photos and story by peter tinucci via ideola's garage
Above is a freshly Rebuilt G31/02 with ZF 40% Limited Slip With 8:33 (4.125) ring & pinion and Euro 5th gear.
G31 type versus 016/083 type transaxles
These transaxles were used in the 924 series cars from 1978 to 1980. (82 for theCarrera GT series and 84 for the ROW 924 turbo) These are commonly called “snail-shell” because of their shape. These transaxles were identical in basic design and had a few internal differences. They were available in multiple designations: G31/01, G31/02, G31/03, G31/30, G31/50, 016Y, 016Z and Group D. (933)
I will break these into five basic versions:
1. 924 n/a
2. 924 turbo
3. SCCA (933)
4. 924 Carrera GT
5. GTS/R/P race cars.
The basic differences are: the 924 n/a transaxle has a 20mm input shaft (016 Y & 016Z) along with the SCCA, (933) the US 924 turbo (G31/02) and European 924 turbo (G31/01) has a 25mm input shaft and are essentially the same but have different ring and pinion and 5th gear ratios, and the 924 Carrera GT (G31/03) has the same gearing as the European 924 turbo except it has hardened gears and a better 1st gear synchro ring. The G31/30 & G31/50 are mostly the same as the G31/03 except that they have a larger differential housing that incorporates an oil pump driven by a gear on the differential. This version also uses 930 synchronizers on 2nd through 5th gears. Also, the G31/50 had two additional final drive ratios to choose from, a 9:33 (3.66) and a 9:35. (3.44)
The 016Y &Z and the G31/01, G31/02, G31/03 could be had with 40% locking differentials (LSD) which are the late fine spline 930 ZF disc type LSD. (P/N 930.332.053.03) The G31/30 and G31/50 had the same LSD but it was set to 40% or 80% locking by changing the order of the discs and plates. The 016Y & Z was the same as the SCCA (933) except the SCCA D (933) had a list of different ratio gear sets available. The G31/03 version had (Carrera GT) was identical to the G31/01 but had hardened gears. The G31/30 (GTS) version was essentially the same as the GT except it has an oil pump and cooler along with 930 2nd through 5th synchros. The G31/50 (GTP/GTR) versions also had an oil pump, cooler and 930 synchros for 2nd through 5th gear but also had the choice of many gear sets for 1st through-5th gear along with 2 additional ring & pinions. The G31/30 and G31/50 transaxle also had a different differential casting that had the provisions to drive the oil pump for cooling. This was done using a gear on the differential along with and nozzles. The casting differences are shown below.
Here is a comparison of the 915 gear set and the G31 gear set. Notice that the difference is in the pinion gear end bearing area. The 915 uses the same front bearing but a different second bearing (the one clamped by the retainer) than the G31. However, the outside diameters are the same so to use a 915 gear set in a G31 you would need the 915 bearing. This means that you could probably use any of the G31 gear sets in a 915. Incidentally, I used the one piece 930 bearing retainer in my G31 in place of the two piece retainer that is standard.
Here is a list of the factory listed synchro rings:
Possibly could be made from 930 dog teeth
The 933 used 928 synchro rings on 2nd and 3rd gears. The GTS and GTR used 930 synchros on 2nd through 5th gears. The 930 synchros are the same design as the 928 synchro with the notch around the bottom outside of the ring. These would probably fit 2nd and 3rd gear on the 016Y & Z along with the G31/01-02-03. The 930 4th and 5th gear synchros need different dog teeth for these synchros to fit. If you tried to put the 930 4th and 5th gear synchros on the standard dog teeth the installed diameter would be too small.
It is interesting to note that the 915 input shaft is only slightly larger in diameter than the early 924 n/a. The 915 has a 22 mm (7/8”) 20 spline and the early 924 n/a has a 20mm (13/16”) 24 spline. The 924 turbo and 944 used a 25mm (1”) 23 spline. It is possible that these transaxles have more torque capacity than a 915. The 911 based transaxle used a 25mm (1”) 23 spline input shaft. The 915 has been rated by Renegade Hybrids to handle 450 HP and Rod Simpson Hybrids rates it at 450 Lbs. Ft. of torque so the same should be true of the snail-shell transaxles.
G31 Ring and Pinions
9:33 (3.6666) Special Competition Ratio
9:31 (3.4444) Special Competition Ratio
G31 Standard Ratios
The G31 and O16Y/Z use different 1st gear synchronizer parts. The G31 uses synchro parts from a 915 and the 016Y/Z use synchro parts from the 928. The design is different enough so that they are not interchangeable. The dog teeth for 1st gear on the G31 is specific to the G31. It is similar to the 930 dog teeth and the 930 dog teeth can be modified to fit. The 016Y/Z dog teeth are the same for 1st gear as 2nd and 3rd.
There is a different tooth count between the G31 and 016Y/Z and the G31 uses a asymmetrical tooth design where the 016Y/Z uses a symmetrical design. (see below)
These were available in multiple configurations. These can be broken into 5 groups. 924 4 spd, (actually an 088); 924 5 spd (early); 924 turbo 5 spd (81-82 US)/944 (016); 924S (86-88); and 944S/S2 / 944 turbo. (083) (early = 76-82 US & 76-84 R.O.W.)
