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Porsche 911 pulley ratio calculation
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ottobevis
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PostPosted: Fri Mar 12, 2021 12:16 am    Post subject: Porsche 911 pulley ratio calculation Reply with quote

Hello,
I'm trying to understand how the commonly quoted ratios for 911 pulleys are calculated. I searched but could not find an explanation.

For example, 1965-74/75 use a 110mm crank pulley and 95mm alternator pulley. The ratio usually quoted for this is 1.3:1. When I calculate it (1/95*110) I get 1.16:1.

1980-89 uses a 120mm crank pulley and a 83mm alternator pulley. The ratio quoted is 1.67:1. The ratio I get is 1.45:1.

I'm assuming the measurements above are outside diameters. Is the difference due to using inside diameter measurements or using an average for the alternator diameter because of the spacers?

Otto
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Starbucket
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PostPosted: Sat Mar 13, 2021 4:11 pm    Post subject: Re: Porsche 911 pulley ratio calculation Reply with quote

You use the middle diameters of the pulleys, calculate outer diameters then calculate inner diameters add the two together then divide by 2 and you will get their numbers.
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Glenn Premium Member
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PostPosted: Sat Mar 13, 2021 4:16 pm    Post subject: Re: Porsche 911 pulley ratio calculation Reply with quote

Please use your original topic: https://www.thesamba.com/vw/forum/viewtopic.php?t=751300&highlight=

If you want it moved, PM me and i'll move it.
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