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Old 08-22-2005, 07:02 AM   #1
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Join Date: Aug 2005
Location: Red Bank NJ
Posts: 9
987S gearing

I put together a spreadsheet to analyze the gearing of the 2005/6 987S, which is attached. There are two plots, RPM versus speed in each gear, and rear-axle torque versus speed in each gear. The latter neglects any frictional losses. The spreadsheet is attached (as a .zip file, since apparently .xls files aren't supported). Here are some observations:

From the plot of speed versus RPM the gear spacings looks pretty well chosen except for 6th gear. From a pure performance perspective 6th gear should be somewhat shorter, and I expect might extract another mile per hour or two of top speed. Having said that, for a street car where noise and wear at highway cruising speeds is also relevant, Porsche probably made the right choice by stretching 6th gear out a bit. From a performance perspective the gearing looks pretty optimal up to ~150 MPH where 5th gear hits redline, and that probably covers most performance needs except for full-bore racing.

From the plot of rear-axle torque versus speed, the optimum shift point for 1-2 and 2-3 is quite obviously the redline (there is lots more torque available at the redline in the lower gear, due to the multiplication by the gear ratio). 3-4 is about a wash, with the torque at the redline in 3rd amost exactly equal to the torque in 4th at the same speed, so shifting at the redline is also about optimal. For 4-5 and 5-6 shifts there is less torque at the redline in the lower gear than in the higher gear (due to the drop off of torque near the redline combined with the close gear ratio spacing), so the optimum shift point in both cases is perhaps 6800 RPM. Keep this in mind the next time you are shifting from 5th to 6th at 140+ MPH
Frank

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