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Old 07-10-2014, 09:31 AM   #1
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Ims rpm

Does anybody know the ratio of IMS rpm to crank RPM?

Might be a long shot, but without having a ruler to my crank cog at this moment, does anybody also know the ratio of RPM for the idler pulleys/alternator/PS pump?

Thanks

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Old 07-10-2014, 12:11 PM   #2
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The intermediate shaft is driven by the crankshaft via a duplex roller chain with a transmission ratio of 0.67:1 (24 teeth on the crankshaft and 36 teeth on the intermediate shaft).


Porsche Workshop Manuals > Boxster (986) F6-2.5L (1998) > Engine, Cooling and Exhaust > Engine > Cylinder Block Assembly > Auxiliary Shaft > Component Information > Description and Operation
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Old 07-11-2014, 08:14 AM   #3
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Thanks, that answered almost all my questions.

Do you happen to know the ratio of alternator to crank? The workshop manual and the bently book dont cover it.
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Old 07-11-2014, 10:13 AM   #4
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I have found these references saying that the stock crank pulley circumference is 454mm and the alternator pulley circumference is ~166mm (taking the 53.9mm diameter and converting to circumference). This would give a ratio of ~2.7:1.

This was the best that I could find.

Mantis versus BBI pulley size

Amazon.com: NEW ALTERNATOR PULLEY PORSCHE 911 SERIES BOXSTER 0-124-515-073 996-603-012-02: Automotive
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Old 07-11-2014, 11:15 AM   #5
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Awesome, thanks!

Now for the kicker...do you know how many balls are inside the main bearing inside the alt?
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Old 07-11-2014, 11:26 AM   #6
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Awesome, thanks!

Now for the kicker...do you know how many balls are inside the main bearing inside the alt?
Hah! You got me. I have no idea.
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Old 07-11-2014, 01:05 PM   #7
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Quote:
Originally Posted by thstone View Post
The intermediate shaft is driven by the crankshaft via a duplex roller chain with a transmission ratio of 0.67:1 (24 teeth on the crankshaft and 36 teeth on the intermediate shaft).


Porsche Workshop Manuals > Boxster (986) F6-2.5L (1998) > Engine, Cooling and Exhaust > Engine > Cylinder Block Assembly > Auxiliary Shaft > Component Information > Description and Operation
Can't argue with your math, but it doesn't make sense. The IMS is run off the crank and drives the cams via a chain drive. Since the cams operate at 1/2 or 2x (can't remember which) the crank rotations to ensure proper timing, .67/1 ratio would mean the cams would be wildly out of sync with the crank. What am I missing?
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Old 07-11-2014, 01:13 PM   #8
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Can't argue with your math, but it doesn't make sense. The IMS is run off the crank and drives the cams via a chain drive. Since the cams operate at 1/2 or 2x (can't remember which) the crank rotations to ensure proper timing, .67/1 ratio would mean the cams would be wildly out of sync with the crank. What am I missing?
Good point. I'll take another look...
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Old 07-11-2014, 04:42 PM   #9
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The cams are reduced again to get cam->crank ratio of 0.5:1. Its shown in the cam section

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