OK, here's what I know, and would like to know. Our 901 has a 2nd gear that can't be changed w/out alot of $$$. If I want a shorter 3rd gear, the shade tree method would be to "flip" a 5th gear set.

If I want a taller 5th gear, would a "flipped' 3rd gear in the 5th gear location be taller than the factory 5th gear, and calm things down on the highway at 80 mph?

Yes, it might make for some odd gear spacing, I'll have to think hard about that as well, but as the car is mainly an auto x toy, the short 3rd would be nice, and a taller 5th for the trip to and from the events would be welcome, although the 3,4,5 spacing might not be so good for a track day.

I have a GA mainshaft & gear F/S in the classifieds. Can't help with 3rd.

The ZD 5th gear in the 914-4 trannies is the highest 5th that Porsche made for 901's. That's the one that people flip around for a KA 3rd which is almost like an M 3rd. The 914-6 5th is a ZA which is even lower. If you want a higher 5th you'll need bigger OD tires or a different ring and pinion and either will make all of your existing gears seem higher. There might be aftermarket 901 gears available at some point, I head a rumor that a tranny parts manufacturer will be producing brand new gears for us in '09 but I don't think that they'll be cheap, nothing is except Top Ramen.

The best way to get a taller 5th is to flip an H second and put it in the 5th spot. A stock ZD is a .710 ratio, a flipped 3rd (N) is a .793 ratio. You want a smaller number, thus a flipped N will give you a shorter gear. A flipped H is .594 and a flipped HB is .613. The H series are the easiest, most straight forward and cheapest (about $500 or less) way to get taller gearing in 5th.

Thanks for all the replies:

JP: I'm trying to give all my available $$$ away to Jake for engine parts to build a better turbo beast- I'm a little short for your parts...

SWW: taller tires or different r/p will help the highway and hurt the auto cross, so good idea but not the right solution right now.

Dr Evil: just what I was looking for. Sadly there is no cheep-skate flip this and that mix-n-match way to make 5th taller. $500 will go quite aways for motor parts, so it looks like I'll tear into my mostly dead trans and swap the 5th in to the partly dead trans that is in the car and make due with that set up for awhile.

Back to wrenching.

I think you cannot flip an H gear onto the 5th gear spot.
The H gear has a smaller hub than the 5th gear.

Rich

Well OK, be that way.
After youz' got a motor you may someday realise that's AX is all about speed in gears.....shifting is bad .....particularly down to first/up to second.
I think I have the solution but if it works (gonna try it for a season) I'll keep it to myself.

There is H, HA and HB. IIRC, you can not use HA as it is too small of an internal fit, but the H, and I know for sure the HB are perfectly fine.

Here's the '901 gear flip chart'

Click to view attachment


This chart, along with the gear ratio worksheet is available here:

http://www.914world.com/bbs2/index.php?showtopic=79893

Coincidentally, the Re-gearing tech article includes an example auto-cross setup that shows flipping 3rd & 5th making a A-F-Ka-V-Z

I just bought R_Towle's 904 mainshaft & Ha 2nd gear from the classifieds here and plan to do some re-gearing myself!