928S4 Automatic Transmission to Manual Transmission Conversion

Well I never thought we would do one of these conversions , but this ( most likely ) one off event is happening at our workshop right now , the 928 getting the manual transmission conversion is a relatively low km 1990 928S4 with automatic transmission that the owner has owned since circa 1992 ( a real garage queen / looks like new )and the owner has always wanted a manual transmission 928S4 after he drove one decades ago , but time slips away and before you know it decades have gone by and interestingly even though the owner of this particular 928S4 ( was automatic ) has quite a stable of much later supercars it was always his dream ( for want of a better expression ) to make his 928S4 a manual transmission one . Note } This particular 928S4 has been in the current owners family for 33 or so years and is in new condition ( a garage queen + )  with very low kms for the age and a very strong like to the owners late father ( never to be sold ) while the current owner is alive So a suitable "written off " but very drivable 928S4 with manual transmission was purchased & yes this purchase took years to come across the best candidate because we did not want a bad one with mechanical damage so the candidate just had body damage and nothing else , it even drive perfectly straight , not even suspension damage , it was mainly written off because it had a creased rear boot floor area and a slight kink in the roof from a rear end impact , even the rear hatch closed and opened OK . Naturally the 1990 928S4 ( be it ) automatic transmission or manual transmission was the first year model of the 928 series to receive the PSD ( computer controlled , hydraulically activated ) limited slip diff system , for 1990 it was and still is very sophisticated and works extremely well on a road car , but the donor  928S4 with manual transmission was / is a 1987 928S4 with manual transmission which did not have the PSD system , but with prior knowledge of the 1987 and 1988 928S4 with manual transmission we knew it was quite feasible to convert the manual transmission diff of the 1987 manual to accept the PSD system The 1987 manual transmission already had the "later" entire transmission one piece cast alloy projections ( but not machined or drilled ) in readiness for the 1990> PSD hydraulic slave cylinder and the lower PSD pressure plate set position bolted lock nut & yes the PSD LSD unit has its own pressure plate but naturally quite different to a normal clutch pressure plate ( but in principle the same ) , but these cast projections are NOT milled flat or drilled or threaded for bolts or anything , they are just there with their casting imperfections in readiness for machining  for the PSD system that was to finally turn up a few years later in 1990 But first things first , we were always going to totally dismantle the 1987 928S4 manual transmission anyway because we were going to vapour blast the entire ( totally stripped ) manual transmission/diff case and top cover and anything going into the vapour blaster has to be completely bare ( no gearbox internals at all to be exposed to the vapour blasting ) and naturally the quality of the alloy finish when the vapour blasting is completed is unsurpassed , particularly when alloy components are heavily stained from decades of being driven on the road We did some small repairs to the 1987 928S4 manual gearbox , be it new reverse lock out and new selector shafts detents and springs and one new selector fork ( 1st and reverse fork ) , we already knew the gear change quality ( meaning dog teeth and synchro components was quite good anyway ) Then came the dismantling of the 928S4 1990 year model auto transmission PSD differential , we then carefully measured where Porsche had machined different surfaces on the inside of the diff alloy housing and on the outside of the diff housing , then after lots of measurements we set about machining the same points / areas within the 1987 928S4 manual transmission differential housing and then once the individual flat surfaces need were done to the same thousandths of an inch depth/dimensions  we then set about drilling out and tapping the large dia hole in the lower projection to then tap the thread for the PSD pressure plate fork adjustment bolt and installing a Time -Sert steel thread into the alloy case for the adjustment bolt and then we machined the hole for the PSD slave cylinder in a stepped bore configuration , the same exact measurements as the stepped bore in the Auto Trans diff alloy housing that's for the PSD slave , then the two holes and threads for the M8 PSD slave securing bolts were completed . Once all that was completed and the bare manual transmission alloy housing/ casing was washed inside and out many times and tired we re confirmed all the machining measurements were exact , then finally the entire manual transmission bare case was vapour blasted , same with the  alloy top cover plate and the alloy diff end plate and the two large alloy differential side bearing carriers then washed many times and then a final wash in solvents and dried with compressed air , this is important because we can not leave a single grain of the medium used in the vapour blasting process to be present anywhere , like in the detent or select shaft galleries , you can not spend too much time in this important preparation , it has to be perfect Finally we can start assembling the manual gearbox part of the transmission ( but not the diff section ) with a selection new parts , then finally we are ready to dummy fit the PSD / LSD diff centre without its crown wheel , because this will be the start of the process of diff side bearings preload adjustment ( with shims ) Note }.  