change 4 speed to 5 speed

[rac4bux]

hello all,i have a 4 speed 901 and need a 5th gear from 914 or better for highway speeds.can my 4speed case and all its gears be converted to 5 speed with the addition of a 5th gear.i have bought 31;21 =.0677 overdrive gear set i would like to have installed.is this a big deal?do i need to start with a 5 speed?how much would this cost?and who around so.cal. can do this asap....any one out there have trans experience.i have hered of this done before but need the facts.thank you much.

[mike curnow]

I haven't looked into this question for 30 years, but I know there is more to it than a gear set. I think you need a 5 speed mainshaft & gear and other items.Buying a good 5 speed that dosen't have to be taken apart is probably cheaper & faster.

[68911USA]

Just out of curiosity, what benefit to highway speeds are you looking for with the 5 speed? It's not an overdrive, so the cruising speed in 4th on your current tranny and the 5th in a 901 box are identical on a 911, I thought. I get the enhanced gear splits you get with a 5 speed, so if that's what you're going for, I would go for a +1 on just buying the 5 speed. Is the 5th gear for a 5 speed equipped 914 not a 1:1 ratio?

[68911USA]

Sorry, I missed the overdrive part somehow. That would be neat to have an overdriven 5th- I'll restate my question to "are the 914 5th gears different than the ratios in a 911 5th gear, or is that overdrive gearset you've got a non-stock application or for road race gearsets or something?"

Sorry for the confusion. I should read more closely!

[304065]

A 914 has a ZD fifth a 901 has an early Z, late X, 911 has ZA.

With a 7:31 R&P and 25.2" tire, 80 MPH is

Z- 3748
X- 4051
ZA-3585
ZD- 3354

[68911USA]

That's interesting. I'm at 4000 rpm at 75 mph on the highway, and the speed matches my GPS. So I guess my tires are still not as tall as the original ones, even with 165/80R15's on 4.5 inch steelies. The closest ratio for me would be the X, I guess, for my '68 built in Oct of '67. Does that sound right? Sorry for hijacking.

[rac4bux]

i don't what i said wrong.but most of you think all 5 speeds have the same gear set they don't.i know the 901 4speed 4th are the same as the 5speed 5th 28;24=.085 which is an overdrive gear.with my 912 with 2.7 type 4 i am reving 4000 rpm at 74 mph.i would like change to install the gearset i have which 21;31=.o67 which would give me 74 mph at about 3100 rpm.what is so difficult about that.the 5 speed 901 in a 914,the 5th gear is 22;31=.071.the 4th and 5th gear 901 in a 911 or 912 is 28;24=.i called carquip and ebs and they said no problem that you can change the 4 speed to a 5 speed,and they have done many.you don't need a different main shaft and don't need to start with a 5 speed case.i just thought maybe some local new of a good trans man.i did not mean to wake up the porsche police.most people consider anything over 1:1 an overdrive,usa 911 your 5th gear is an overdrive.so to go out a buy a 5 speed would not be the cheapest way because i would just get what i have now.hello.in a 901 914 trans 4th gear and 5th gears are overdrives.

[911quest]

It can be done.....

The four speed main shaft is different it has first gear made on to it. Unlike a five speed mainshaft which has second gear made on it. It can be done I just converted a 914 5sp. to a 4speed. But you would be better served to sell the four speed it could have some valuable gears possible H & Q gears. and get a decent 5speed. and have fifth switch out to gear that you find more comfortable for cruising.

This picture shows a a four speed mainshaft and a five speed if tou use your fourspeed mainshaft you will have an A second not sure what your going to find for a first gear.

[JohnT]

Remember they're VW type IV motors. Consequently those cars require taller gearsets to reach Porsche road speeds.

My understanding of the 901 4 & 5 speeds (&923) is not as good as many, but I'll have a go at it.

The 901 5 speed has 1st gear in the nose of the tranny with reverse and second gear is fixed to the mainshaft. In this transmission one can put 5th gear in the 4th position or one can reverse the ratio an put it in 3rd (i.e a 27:25 becomes a 25:27). 4th will move to 5th or can be "reversed and put in 3rd. a 3rd gear can interchange with 5th & 4th in the same manner. Noting can interchange with 2nd because it is fixed to the mainshaft. Now don't take my "can" as anything but it is physically possible - I'm not suggesting or recommending anything.

So on a 4 speed, 1st gear is fixed to the mainshaft (in the 5 speed's 2nd gear position) and 2nd, 3rd and 4th occupy the positions of 3, 4 & 5 in a 5 speed. Once could play the same tricks on 2, 3, & 4 in a 4 speed that one can with 3, 4, & 5 in the 5 speed.

So to answer your question, yes you could replace your current 4th gear with a 914 or 911E tall fifth, but you cannot add a 5th gear to your box without a mainshaft change and a new first gear.

Perhaps the real experts will chimne in and we can all learn something?

johnt