915 to G50 conversion on '85 911 - What's needed guide‏

[ALEX P]

I’m running a 3.2 chassis with a 964 3.6 engine mated up to a 915 gearbox. The gearbox has done around 120-130k miles without rebuild so with the age, mileage and the additional loads placed upon it from the 3.6 conversion I did around 10k miles ago, needless to say it is becoming pretty stretched, crunching more and more and generally worn. It has always had a factory short shift which possibly exaggerates this and more recently a Seine gate shifter. Ideally I would have dealt with it when I did the 3.6 conversion but time and money constraints did not allow.

Now I’m not going to get drawn into any debates over 915 v G50 because it has all been discussed far too many times before. What I would say is that I have owned both and personally feel that the G50, albeit slightly heavier with more complications such as hydraulic operation does allow for a bit more clumsiness, faster shifts and is arguably a slightly more robust unit. I am trying to compile a rough argument against rebuilding my 915 verses swapping it for a G50 unit from the ’87-’89 Carrera.

Trawling through many old threads, the thread that stands out the most for good information is this:

Fitting 6 speed G50 to '89 Carrera

And the thread with the most amount of promise but eventually fizzled out was this:

(Really wish you’d carried on with that one Blackbyrd!)

915 to G50 Conversion on my Euro 3.2 911 SC

This for sale details some of the parts with photos that need to be modded:

FS: G50 Shortened Bell Housing/Main Shaft/Fork

All I want to do is try to seek some answers from people who have either done the conversion or have knowledge of it without cluttering up anyone else’s thread so that I can decide whether to start hunting for a G50-00.

I would like to retain the torsion bar set up instead of going to coilovers and would really like to not have to cut into the torsion tube. So basically a nut and bolt conversion (apart from the shortening to the ‘box). I have read that some do have to and some don’t which always puzzles me as I thought it would be a bit more of a black or white answer!

As far as a parts list goes, I’ve figured it will need:

G50-00 Gearbox (Not sure if this is the only G50 Variant that fits but appears to be the shortest)
G50 Pedals and master cylinder (possible aftermarket – Tilton?)
3 x Hydraulic lines
G50 Shifter assembly
G50 front mount / adaptor to 915 chassis (Patrick Motorsport?)
Reservoir and line
Slave Cylinder
G50 starter motor
Clutch kit
Flywheel (Patrick Motorsport?)

And the work involved other than fitting the above is:

Shortening the bellhousing of the G50 by the 29mm or so – Are there any threads or info out there on the best way to do this?

Shortening the main shaft of the G50 by the same amount - Are there any threads or info out there on the best way to do this?

Do you have to completely dismantle the G50 to remove the main shaft to shorten it? I've seen people quote $1000 for the work which seems a lot for simply doing the machining work so presumably the rest is in the labout to strip and reassemble the 'box?

Any advice on offer or anything that I have missed out there would be great as even if I don’t do it and live with/rebuild my crunchy 915 - It may be useful to others

[Northy]

Alex, have you had a look at brummie's thread on impact bumpers? He changed his sc to a g50. A lovely job too, if I remember correctly.

[ALEX P]

Originally Posted by Northy

Alex, have you had a look at brummie's thread on impact bumpers? He changed his sc to a g50. A lovely job too, if I remember correctly.

Hey Lewis, yes I've read that through a few times and he did a cracking job but he really went to town on it and fitted a 6 speed G50 as well as cutting the torsion tube although retaining the TB setup. He also cut and reprofiled the rear bulkhead as well as fabbing a new centre tunnel etc etc

Brummie's G50-into-an-SC transplant - Gruppe IB: The Hot Rods - Impact Bumpers

What he's done is fantastic but it's a lot of work - If I press the button on this I was aiming/hoping to do no real chassis mods at all otherwise I may reluctantly swing back towards rebuilding the 915.

[spuggy]

I see no mention of an LSD. IMHO, you'd really appreciate a 60/40 LSD set to about 80 ft/lbs in that car. Best money I've ever spent on either of my transmissions.

A Guard Transmission (GT) clutch LSD is probably the very best available, but pretty spendy in the UK with import duty/taxes.

The Gripper guys told me they've made G50 LSDs before (and will again if asked), they just don't list them on their web site.

However, with a Gripper, you need someone who knows how to tune one to set it up for you for your intended use, as well as to install it. Because those guys just make diffs.

Mike Bainbridge has worked with those (Gripper) diffs extensively.

Originally Posted by ALEX P

would really like to not have to cut into the torsion tube

You may still have to at least cut the cover away to get the extra half inch you need. My '77 did.

