LammMorada

Hi, All:

I'm sure this has been asked before, but how difficult is it to replace the 3-speed automatic in a 1980 Porsche 928 with a 4-speed from a 1983 model?

I'd be grateful to hear from anyone who's done (or considered) such a swap.

Thanks.

-Mike in California
1980 Porsche 928, white

Jim bailey - 928 International

You need the 4 speed torque tube as they are different lengths. Brendan C did this liked how it drove, disliked that the "good used" 4 speed.....died shortly after the swap

rixter

Jack Riffle did this last year, you should talk to him

LammMorada

Phone, email address? Thx, Mike

rixter

his screen name is Jack Riffle, pm him

BC

I think what I will do is take a vacation - you can just answer these sort of questions for me - so I don't have to. Spot on, really - loved what it did to the car - but it went silly after a while. Learned after that it was probably the B2 piston - which may have been an in-car fix.

BC

The 83 4spd box would be perfect. You need a new bowden cable. You kind of need to customize the shifter cable. There are some other things, but the main issues are the TT and the box.

Oh, and the trans fluid lines need some changing.

Jim bailey - 928 International

See there Brendan we did need you to comment I just remembered how much you liked the lower low !!

BC

It really wakes up the car ALOT. As I recall, the first 2 gears are lower than 1st in the 3spd.

Jim bailey - 928 International

Yes ....3 spd the first is 2.306 2nd 1.460 3rd 1 to 1 into the 2.75 final drive.....USA 4 speed is 3.676 , 2nd 2.0412, 3rd 1.4360 , 4th 1 to 1 BUT INTO A 2.20 final drive. SO for the final answer you need to multiply each gear ratio by the final drive ratio..... 3 speed first is 6.34 engine revolution to turn the wheel once , 4 speed first is 3.676 x 2.20 for a 8.08 ratio. Second for the 4 speed is a 4.49 compared to the 4.015 second for the 3 spd !! The 4 speed with the 2.20 final drive should get better fuel economy in drive than the 3 speed 2.75 since the engine turns about 20 % SLOWER at 75 mph !!! Yea thats it do it for the fuel economy ....

IcemanG17

Mike
Its possible, but like most things 928 it can get quite pricey with a new tranny and torque tube + the installation... "if" your tranny is going out, it will be far cheaper to just replace-rebuild that one... send me a PM and I can give ya some local shops that could do it for ya

Guys I met Mike at the PCA Concours last month at my work....Mike is a very nice guy and has a pristine OB in excellent condition!! Its very clean and drives very well!

harpermf

OK, I know I'm gonna get some flack. I'm planning to swap the 16V 4.5 in my 82 for either a LS-1 or LS-3. I have an 88 that I am keeping stock. So, would most people swap the 3 speed for a 4 speed too? I'm not looking for a 1/4 mile car. I looking more for top end and reliability. Does anyone know if the 3 speed will handle 650 HP? Thanks.

The Forgotten On

I don't think a 3 speed will handle 650 HP. I know a 4 speed can, but it would be best to send it to Greg Brown and have the internals beefed up to handle the extra power.

Plus you will have a fresh trans that will last longer than a normal used one.

FredR

A French chap working over here did a huge rebuild on an early 3 speed [heavens knows why] and amongst the mods he converted it very successfully to a 4 speed with a transaxle/torque tube from a later model 928.

I do not know the ins and outs of this but as I understood from the local agents who did the work it was a pretty straight forward thing to implement and did not involve much more than dropping the transaxle/torque tube and bolting in the new with the requisite parts.

I would recommend you avoid the US spec final drive units with the 2.2 ratio unless you are fronting it with a motor that can pull this ratio. My S4 is reasonably well sorted but I do not like the 2.54 final ratio on a 4 speed box- to me it feels lethargic in top gear- unfortunately as I know there is not an automatic 5 or 6 speed available for our cars. Boost the motor or put a GB stroker into the equation a whole different ball game.

It all boils down to a question of what you want to achieve.

Rgds

Fred

harpermf

Thanks. Any idea on the price tag to "improve" it? My plan is to pick up a late model 4 speed and get it ready before dropping the 3 speed. Didn't know about the torque tube, but it makes sense. Is there any difference in the wiring harness? I understand that the kickdown cable from the throttle has to be modified and I imagine that the cooling lines need to be shortened. Any comments on LSD vs Non LSD. Again, it's not going to be a 1/4 miler. Thanks again.