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914World.com _ 914World Garage _ Tranny ratios, diff, tire sizes and RPM's

Posted by: boxsterfan Sep 16 2003, 10:23 AM

First of all, I will state that I am not a mathematical wizard. From my previous days restoring Datsun Z's, I would use this calculator to get my engine RPM and MPH on a Z-car.

http://www.geocities.com/z_design_studio/transmission.html

Is there such an animal for the 914? Should be able to plug in the tranny ratios and diff ratio and tire sizes on this one to get approximate results.

Posted by: porsche735 Sep 16 2003, 10:26 AM

I have one. Let me see if I can get it uploaded...

Chris

Attached File(s)
901transmissionratios.xls ( 21.5k ) Number of downloads: 83

Posted by: porsche735 Sep 16 2003, 10:29 AM

Here is another one I used for scratch....

With this one I checked the % difference between the gears to choose the optimal setup. You can see we did the swap between 3 and 5 and then put an S gear in place of the old 4th. You could leave 4th alone, as you can see by the math, but it works out very nice with the S there.

Chris

Attached File(s)
Gears_on_Porsche.xls ( 19k ) Number of downloads: 121

Posted by: john rogers Sep 16 2003, 10:31 AM

You could go over to the Pelican site and download the GEARS.exe program I wrote a few years ago, it is in the 914 tech section. It lets you input gear ratios, tire diameter or turns per mile or circumference (most accurate) and final drive ratio and the slider will show RPM in each gear vs speed. Good luck.

Posted by: porsche735 Sep 16 2003, 10:44 AM

John,
Looks like the same one I just posted. Mine has the gear ratios we tried highlighted.

Boxsterfan,
Take a look at the scratch sheet. What you want to do is make your percentage changes very even through the gears. We chose to make our 4th to 5th a lower percentage and chose a lower top end gear. This will give you close to redline (5700rpm) at the end of the longest straights on the West Coast (Thunderhill and Laguna Seca). The ratios are perfect for just about every track out here....

Chris

BTW. the 22.8" diameter is a 205/50/15 bothe Hoosier and Kumho

Posted by: Brad Roberts Sep 16 2003, 11:12 AM

We have multiple versions HERE on this site.

http://www.914world.com/downloads/Gears/

B