The Desktop Dyno

Dyno Spreadsheet   |   National Dragster Article   |   The Paper Dyno   |  

I wanted to follow up on The Paper Dyno with an article called Desktop Dyno that used spreadsheets to do the same thing. Unfortunately "Desktop Dyno" was already being used and I got busy raising a family and never did it – that's my story and I'm sticking to it. In today's world of iPad apps and such the whole thing has become irrelevant. But, you know me. I'm all about yesterday's technology.

So I recently developed this spreadsheet to replace the chart. It also includes a utility for graphing horsepower based on torque and a quick sheet you can just take in your car and do a down and dirty estimation of your power by timing.

One of the handiest worksheets in there is the thrust curve. On this you plot the torque curve, then based on your gear ratios the sheet gives you a thrust curve. This basically tells you when to shift—whenever the next gear up gives you more thrust at the same speed.

I apologize--these are in that horrible Office 2007/2010 .xlsx format. If you have 2003 you can run it in compatibility mode.

Comp-CFM: I just threw this in. It's a utility to calculate your engine's displacement, compression ratio, and airflow in CFM

Tire-Size: This calculates the width and diameter of your tire based on the size callout. It also compares tire sizes to the original reference, so, for example, you can get an equivalent height with a different aspect ratio. It also has the start of a table of ratios for transmissions. You can probably just find these for your particular car easier over the internet. The diameter of your tire will be input int he later sheets to get a 'thrust index' which you can mess with to get different effective gear ratios by changing tire size.

Thrust_curve: Draws curves for thrust in each gear based on the torque curve and gear ratios. Input weight to find the acceleration in g's. Your shift point is where the curves cross; that's when the thrust for a given speed becomes higher in the next gear. If the curves don't cross obviously you shift at redline.

Dyno_by_RPM: This is the chart for calculating your torque curve using acceleration runs based on running from one RPM to another. For coastdown you'll have to use a GPS or speedometer, but I prefer this method because you typically graph based on RPM anyway.

Graph-RPM: Just the graph for the curve you develop from the test runs with the above sheet

Dyno_by_Speed: This is the chart for calculating your torque curve using acceleration runs based on running from one speed to another. It converts RPM so the intervals aren't as clean, but it might be easier for nice even breakpoints in speed.

Graph-Speed: Just the graph for the curve you develop from the test runs with the above sheet

QuickTable: A down and dirty chart you can take in the car and use to do the test runs without doing the full set of runs. Just start slightly below the initial speed, romp on it (don't allow the gear to change if you can help it) start the stopwatch at the initial speed and stop it at the final speed. Find the time on the chart in the column under the final speed, track it along the row to find the horsepower.

Graph-utility: Draws a torque and horsepower curve based on values you input for torque at various engine speeds

I hope to get better instructions on all of the worksheets in there. That would make it more obvious what I was trying to do with this, but if you play with it a minute I think you can figure it out. Just messing with the numbers even without doing the runs should give you a better feel for what power is.

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