There's a one photo on this thread that'll give you an "oh, OK" moment: Verify a G50 is shortened?

The welded cover is just a welded cover. It's OK to cut some of that away.

Some cars don't need it; I've heard of earlier ('74) and later (SCs) with G50's that didn't need this, no-one seems to know quite why some do and some don't....

G50-00 Gearbox (Not sure if this is the only G50 Variant that fits but appears to be the shortest)

All the N/A, 2WD, 5 speeds are same length I believe, if fitted with the early (pre-964) nose cone. You can't convert the 964 4WD box to 2WD nicely, don't bother even though they're cheap as chips.

G50 Pedals and master cylinder (possible aftermarket – Tilton?)

87-89 pedal box. 964 one doesn't fit.

3 x Hydraulic lines

Shortening the bell housing moves the slave further away from the master - in the same way that the shift coupler also needs to be extended by the same amount/same reason.

Patrick Motorsports make an extended, full-length hose. Or a short extension hose (just source new stock hoses locally).

PMS also make a coupler extension. They can provide a full package of parts/fiddly bits.

G50 Shifter assembly

Replace internals of the factory shifter with the J West short shift while you're in there - and replace the factory coupler pillow with the J West one.

Fabulous stuff - you'll not regret that choice.

G50 front mount / adaptor to 915 chassis (Patrick Motorsport?)

This is an excellent piece - my tranny guy was quite taken with it. It also lets you use 915 engine/tranny mounts - including WEVO ones - instead of the bonded-in, non-replaceable ones.

Reservoir and line
Slave Cylinder

Yup.

G50 starter motor

When the 915 was built up for the 930, I bought a hi-torque starter motor (Edge) from Cambridge Motor Sports in 2006 - and just re-fitted that to the G50.

Clutch kit
Flywheel (Patrick Motorsport?)

Probably best to buy a package from PMS; there's some funkiness around the throwout bearing/flywheel bolts/release fork tube that they've got down pat.

Unless you really want to go DIY. Basically, you're just converting to a 76-77 spring-centered 930 clutch (factory friction plate good for around 500 ft/lbs, so you don't even need "trick" plates like Center Farce or whatever), but you do need to get the flywheel and bolts, TO bearing right.

And the work involved other than fitting the above is:

Shortening the bellhousing of the G50 by the 29mm or so – Are there any threads or info out there on the best way to do this?

Shortening the main shaft of the G50 by the same amount - Are there any threads or info out there on the best way to do this?

Give it to someone who's done it before. Preferably some who's done it many times.

Do you have to completely dismantle the G50 to remove the main shaft to shorten it?

Most certainly. I don't know if the weld-on 1st gear needs to come off, but I believe you should certainly re-harden the main shaft after machining.

I've seen people quote $1000 for the work which seems a lot for simply doing the machining work so presumably the rest is in the labout to strip and reassemble the 'box?

LOL. No, that figure doesn't include stripping the box. 600-1000 USD is pretty much the price for performing the machining on your parts. Or, in other words, parts not included.

BTW, even though 915 rebuilds have been edging up for years, G50 boxes are way more expensive to go through. A G50 with 120,000 may actually need a fair bit if the spec is "replace wear parts if questionable".

But then I expect to be stuffing over 500HP through mine.

Any advice on offer or anything that I have missed out there would be great as even if I don’t do it and live with/rebuild my crunchy 915 - It may be useful to others

If I were you, I'd talk to Mike Bainbridge of MB Engineering. I'm sure I discussed the machining for this with him years ago when he built up the 915 I used until the G50 became necessary - and he could do pretty much all of it.

PM me if you aren't clear how much all this costs, or need more details.

Frankly, with "only" a 3.6, I think I'd put the car on an aggressive diet, build up a couple of serious 915's with LSDs, internal oiling (spray bars) and stiff cover plates/bearing retainers etc, uprate the stub axles so you can use G50 axles. And still have multiple buckets of cash left over instead...

Unless you drive it seriously hard (like competitively), it probably just doesn't make sense for that power level.

[ALEX P]

Hey spuggy, firstly, big thanks for taking the time to answer my questions in such good detail, it is much appreciated.

Originally Posted by spuggy

I see no mention of an LSD. IMHO, you'd really appreciate a 60/40 LSD set to about 80 ft/lbs in that car. Best money I've ever spent on either of my transmissions.
A Guard Transmission (GT) clutch LSD is probably the very best available, but pretty spendy in the UK with import duty/taxes.
The Gripper guys told me they've made G50 LSDs before (and will again if asked), they just don't list them on their web site.
However, with a Gripper, you need someone who knows how to tune one to set it up for you for your intended use, as well as to install it. Because those guys just make diffs.
Mike Bainbridge has worked with those (Gripper) diffs extensively.

