1986 944 2.5 Lux - Turbo Trans Swap Questions

[Fatboy]

HI Everyone,

I've got an intermittent knocking noise coming from the back end of my 944 lux, which internet research leads me to believe is probably the driveshaft on its way out - and where one is on the way out the other is probably not too far behind it.

Now, as I'm set on keeping this car for the forseeable future, I've been thinking about upgrades (long term plan is a V8 conversion), and I'd always assumed that the various transmissions fitted to the post '85.5 944s were interchangeable. Looking at replacement driveshafts from Euro Car parts etc shows the turbo being a different part number to the 2.5 8v n/a driveshafts, so rather than go and just replace the driveshafts I was thinking now might be a good excuse to upgrade to a turbo transmission, as I'd quite like the LSD, and it would be a better prospect for putting up with V8 torque...

What is involved in swapping the turbo transaxle into a 2.5 8v N/a - are the mounting points different (found a thread on here that suggested 3 mounting points for the turbo but 5 for the n/a - are these in different places or would it just bolt up)? And with the different driveshafts, will these need turbo hubs/drive flanges etc?

I know it'll be a higher ration diff, but that doesn't bother, I can live with loosing some of the off-the-line speed (or fit nitrous )

 

[Eldavo]

I appreciate your willingness to get involved but this could be a massive waste of cash. The V8 conversion is tricky on a RHD car to start with and involves a few challenges that whilst not insurmountable would need some time and money throwing at them. Also, a Turbo would be a better base for a V8 conversion anyway as you'd already have the bigger brakes, transmission, etc.

Fix the driveshafts, run it and enjoy it. If in time you still have an itch to scratch then sell it and buy a turbo. Transmission swaps and nitrous systems, etc. will get you half way to what a turbo would deliver and probably cost you more than just selling up and buying one.

 

[Fatboy]

Cheers for the reply, I appreciate that I'd be financially better off just buying a turbo (V8 conversion compared to a tweaked Turbo isn't remotely worth the cost of the LS2 etc until you're talking silly power levels, and that I'd never get the money back selling it), but there are a couple of mitigating factors ]] :

1) I already have this car, and I'm quite attached to it, not a good reason, but still [] I've always looked at is as an ongoing project (how I tend to look at my cars really, I love tinkering and if there's nothing to work on/improve then I'll start to lose interest)

2) It's cosmetically quite tatty so wouldn't sell too well, although I know the bodywork is solid as I've done a bit of work on the sills and tidied up underneath etc (my thread on Retor-rides: http://retrorides.proboards.com/thread/153657/1986-porsche-944-lux-2)

3) I can do bits and peices rather than do it all up-front as per buying a new turbo (values have gone up a fair bit since I bought mine!) - if all I need the is the turbo trans and driveshafts then it's under £1,500, and it'd be worth that to me for the LSD (had a couple of LSD equipped cars in the past and did like the way they drive, pluss living out in the sticks an LSD makes a big difference in a bad winter)

Given that the newest turbo is over 20 years old I figure there would be a fair amount of suspension etc needing replaced anyway so dependent on what is actually different n/a to Turbo then the overall difference in outlay might not be too bad (mine certainly could do with tightening up - still rides really well, but plenty of the rubber will need changing in the next couple of years, so why not uprate a bit while I do it?)

 

[JM1962]

I think its a straight swap with the drive shafts. In the PET all the transmission suspension parts are the same for the manual cars and the early turbo. You may need the support for the oil cooler. Torque tube is the same part number for 944/turbo.

 

[Fatboy]

Thanks for that Jim, I'll have a bit more of a look for a turbo trans...

 

[colin944]

The only problem I can see with this is that in the short term ie before you do the V8. You might find the 2.5 struggles to push the big long gears of a Turbo box and the car becomes not nice to drive.

 

[Fatboy]

I'd thought about that, but really all it will mean is it's a bit slower off the line in 1st, which is pretty low geared, so I'll just have to hang on to a lower gear for longer, but it is something to think about...

One other thing I suppose is whether the speedo drive will be the sme or if my speedo will be out as a result - I'd best check the Porsche Parts list for the sender unit...

 

[944 man]

I haven't had chance to read the other replies, but I will add that the longer final drive in a Turbo transaxle means that a Turbo isn't nice to drive in town and I suspect that a 160bhp car will be extremely lethargic.

Remember too that LSDs are far from common in Turbo transaxles.

 

[Fatboy]

Hi Everyone,

I've gone and had a dig for looking at the transmission ratios, to see what the relative difference is between the Turbo and the 2.5 N/A transmissions, and using the information from Clark's Garage (http://www.clarks-garage.com/shop-manual/trans-04.htm) I've put an excel spreadsheet together to look at the various options, and there aren't that many variants, so I reckon the figures are:

Ratios
FDR 1st 2nd 3rd 4th 5th Rev
1986 Lux 3.8889 3.6 2.125 1.4583 1.0714 0.829 12.42
Turbo (all years) 3.375 3.5 2.059 1.4 1.034 0.829 12.42

Overall Ratio (inc FDR)
1986 Lux N/A 14.00004 8.2639125 5.67118287 4.16656746 3.2238981 48.300138
Turbo (All Years) N/A 11.8125 6.949125 4.725 3.48975 2.797875 41.9175

Relative Difference 1.152266667 1.028571429 1.032054395 1.041642857 1.036170213 1 1
"Combined Effect
(inc final drive)" 1.185188571 1.189201878 1.200250343 1.193944397 1.152266667 1.152266667


Pic added in case the formatting is all out for the table [] If anyone wants the excel doc just ask and I'll email it you - or can I post it up here?

Basically it looks like the Turbo trans will be between 15 and 20% higher geared than the standard transmission, so it's not like 1st will suddenly become 2nd - given that 1st is very short in the 2.5 N/a anyway, and as it's a good torquey lump I don't reckon there will be a problem...

So the only things to check now are the oil cooler mounts and speedo drive []

 

[944 man]

Its the final drive ratio that is the issue. The Turbos final drive is too high for a 160bhp car and it wont have the power to pull it. I did this twenty years ago.

 

[Fatboy]

Surely all that means is that I'd need to hold on to the lower gear for longer - i.e. instead of changing from 2nd to 3rd at 30mph, hold on to second until 35/40 mph? I'd lose initial off the line, and response in 5th would be significantly worse, but in normal driving, using the gears, would it really make any difference?

 

[944 man]

I fitted a 220 transaxle with an LSD into a hillclimb car and it was terrible.

 

[colin944]

I'm with Simon on this one. I can't help but feel that you'll end up with a lethargic car, it just won't pull.

 

[944 man]

I have a 944 Turbo and Im looking for an S2 transaxle with an LSD! [] Reason: lower final drive.

 

[924nutter]

It will be like putting the 12 inch wheels on a ten inch wheel mini. (barring the extra gear) Suddenly you wil find you have a four speed box with overdrive 5th so high the car will slow down

 

[Fatboy]

Thanks for the input guys, I'll just go with new driveshafts and leave the turbo trans upgrade unt I've got the V8