There are two groups as far as strength. The strongest is the 944S/S2/turbo (083). All of the others are essentially the same. These are identified as follows:
924 n/a 4 spd. 088 --- YR; XT
924 n/a early 016 ---codes ME; MF; VQ; VR; UV; MD
924 turbo 016 ---codes MB; MX
924S 016 ---codes MQ; 7Q
944 016J; 016K---codes QM; 8Q; 5Y; 5Z; 5S; 7V; ASG; ASH
944S/S2 083D; 083F---codes AGP; AGR; AOS; AOT; ASV; ASW
944 turbo 083 ; 083 ---codes AOR; 5R; 5P; UY; 9U; ASG; ASH; AOQ
(ASG; A0Q; 5R; 5P have oil cooler)
924 n/a 4 spd. has YR 9:31(3.44); XT 9:35 (3.89)
924 n/a early has 9:37 (4.11) R & P
924 t has 9:35 (3.88) R & P
924S has 9:35 3.88) R & P
944 early has 9:35 (3:88) R & P
944S/S2 has 8:31(3.875) R & P
944t has 8:27 (3.375) R & P
The model 083 transaxles differ from the 016 transaxles in a few key areas. First, the ring & pinions are stronger. There is added strength here because the front pinion bearing is larger, the ring gear is .118” larger in diameter and the gear pattern was redesigned. 2nd, 3rd and 4th gears are wider. This makes the cast iron case along with the pinion shaft, main shaft and shift rods 0.412” (10.5mm) longer total. Third and fourth gears on the 85.5 and newer 016 & 083 transaxles are not just an interference fit to the pinion shaft as the previous 016. Both gears now have splines to increase torque capacity. The turbo version transaxles are available with or without coolers and all have the option of limited slip differentials.
The 944, 924S and 924 transaxles are essentially the same with the exception of the early 924 n/a transaxles, which have a 20mm (13/16”) input shaft and 3rd & 4th gears on the 924S are splined. The smaller shaft does not appear to have much impact in the strength. It is interesting to note that all of the AUDI 016 transaxles also share this size (20mm). The Audi 016 is a very popular transaxle for GT40 and Lamborghini replicas and the input shafts have no history of breakage. This input shaft size is also only slightly smaller in diameter than the Porsche 915 transaxle. The 915 has a 22mm (7/8”) input shaft.
The parts that typically break on the Audi 016 when used in V8 powered replicars are in this order:
1. Pinion pushes through the rear pinion bearing seat on the iron
casting—R & P failures in 944’s are related and are likely from worn pinion
bearings and not changing the oil.
(the pinions pushing through the rear casting is a function of dropped
clutch starts [catastrophic] and the common 944 failure is from long term
usage with worn [loose] bearings and no oil changes.
2. Axle shafts
3. Differential spider gears
The GT40 and Lamborghini Replicars don’t see the ring & pinion failure that the 944’s have. The talk is that this is caused by lack of maintenance (routinely changing oil) and worn pinion bearings. They only see the catastrophic failures.
The only differences that I can find between the Audi 016 transaxle and the early 944 transaxle are the input end of the mainshaft is smaller in diameter (20mm vs 25mm on the 944) and machined for a pilot bearing, the ring & pinion ratio along with some casting differences allowing bosses for a slave cylinder, fork pivot ball and t/o bearing guide sleeve.
The GT40 and Lamborghini Replicars also use high HP engines and the 016 holds up. A strengthening plate was designed to give added support to the rear pinion bearing which solved the casting breakage. The next area of breakage is axle shafts. When these are strengthened, the spider gears in the differential break. This, however, is fixed by using an LSD. (4 spider gears instead of two)
The 944 turbo ring & pinion has been retrofitted into the Audi 016 (should be possible in the early 944 too). Since the pinion shaft is .412” longer, a spacer must be made to go between the front case and the cast iron case. Also, 4th gear has a shoulder of approximately .15” wide. You can turn it around to keep it lined up with the matching gear. Some shimming is necessary for 3rd gear. The shift shafts also need to be lengthened by .15”. The front pinion bearing hole in the case needs to be machined larger .3.497±.001. Then the case needs to be relieved around the ring gear since it is .3” larger in diameter. This will strengthen the 944 transaxle by the addition of the stronger ring & pinion along with the larger front pinion bearing. The 944S/S2 cases already use the larger bearing and have been machined in the ring gear area and have a new 8:31 (3.875) ring and pinion the same diameter as the 944 turbo, 8:27 (3.375). The turbo ring & pinion can just be a swapped into these.
016/083 Standard Ratios
There are many differences between these two transaxles. Some are significant and related to strength. The most obvious difference can be seen by just looking inside of the car. The 016/083 has a conventional shift pattern and the G31 has a racing shift pattern with first gear to the left and down with reverse above it. There are major differences in the mounting, the shift mechanism, weight distribution, synchronizers, differentials, ring & pinion gears, and bearings.