If I do this then I was only planning on tacking one project at a time so LSD wasn’t really going to be on the list until maybe a bit further down the road. I know it makes perfect sense to do it while you’re in there and the box is apart but it would be what, at least another $1500 or more. I’ve spoken to Mike a couple of times before (he used to work in the same town I live in) and he or Nick Fulljames of Redtek would be top of my list to do any gearbox work.

Originally Posted by spuggy

You may still have to at least cut the cover away to get the extra half inch you need. My '77 did.
There's a one photo on this thread that'll give you an "oh, OK" moment: Verify a G50 is shortened?
The welded cover is just a welded cover. It's OK to cut some of that away.
Some cars don't need it; I've heard of earlier ('74) and later (SCs) with G50's that didn't need this, no-one seems to know quite why some do and some don't....

It’s funny isn’t it – you would think they were all the same!
So if you cut the torsion bar cover away as in the photo below then do you just leave it open?
Does it have any adverse affect on the strength? If it literally is just cutting a window in the tube and treating it to some rust prevention then that’s not too bad but didn’t really want to start breaking out the welded as it always seems a slippery slope!

Originally Posted by spuggy

All the N/A, 2WD, 5 speeds are same length I believe, if fitted with the early (pre-964) nose cone. You can't convert the 964 4WD box to 2WD nicely, don't bother even though they're cheap as chips.

Yeah, shame that ‘87-‘89 911 G50 it would be then.

Originally Posted by spuggy

87-89 pedal box. 964 one doesn't fit.

I think for most parts I would find a ’87-’89 G50 car being parted out.

Originally Posted by spuggy

Shortening the bell housing moves the slave further away from the master - in the same way that the shift coupler also needs to be extended by the same amount/same reason.
Patrick Motorsports make an extended, full-length hose. Or a short extension hose (just source new stock hoses locally).
PMS also make a coupler extension. They can provide a full package of parts/fiddly bits.

Patrick Motorsport do seem to be the place to go to get all the conversion bits, maybe I’ll fire them an email. I run their flywheel already on my 3.6 conversion.

Originally Posted by spuggy

When the 915 was built up for the 930, I bought a hi-torque starter motor (Edge) from Cambridge Motor Sports in 2006 - and just re-fitted that to the G50.

I run a high torque starter on my 915 but didn’t realise they were transferrable – Aren’t G50 starters different?

Originally Posted by spuggy

Most certainly. I don't know if the weld-on 1st gear needs to come off, but I believe you should certainly re-harden the main shaft after machining.

Yes, I read that the main shaft needs to be hardened and ground to 15mm dia

Originally Posted by spuggy

LOL. No, that figure doesn't include stripping the box. 600-1000 USD is pretty much the price for performing the machining on your parts. Or, in other words, parts not included.

Damn! Maybe I’d end up doing more than planned myself!

Originally Posted by spuggy

BTW, even though 915 rebuilds have been edging up for years, G50 boxes are way more expensive to go through. A G50 with 120,000 may actually need a fair bit if the spec is "replace wear parts if questionable".
But then I expect to be stuffing over 500HP through mine.
If I were you, I'd talk to Mike Bainbridge of MB Engineering. I'm sure I discussed the machining for this with him years ago when he built up the 915 I used until the G50 became necessary - and he could do pretty much all of it.
PM me if you aren't clear how much all this costs, or need more details.

Thanks spuggy

Originally Posted by spuggy

Frankly, with "only" a 3.6, I think I'd put the car on an aggressive diet, build up a couple of serious 915's with LSDs, internal oiling (spray bars) and stiff cover plates/bearing retainers etc, uprate the stub axles so you can use G50 axles. And still have multiple buckets of cash left over instead...
Unless you drive it seriously hard (like competitively), it probably just doesn't make sense for that power level.

The car isn’t driven especially hard (i.e. competitively) but she does like to get out and stretch her legs. Last time she was on the scales it was 1030kg (270lb). The engine is ‘only’ a 3.6 but it has a few little extras so puts out a little extra!

The thing that worries me about rebuilding the 915 is a friend recently rebuilt his at Mike Bainbridges and did some sensible upgrades, LSD, spray bars, stiffening etc and do do that sort of job I would be looking at $6500-8000 and I'd want to be pretty damn sure that I'd be very satisfied at the end of it.
Obviously buying a used G50 is also rolling the dice in a big way and as you say they're expensive to rebuild.
It's kinda why I'm trying to do a side by side comparison to help me make my mind up!

