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Rob 930 Rob 930 is offline
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Join Date: Dec 2002
Location: Seattle
Posts: 661
If you're interested in flow numbers for Carrera manifolds, I posted a thread about that here, in case you didn't see it:

http://www.turbo911.org/forum/forum/forum_posts.asp?TID=1126&KW=extrude

I took stock Carrera manifolds and sent them to Extrude Hone, and after a couple swipes they came back with 285 cfm +/-1 cfm runner to runner. It appears that your numbers are higher than mine, which may be because you flowed them at something higher than 25 in H20. I haven't looked back in your thread to investigate. Given how much was done to mine, I'd be surprised if there are any stock-configured Carrera manifolds that flow much better than mine do. The Extrude Honing process is pretty impressive. Of course, the only fair way to compare is to flow them on the same bench with the same operator.

About your change to an 8:39 R&P: If you're satisfied with using just 2nd though 4th, it's a good solution, I think. For a track car, it makes 4th a more usable gear. Problem is, the spacing between the gears, which is fairly distant, isn't improved, and it makes 1st almost worthless (especiallyin your car, where it will just be a tire smoking extravaganza). Also, the strength of the 8:39 is lower than the 9:38, though I'm not sure how significant that is. I chose to do something different here; I stayed with the bulletproof 9:38 R&P and regeared my stock 930 gearbox to the following ratios:

1st 19:33 (taller)
2nd 23:30 (stock)
3rd 26:26 (shorter)
4th 30:23 (shorter)

This makes for a close ratio gearbox, or an "active" gearbox, where all four gears are road racing gears (which was my goal). If I recall, it will rev out in 1st gear at 72 mph. That makes for some clutch slipping to get it going, but it's not objectionable to me (of course, if drag racing is your thing, it won't work). It revs out in 2nd at around 95, third at around 125, and fourth at 161 or so. This choice of gears isn't for everyone, but it should be right on for my purposes. It's essentially the same gearing as a 915 gearbox, but without 1st gear!

The cost of doing this is not trivial, but neither is an 8:39. I'd say the cost is equivalent. If you've got your box apart, and are doing your own work, that certainly helps. I got some used factory 962 gears (1st and 4th) for $650 each or so, and a new aftermarket 3rd gear for $750 or so, if I recall.
Old 05-16-2005, 03:58 PM
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