All Crown Wheel and Pinions are matched and can NOT be mixed , so the Crown wheel from the 1987 manual transmission must be used because the 1987 gearbox and pinion head/shaft is from the same 1987 gearbox ,so that means the 1990 crown wheel is naturally matched to the 1990 auto transmission diff , this is all for noise control Note 2 }. The actual size of the differential or put another way , the ability to swap diff internals is from the 1985 928S all the way through to the 1995 GTS , meaning the basic overall dimensions are the same , the 1978 to 1984 928/s are a different internal size , remember I am only referring to the mechanicals and not the transmission case Once the diff side carrier bearing preloads are completed to within spec and using new carrier bearings , we then install the 1987 diff crown wheel with new lock bolts and lock washers to the PSD diff centre and then do a dummy fit , because we now need to adjust diff backlash & this time we must NOT change the preload but we do the mathematics to determine what shims we have in both side bearing shell carriers and swap one to two shims or a fraction of a shim to obtain the correct back lash , too much back lash = noises and too little back lash = wearing out of the crown wheel and pinion teeth but the "total" thickness of both shim packs must not change for preload purposes Once everything is checked and remeasured , finally we can install the diff and its PSD components and set the PSD mechanical adjusting bolt length and fit a new PSD slave cylinder and install all new gearbox seals and gaskets & check the gearbox gear change operation etc We had already overhauled the internals of the manual Torque Tube with new components and while the T Tube was apart we stripped off the old external paint and primed and repainted its exterior .We then vapour blasted the alloy bell housings from the manual transmission done car and installed new clutch components and new flywheel etc & installed the drive line with new selector shaft knuckle and selector shaft ball joint on the repainted selector shaft from the 1987 manual & a new genuine Porsche 928S4 leather gear stick assembly and the new plastic base frame etc Note } All the 1990 Auto Transmission auxiliary components had already been removed from the car and we had already installed the manual pedal box into the 1990 S4 from the 1987 manual 928S4 and we had cut the holes in the firewall for the clutch master cylinder and the 1990 received all new clutch hydraulics and extra heat protection sleeving for the new clutch pressure hose Note 2 } The all alloy rear axle subframe is quite different from the Auto Trans version compared to the manual transmission version ( different casting positions to suite the manual transmission mounting points) , we vapour blasted the rear axle subframe from the 1987 S4 and installed it into the 1990 S4 along with a large number NEW genuine Porsche rear axle ( area )nuts and bolts and washers and brackets from Porsche Germany and they were mostly still available new , which finishes off the job very nicely and makes for fantastic photos ( lots ) for the owner of the 1990 S4 & we installed 2 new transmission mounts as well Note 3 } While we had the rear suspension apart we removed the upper and lower control arms from the axle hubs ( uprights ) and pressed the hubs apart and vapour blasted them and installed 2 new rear wheel bearings and circlips , pressed the old bushes out of the rear suspension upper arms and pressed in 4 x new genuine Porsche upper arm bushes On test driving the car , we had no issues at all , she drove perfectly , the PSD worked perfectly and everything was perfectly quiet mechanically , even I was impressed , its a very nice and very quick and very stable 928S4 manual & we are securing all the auto transmission components on pallets so the owner can store at his warehouse incase that in the future it might be converted back , a bit unlikely but we were very careful to protect all the hundreds of large and small parts so it can be converted back ( which is nice ) Then when we were completely finished & all photos completed , we coated ( sprayed ) a clear cavity wax over every under the car component to protect from salt air and road grime ( just like Porsche did when the car was new)  , the beauty of a good quality cavity / body spray on wax is completely seals everything its covering so in 10 or 20 years later you just spray the wax covered alloy and steel components with a solvent as simple as kerosene  and the wax falls off leaving the new look that it was 10 or 20 years before So as you can see this was not a " chuck some old 2nd hand parts at a car " , this was a carefully planed mechanical restoration with a twist  & as a side note we did the engine up on this very car only a few years ago ( it was on the verge of failed head gaskets ) and the engine looks cosmetically new since then , so this transmission and suspension work just finishing touches of a very special Porsche to us and the owner The above is just a summary , there was a lot more detailed work done , far too much to put into this post Regards Bruce & Sean Buchanan Buchanan Automotive , Sydney , Australia Reply Like More 1 Person liked this

Thanks for sharing! Lots of effort just in your typing! Sounds like a lovely car and lovely work.

Here are some before, during and after pictures.

Quotes source post: 406793

Beautiful. I hope the owner puts many of kms on the car rowing through the gears and using all limbs! Sounds like a challenge doesn’t it- he who dies with most kms on the clock wins!

Love your work guys, looks great.