[Tippy]

The G50 will handle well over a 1000 hp with a billet side cover. A 915 seems to be strained past 350.

Point is, in stock form in a naturally aspirated 3.2, the G50 will have never been pushed to its limit.

Not even close.

So, as long as the fluid was changed properly and the owner didn't abuse the synchros, I wouldn't worry about a used G50. It'd be worth the gamble.

[Matt Monson]

A used G50 these days is a giant question mark. You might be in for $1000 worth of synchros. 3/4 slider/hub assembly is another $1000. Factor a $3000 repair bill into your budget just in case.

My G50 LSD is almost $3000. Don't know what a Gripper costs. It is less.

[ALEX P]

Ok, so I had a reply from Patrick Motorsports and the conversion parts from them came out to $3670 broken down as below:

$40 Engine case to g'box studs
$285 Mount adaptor
$90 ARB/Sway bar adaptor
$690 Hydraulic conversion kit inc reservoir, brackets, hoses, sleeves, clutch m/c, slave cyl, lines etc
$225 Shift rod adaptor
$650 Flywheel
$22.50 Flywheel bolts
$1400 Clutch set
$245 Starter ring gear
$22.50 Bolts

They also sell the guide tube for $130 - lots of info here:

Transmission > Porsche G50 SBH (Short Bell Housing) By Patrick Motorsports Porsche & Mid Engine Performance Specialists

I alsodiscussed it with an engine & gearbox specialist in the UK who thought the modification work to the the gearbox would probably be around £2000 / $3150. This is to deliver him a gearbox and for him to strip it, carry out the machiing work and rebuild it. Anything worn or broken whilst in there would obviously be extra.

So as a quick tott up for the assumed total:

$3670 Patrick Motorsports conversion parts
$3150 Gearbox modification
$4000 (approx) for G50 Gearbox
$1000 (approx) Pedals, shift rod & sundries

It looks to be approximately $12,000 / £7700 for the conversion + a boat load of work + the unknown theoretical expense of a used G50.

all of a sudden I'm developing a new love for my 915! I simply couldn't justify that sort of expense. On the plus side it does give a new justification for dropping some coin into a killer 915 build

Hopefully this is of some use to anyone else considering the same conversion.

[quattrorunner]

Crap!

[ChrisP911]

cheaper to buy a tub

[safe]

What is the difference between the 915 and G50 shifter, is it really necessary to use a G50 shifter?

[safe]

Peter, yes I know the difference between the 2 boxes.
I like the 915 too, but even with a good 915 you need to pause between gears and going into 1st from second with any speed can't be done without brute force.
With a bit of torque, they do brake if you're not careful...

But why in a G50 conversion do you need to change the shifter?
What is the difference in the shift pattern?

If the change is really a must, can you use a 964/993 shifter in an early tub or do you need a 87-88 shifter?

[spuggy]

Originally Posted by safe

What is the difference between the 915 and G50 shifter,

Completely different console, shift pattern & shift coupler rod.

The 915 shift pattern has reverse straight back from 5th - if I recall correctly, there's a lockout mechanism in the console itself? - where the G50 is left/forward for reverse, like an early pattern dogleg 1st.

is it really necessary to use a G50 shifter?

You can make anything fit if you want it badly enough, lol. Bolting in a G50 shifter is a 15 minute job on an install that'll take a week or two. Most seem to settle for that.

[winders]

Yes, the change is a must. Reverse is outside the pattern to the left. The g50 springs are internal to the transaxle and not in the shifter like with The 915. I am sure the throws are different too.

[safe]

aha, reverse!

So there is no springs or lock out in the G50 console?
Just a simple linkage pulling/pushing/twisting the shift rod, like on the 944?

[KTL]

No springs or anything inside the G50 shifter. Like Scott said it's gating and spring centering is all contained inside the transmission

FS: G50 Shifter and rod

[safe]

Originally Posted by KTL

No springs or anything inside the G50 shifter. Like Scott said it's gating and spring centering is all contained inside the transmission

FS: G50 Shifter and rod

Cool. Found pictures of a 964 shifter, thats even more simple.

I think I could get a 944 shifter dirt cheep and modify that, moving it back and up.

[-Levi-]

any more additional information ever come from this thread?

[GH85Carrera]

Like most major modifications, it just does not make economic sense.

Sell the current car and get a G-50 car. You will be much better off. Just my opinion, it is your money and time. Do what you want.

[faverymi]

Long shot but.

Would a 996 transmission work for this conversion.?

My 3.8/ 915. Will eventually need something. I'm following this thread closely