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"Butt Dyno" Instructions

Old 04-11-2003, 06:20 AM
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Default "Butt Dyno" Instructions

Here's a "Butt Dyno":
<img src="http://www.ross-tech.com/vag-com/cars/Examples/dynoplot4.gif">

Here's a Real Dyno:
<img src="http://www.ross-tech.com/vag-com/cars/Examples/GIACruns.gif">

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


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


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.
Andy@Ross-Tech is offline  
Old 04-14-2003, 01:54 AM
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Brilliant! I'll have to study this at more length later. Bookmarked!
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Old 12-18-2003, 01:18 PM
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Default Re: "Butt Dyno" Instructions


for the few of us who are having issues generating the TIME STAMP DECEL value, could you please post instructions, and/or formula on HowTo find this set of numbers?!

any help/pointer would be appreciated.
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Old 03-31-2004, 02:01 PM
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Default Re: "Butt Dyno" Instructions

Can you let me in on how to work out decel time stamp
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Old 05-04-2004, 06:59 AM
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Default Additional Instructions

The first thing I do with raw data is make a quick graph of speed-vs-time. This shows what the accel and decel curves look like. I used a Scatter (X-Y) chart type and used:

X-values: $B$8:$B$311
Y-values: $E$8:$E$311

Then, I highlighted the accel data in red, and the decel data in blue.

I made another graph of time-vs-speed for just the decel portion of the data. I used a Scatter (X-Y) chart type and used:

X-values: $E$51:$E$311
Y-values: $B$51:$B$311

Then, I selected that data and chose "Add Trendline". You may need to play with different trendline types to get something that fits your curve well. I found that a 3rd order polynomial fit great.

I selected the trendline and chose "Format Trendline". Under "Options" I checked the choice for "Display Equation on Chart".

Now that the equation was on the chart, I selected it and used the "Increase Decimal" command in the Excel toolbar to give the equation more decimal places. Just crank it way up there, maybe 12-15 places showing should be fine.

y = -0.00001966013549x3 + 0.01010649042580x2 - 2.11664390630149x + 171.35689830770700

Now I copied that formula and pasted it into cell G8 with the heading "Decel calculated". Now, it doesn't yet work as a formula until we replace the "x" with the cell that contains the X-value, E8 in this case. Remember to replace 9x3 with 9*E8^3 for example. So, in cell G8, I have the formula:

-0.00001966013549*E8^3 + 0.0101064904258*E8^2 - 2.11664390630149*E8 + 171.356898307707

Now that we have decel stamps that correspond to each accel speed, they can be pasted into the Butt Dyno.<ul><li><a href="http://www.ross-tech.net/andy/vwfiles/butt-dyno/crunched-data-01.xls">http://www.ross-tech.net/andy/vwfiles/butt-dyno/crunched-data-01.xls</a</li></ul>
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Old 05-07-2004, 10:05 PM
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My VAG sayd Key-com is Hex com better?
NJ4.2GUY is offline  
Old 05-10-2004, 07:38 AM
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Andy@Ross-Tech is offline  
Old 12-12-2004, 03:49 PM
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Default Windows program

There even is a Windows software that generates a chart from your VAG-COM logs.

You can download it here:


It's called WhOOPSiE's Popometer. It's helpful if you understand some German though.
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Old 02-17-2005, 01:27 AM
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Default How about an english version?


we developed an improved version of Whoopsie's 'Popometer' software: KPower

New are:
- better support for VAG-COM log imports
- improved automatic acceleration/deceleration recognition
- availability to save the resulting curve as bitmap
- some bugfixes

If enough interest exists, I would make an english version. Should I?

Best, Rainer<ul><li><a href="http://shop.dieselschrauber.de/download/KPowerSetup.exe">Download KPower</a></li></ul>
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Old 10-25-2005, 07:00 AM
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Default Re: "Butt Dyno" Instructions

Where was the real dyno done. It says Cape Cod Dyno and High Performance. Was it done there? I just hapen to live there and I need to do a dyno on my TDI. And, besides, excellent page, great help, thanks. Dimo
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