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Jeff,
Congrats!! I am sure you'll have lots of fun. It will be somewhat different driving on the track between the 2, depending on how your turbo comes on.

A few comments from my own experience -
a) Friend of mine used 915 with his turbo motor when the motor was making (my guess) ~450hp and it lasted about a season. 3rd gear literally shattered on the track.

b) Changing 2nd/3rd/4th on the 930 box will make a HUGE difference for the track. I shortened mine and now have no complaints for, say, Mosport & the Glen.

c) I actually find 4 speed is not that bad... less gear = less shifts the 930 is a SLOW box and I lose ~1.2 sec in every shift, so less shift = faster time (G50 shift I think can be around .7 sec)

Good luck & have fun!
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87 930 K27HFS/B&B/Twin-Plug... Megasquirted
Old 05-22-2010, 06:47 AM
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J, If you do a 930 box look for a 76/7 a short box to keep your motor a the same location. I would run it first so you can get a deal for what you need for your track. Also, I would want to have your dyno plots.
Old 05-22-2010, 08:36 AM
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Thanks for the insight guys!

I am somewhat resigned to replacing the 915.....

911st,

I will dyno the motor and tune it to my liking for sure. The gearing is always a compromise depending on the tracks one runs and finding the gearing that is a good mix on the most common tracks can be a real mystery....

Cheers
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Old 05-22-2010, 11:55 PM
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Quote:
Originally Posted by Jeff Alton View Post
The gearing is always a compromise depending on the tracks one runs and finding the gearing that is a good mix on the most common tracks can be a real mystery....
Jeff,
If you know the entry & exit speed of each corners, you can then calculate which gear and what ratio you want/need. Its a bit tedious but if you do it across the tracks you typically go to, you can then optimize (or find the best compromise) at least mathematically.

One more point from my experience - your car may (will ? develop more power in the future so don't go too short on the top gear
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87 930 K27HFS/B&B/Twin-Plug... Megasquirted
Old 05-23-2010, 05:16 AM
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I wonder if for a track car it just moving first up so the rpm drop is not almost 3000rpm would work as a budget box. The other gears have about a 1900rpm drop which is not a close ratio but a good build might have a fat enough power curve for it to work well.
Old 05-23-2010, 08:09 AM
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Quote:
Originally Posted by hobieboy View Post
Jeff,
If you know the entry & exit speed of each corners, you can then calculate which gear and what ratio you want/need. Its a bit tedious but if you do it across the tracks you typically go to, you can then optimize (or find the best compromise) at least mathematically.

One more point from my experience - your car may (will ? develop more power in the future so don't go too short on the top gear

Good points, I can pull that data now out of my files...

Cheers
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Old 05-23-2010, 08:58 AM
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You know the drill. Most important is the corner that enters the longest straight(s).
Old 05-23-2010, 01:42 PM
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Yes, absolutely! The speeds are similar, though not the same for the two tracks we run the most. Everything is a compromise. But first I need to get the motor into the car and go do some testing to see how it goes.


Cheers
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997 S 4.0, Cayman S 3.8, Boxster 2.7, Boxster S race car, 996, 955 Cayenne TT, 958 Cayenne TT
Old 05-23-2010, 09:27 PM
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Again the weakness of the stock 4 speed is after the 1 2 shift at about 60 where the rpm's drop to the mid 4k's and power drops off about 40 percent on many builds. This is typically about the same speed we get knocked down to just as we enter the big straight.
Old 05-24-2010, 03:35 PM
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