James Bailey

Yes but MORE gears to shift into expect Kibort in two seconds !

dprantl

I'm surprised Mark would not approve of this swap... if you use the 968 box (and it doesn't blow up), you will have 5 useable gears instead of 4, so you will have more Kiborts!

Dan
'91 928GT S/C 475hp/460lb.ft

mark kibort

Well, glad you brough that up. do you need 5 useable gears, not 4? if the gear spacing optimizes HP , there is NO sense in gainng another gear at the expense of another shift. So, actually, the most "Kiborts" (aka Hp-seconds) is at a RPM range that maximizes HP. if you are already maximized, you cant do any better than that. But most importantly, the gears, if i remember correctly, were not that much closer. currently, a S4 has 50, 80, 120, 150 and 225mph shift points. The Vev C5 has a 6 speed, but the gear spacing for usable gears is almost identical to the 928 S4. meaning, there are two very tall gears vs our just 1 5th gear. net net, 6 speed doesn buy you anything down low.

the only swap I would approve is a cup car gear box 6 speed swap. with the sequential would be nice too! Wow, that would be the ultimate!

1, 0,..... read above.

dprantl

I remember from when I owned my 968 that the gears 3-4-5-6 were very close, which would be optimized for a very peaky HP curve. I'm not exactly sure if the extra HP would pay off with the higher number of shifts.

Dan
'91 928GT S/C 475hp/460lb.ft

mark kibort

just plot out the RPM range each gear was responsible to cover. plot it against the HP curve and see. you got to figure .2 seconds loss per shift, for not accelerating, but actually braking. (unless you have a sequential gear box) . just think about that for a second. (no pun in tended) 2 shifts is near .5 seconds. just think how much HP you need to gain to offset .5 seconds off a 1/4 mile time for example. sequential gear boxes with no lift shifting really add a lot of performance!

dprantl

You are telling me? The 7-speed PDK makes the new Cayman S closer to my car than I would like. I wonder if we could fit that into a 928...

Dan
'91 928GT S/C 475hp/460lb.ft

86'928S MeteorGrey

For the cost of a conversion to fit a four banger transaxle into the 928, (If I were to want to do such a thing) I'd look at other transaxles out there... I know the Corvette transaxle won't work without major modification (it's too freakin big and it's upside down), but for around 8K you could explore a Ford GT rebuilt.

Or from FFR they got the Slick 6 that looks small, but will handle big HP/TQ. It runs about 8500 BRAND NEW.

Lets face it. As 928 strokers continue to up the ante in HP, the stock transaxle is not going to cut it any more...

What happens to your racing class with a swap though?

James Bailey

POC /PCA requires that the tranmission case be Porsche......

mark kibort

1st 12:1 unusable
2nd 7.6 kind of a tall, same as 928s first gear, actually a good tall 1st gear! (58mph)
3rd 5.3 very close to 928's 2nd, but taller (85mph)
4th 4.2 exactly to 928's 3rd. (118mph)
5th 3.4 shorter than 928's 4th 146mph
6th 3.0 Taller than 928's 4th 166mph

Doesnt lok like any gains in my book. same gears you would use on the race track, 2nd, 3rd, 4th with a 928 and 3rd, 4th and 5th for the 968 gear box on the track.
these ratios are very close to the M3s I race with . they use 3rd through 5th and I use 2nd through 4th. same same same!

James Bailey

Yes Mark with your flat torque curve stroker having closer ratio gears less RPM drop might not mean much......You run the gears you do because it is not POSSIBLE to change them not because they are BEST !

mark kibort

You just said two different things here. I run the gear box I have because I cant change it (at least from the closer ratio persepctive) but, the gears I have are optimal for the tracks I visit and wouldnt change a thing. So, they are "Best" actually for racing. sequential gears? now that would be best!

and by the way, the 968 gear box is not "a close ratio" one as shown by the ratios. close at the top 5-6th, but that is not a shift needed. actually, 1st to 2nd is unusually long!

Plus, there would be no need to, due to the flat HP curve. So your almost right. it wouldnt "mean much". It also means Im at the max potential to accelerate at any speed im at. it doesnt get better than that. Great example of that is the Audi R10. it only needed 5 gears to put down the 650hp that the 650 other high rever's did with 6. Flat HP curve!

James Bailey

."3rd 5.3 very close to 928's 2nd, but taller (85mph)
4th 4.2 very close to 928's 3rd. (115mph)
5th 3.4 shorter than 928's 4th 146mph " if second is taller and 4th is shorter the three gears you would use look to me to be closer ratios and less RPM drop.....

mark kibort

I didnt go through the numbers, but its all so close, considering that I dont use 4th but a couple of short times at the tracks we visit, and go deep in to 4th as is where it counts. on that same straight, 3.4 vs 3.2 is not that much of a diff, worthy of a change for that amount of time. taller 2nd would be an issue for getting off the turns. 3rd is the wild card. let me check..................

actally its identical. 4.2:1

so, there. 2nd is better at 5.9, 3rd is the same, and 4th is slightly shorter.
over all the gears are closer, but the trade off from 2nd for 4th advantages, cancel out, (hp-seconds) and you end up with not much being changed. SO, why bother. taller 2nd, same 3rd, shorter 4th as far as usable gears.
AND, with the broad HP curve of the 928, it becomes no- change event.

There you go! The HP curve alows you to determine this in a snap.

Mk

mark kibort

Here is the million dollar question. Anyone know why that the 3.4 vs 3.2 wont make any additional accelerative forces at the rear wheels with a flat HP curve in that area?

GlenL

No.

If there are 6 gears instead of 5 won't it be 20% faster?

People are on to the right path: if you're going to go with a different transaxle, go big. The 968 won't fit without lots of mods.