This is the same type of calculation that is done with a DynoJet, except you are using your vehicle's mass instead of the mass of the rollers. It can be argued that this type of measurement is even more accurate than a DynoJet since you are getting real-world airflow unlike the fans used by dyno shops. So, in answer to your question, Group 120 should give you a picture of how the engine perceives torque generation. This can be skewed by some mods and chips. The "Butt Dyno" should give you a picture of how power is being used to accelerate the vehicle. This is a more useful, real world measurement of wheel power, but is considerably more complicated to use.

Official Disclaimer: Noone should break any laws ... EVER ... especially while in possesion of VAG-COM software.

Here's the nerd explanation for what happens:

Velocity = Speed and direction
since we are going straight, Velocity=speed
V is in km/h
Potential energy is constant, at least as far as gravity is concerned, since we are not on a hill

Kinetic energy is 1/2*Mass*Velocity^2

KE=0.5mV^2

Mass is in kg (0.454 kg per lbm)
Velocity is in m/s (0.278 m/s per km/h)

KE is in Joules

Power is change in KE over time

P = (d/dt)KE

or

P = (KE2-KE1)/(T2-T1)

In units, Joules/s is Watts. (0.001341 Horsepower per Watt)

So, at each moment in time, as dictated by the timestamp during the acceleration run, we have Kinetic Energy. The change between any two points is Power.

Here's where the magic happens. During our acceleration run, we were accelerating the car's mass and seeing the results. But, we were also using power to overcome air drag, rolling resistance, etc. The way we account for this is the coast down. During the coast down, we are encountering the same air drag, rolling resistance, etc but instead of overcoming it, we are letting it slow us down.

By measuring the Power being used by drag forces to slow us down, we know how much extra power we used during our acceleration run. Using the GROWTH function in Excel, we can plot a decelleration curve, and more importantly, we can plug in discrete X-values (the velocity values for each timestamp in the acceleration run) to come up with timestamps for the decelleration that match up. So, using the GROWTH function, use the km/h values for the deceleration run as KNOWN X-values. Use the timestamps from the deceleration run as KNOWN Y-values. Use the km/h values for the acceleration run as NEW X-values. This will give you timestamps for the decel run that correspond to each km/h value in the acceleration run.

The purpose of using the GROWTH function in Excel is to generate an exponential growth curve that expresses the speed of the car as it coasts. By logging speed and time while we coast, we get a nice set of points that makes a good curve, but the discrete speed values (let's say 42.28 mph, 42.12 mph, 41.93 mph, etc.) are different from the discrete speed values we got while accelerating (let's say 37.26 mph, 40.07 mph, 43.37 mph). So, the GROWTH function in Excel lets you take a set of known X, known Y, new Y, to get new X values. We are taking the curve that is formed by our decelleration data and plugging in the discrete speed values from our acceleration data.

I haven't tried this with a Tiptronic, but it seems that coasting in Neutral would be pretty similar to coasting in a manual with the clutch pushed in.

• Butt Dyno Version 6

Andy