944 fitting from standard non Limited Slip Diff to a LSD

[jellytott]

Any thoughts on this one....Fitting a limited Slip diff to a non-LSD 944S ?

Winters coming so like others on the forum we all have winter lay-up jobs planned, mine is a whining Diff and excessive backlash add a knackered Transaxle top rubber mount .....so this is my planned job hopefully and to fit a "Limited Slip Diff " from a 944 Turbo complete unit ie the transaxle/gearbox complete to my "Non LSD" 944S Oval dash.

I know (well I've been told) the transaxle's are the same as physical swap out between the LSD and non LSD versions, but I'm trying to work out is there any differences to be had on "Speedo MPH readings" doing it from the Final Ratio info there's difference between the Turbo and 99S Final Drive Ratio's/FDR?
Original Non-LSD for 944S (APG fitment) with Final drive ratio.... FDR = 9:35 (3.889)
The donor LSD from a 944 Turbo (AOR) has a Final drive ratio .... FDR = 8:27 (3.375)

So how did the Turbo Speedo work correctly does it have a different speedo pickup on the Transaxle or does the speedo dial in the oval dash have a different gearing fitted?

Thanks in advance,

John

 

[924Srr27l]

Hi,

You could fit an LSD Unit in your gearbox and retain the same FDR and have no issues on the speedo reading ?

I did just this on a 1987 QK 2.5 8v Gearbox which has the 3.889 ratio which I like as it's short and good for acceleration
and not high top speed.

I fitted a Wavetrac (USA) LSD which I got from EMC
It's quite road friendly although your driving style and instincts need to change as unlike
non locking diffs when the rear loses grip and the car steps out (and you can lift and let it settle), it's best to keep the throttle
on with this LSD and counter steer but not as much.

But it's very good as I get no wheel spin with 205bhp / 205 ft lbs and on a lightweight 1040kg car using 6" wide rims and
205/55/16 tyres.

R

 

[Indi9xx]

One speedo sensor which is gear driven off of the output shaft of the differential (A small gear) so is universally turned at a set amount of revs on the sensor pinion for each turn of the output shaft, so completely independent of the differential or pinion/ring used.

 

[jellytott]

Hi 924Srr271,

Thanks for the reply and Jon too, I really don't want to rebuild the gearbox to retain the FDR shaft, because my plan was fitting the complete donor 944 Turbo ie the LSD complete in its transaxle/gearbox into the 944S, reading John M's reply it looks like I don't need to change anything to compensate for the final drive being different as the pickup is related to output road speed if I've read it right.

 

[blade7]

I think turbo LSD gearboxes are quite pricey these days if that's your plan.

 

[PSH]

correct me if I'm wrong but isn't the gearing a bit long on the turbo for an N/A engine....engine power bands tend to be linked with gear ratios for the best performance...I would expect a 944S to struggle a little, it would certainly have a totally different driving feel even if there wasn't an LSD fitted...however, as I said, correct me if I have this wrong.

cheers

Pete

 

[924Srr27l]

correct me if I'm wrong but isn't the gearing a bit long on the turbo for an N/A engine....engine power bands tend to be linked with gear ratios for the best performance...I would expect a 944S to struggle a little, it would certainly have a totally different driving feel even if there wasn't an LSD fitted...however, as I said, correct me if I have this wrong.
cheers
Pete

Yes I agree, the 16v is known for being dull below 4000rpm compared to the 8v so with longer ratio gears acceleration would be slower but once you got the twin cam in the power band it would pull well.
But it might end up being a 4 speed with overdrive scenerio

R

 

[jellytott]

I've fortunate enough to have a LSD unit already I got it at a absolute bargain price its one of the last of the the Turbo units too low mileage... I've flushed it out the oil fluid was all clean too, no leaks from seals or cooler.

Pete you can see why I asked the question I've checked and all the gear ratio's 1 to 5 and Rev they're the all same its just the final ratio that is different, so I'm trying to work
out any problems doing such a thing, I've worked out that the lower Turbo LSD final drive of 8:27 (3.375) this is approx a 13% difference to the original 944S FDR ....quite huge.

