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.

• http://www.ross-tech.net/andy/vwfiles/butt-dyno/crunched-data-01.xls

Andy