• it does means a lower top end speed,
• changing up the gears will happen quicker.
• the mis-match of engine power bands to the LSD is a unknown will it be awful with a 13% difference?

So is it worth a swap out, Im not a track day racer yeh I enjoy the ride and handling is it worth a punt I would sort of enjoy the benefits of a LSD with a trade off on reduced top end speed and quicker gear changes?

John

 

[924Srr27l]

I've fortunate enough to have a LSD unit already I got it at a absolute bargain price its one of the last of the the Turbo units too low mileage... I've flushed it out the oil fluid was all clean too, no leaks from seals or cooler.

Pete you can see why I asked the question I've checked and all the gear ratio's 1 to 5 and Rev they're the all same its just the final ratio that is different, so I'm trying to work
out any problems doing such a thing, I've worked out that the lower Turbo LSD final drive of 8:27 (3.375) this is approx a 13% difference to the original 944S FDR ....quite huge.

• it does means a lower top end speed,
• changing up the gears will happen quicker.
• the mis-match of engine power bands to the LSD is a unknown will it be awful with a 13% difference?

So is it worth a swap out, Im not a track day racer yeh I enjoy the ride and handling is it worth a punt I would sort of enjoy the benefits of a LSD with a trade off on reduced top end speed and quicker gear changes?
John

3.375 means a higher top speed if you've got enough power to get to the rev limit in 5th which I doubt?
It doesn;t mean it'll be a lower top speed, the opposite
Changing gears with a longer ratio means they will be slower to repsond unless you rev the knackers off the twin cam!
A locking differential stops one wheel spinning and locks the least grippy wheel, unless you have an issue with the wheels spinning
you'll not benefit from a LSD or see any difference or improvement if anything the longer FDR may ruin the car's drivability
especially with the 2.5 16V head, hence Porsche stuck an extra 500cc on the motor which worked much better with 2 cams the power was only 20bhp more but the torque lower down the rev range much better than the 2.5 16v

R

 

[jellytott]

Thanks for the answer 924sr271,

So to match the LSD to a 944S would be replacing the final drive, looks like I have 2 options have the original whinning 944S Diff rebuilt or retro fit a correct FDR shaft in the LSD if thats possible, is this a stupid question
would removing the 944S original shaft/sourcing one and fitting into the LSD be an option would it require setting up pre-loads or is a specialist job too?

John

 

[924Srr27l]

Thanks for the answer 924sr271,

So to match the LSD to a 944S would be replacing the final drive, looks like I have 2 options have the original whinning 944S Diff rebuilt or retro fit a correct FDR shaft in the LSD if thats possible, is this a stupid question
would removing the 944S original shaft/sourcing one and fitting into the LSD be an option would it require setting up pre-loads or is a specialist job too?
John

Ok I'm prob getting myself confused here !

The LSD Unit you have is from a Turbo gearbox yes? I'm not sure if this will be a direct fit into your 944S box. If it is then
you've no issue as the FDR will not be affected.

If it doesn't fit and cannot be made to fit? then you'd need a complete turbo gearbox or buy an aftermarket LSD that
does fit the 944S Gearbox.

R

 

[944Turbo]

The turbo final drive is long, first gear feels high in a turbo with 300+ BHP (some clutch abuse is required for a really quick standing start) and 2nd is good for well over 60, 3rd well over a hundred, 4th to 5th at max revs is around 140, It is possible to fit the s2 final drive to a turbo box as some people do to improve acceleration in a turbo (with a reduction in top speed)

more info here
https://rennlist.com/forums/944-turbo-and-turbo-s-forum/232729-best-mod-yet-s2-ring-and-pinion.html

 

[jellytott]

Hi
I've mentioned that I have a LSD, in fact I have the complete trans-axle from a late 944Turbo which includes G/box+ LSD + cooler + cross member so the complete unit inc bell housing etc, so am I getting a confused here.... If I have the "complete Trans-axale" from the 944Turbo it will fit in my 944S but I would have to consider the changing FDR shaft (ie the 944S ratio and not the Turbo FDR shaft) to match the power range of the 944S engine and retain everything else in the donor trans-axle complete?

John

 

[924Srr27l]

You need to find a Porsche transaxle gearbox specialist and ask them these fitment questions...

R