The 016 early (85 and older) transaxle is mounted through insulators attached to a cross member on the body. The late cars (85.5 and newer) have the crossmember mounted to the transaxle with an insulator and the crossmember is mounted to the body. The G31 is mounted to two beams extending from the trailing arm inner mounts like the 016. Instead of hanging from the mounts, it sits on the mounts.
The 016 shift lever has a ball on the bottom and this fits into a nylon socket mounted to the torque tube. The shift rod attaches to a pin on the lever and extends back to the transaxle.
The G31 has a rod that has a socket that fits over a ball that is mounted on the rear of the torque tube. This attaches to a pin on the shift lever. The rod that attaches to the shift rod on the transaxle also attaches to a pin on the shift lever and also extends forward to a rubber insulated mount mounted to the tunnel. This was done to try to eliminate drivetrain vibration in the shifter. This mechanism works well unless components become worn and loose.
The 016 has the transmission integrated behind the differential placing the weight behind the axle. The G31 has the transmission integrated in front of the differential which places the weight in front of the axle. The G31 is also lighter at 99 pounds VS 129 pounds for the 016. This does effect the weight distribution of the car slightly.
083 - 016 similar
The gears between the G31 and the 016 are close to the same size as far as gear thickness and tooth design. The diameters and tooth counts differ. The G31 gears have much more finishing work performed. The teeth are all chamfered on the sides where the 016 gears do not. The G31 gears look as though they have had some kind of surface finishing performed too and the surfaces are all smooth and free from any sharp edges. The gears from the 016 do not have this extra finishing. The G31 also has a tooth helix about 3 degrees less than the 016. This increases the area of tooth contact as well as reducing the thrust loading between the gears, surfaces and bearings.
The 016 uses the Borg Warner cone type synchronizers which work quick and positively. The G31 uses the Porsche balk ring type synchronizers, which makes shifting a little slower. Many people do not like the way these shift. Too much speed shifting also causes premature wear on the synchronizers, which contributes to their bad reputation.
balk ring type
The mainshafts are very different. The 016Y/016Z/G31 is short without many diameter changes. This is a strong design. It also limits the amount of twist from one end to the other. The 016/083 has a long mainshaft with many diameter changes. It is about 1½ times as long as the 016Y/G31 which means that there will be about 1½ times the twist. The diameter changes also add high stress areas. However, there is no history of mainshaft breakage in either.
The G31 uses the same type of bearings as the 915 and 930 transaxles. The mainshaft has a cylinder roller bearing and a four point ball bearing at the differential end and a cylinder roller bearing on the input end. The pinion has the same types and placement, a cylinder roller bearing and a four point ball bearing at the differential end and a cylinder roller bearing on the input end. These bearings are all larger than the bearings used in the 016. The four point ball bearings bear the axial loading and the cylinder roller bearings bear the radial loading. This is a very robust and high load set up.
The 016/083 mainshaft uses a smaller four point ball bearing on the back end, a small cylinder roller bearing in the center housing and two needle roller bearings on the input end of the mainshaft. The bearings are all smaller than the G31 and the shaft is much longer. The pinion has a tapered roller bearing on each end. These take the axial and radial loads together. The proper preload is essential for long life and reliability. The problem with this is it only takes a slight amount of wear to loosen the preload to the point that it will affect the life of the ring and pinion.
These pictures are self-explanatory. This also gives you an idea of the difference in size of the shifting sleeves.
For anyone that is familiar with the DANA 44 axle, here is a comparison of the 016, G31 and DANA 44 ring and pinion gears.
As you can clearly see, the DANA 44 ring gear is larger in diameter but tooth width is narrower than the G31 and is very close to the 016. The pinion gear shaft is smaller in diameter than both the 016 and G31. The DANA 44 is a very strong gear set and equivalent to the GM 8.5” corporate gear set that was used in 1000’s of cars including the 60’s and 70’s Camaro and Firebird.
The G31 uses spiral bevel gears and the 016/083 uses hypoid gears. The spiral bevel gear set has the pinion centerline in the same plane as the ring gear and the hypoid pinion is offset (usually lower) from the ring gear. There are pros and cons for each type.
The spiral bevel type has larger teeth (for the same size gears) than the hypoid and is a more efficient design but noisier. The hypoid gears are quieter but are less efficient. The hypoid is somewhat stronger than the spiral bevel because there is more tooth contact than the same size spiral bevel and has a wiping like contact as the gears mesh. But this is what makes them less efficient. It also generates more heat due to the added friction----this is also what lowers the efficiency. But the hypoid design also allows the final drive output to be higher than the driveshaft which allows a more compact drive train.
But as for the G31 and the 016/083, the G31 gear teeth are much larger and have slightly more tooth contact area than the 016/083.
The spiral bevel design is what is necessary to be able to flip the ring gear, (as with the 915 and 930) which allows the transaxle to be turned 180° for use in mid-engine cars.
ring gears and attaching bolts
The differential case for the G31 is considerably larger than the 016/083. Notice the larger cross section in the center part of the G31 differential case. You can also see the larger side bearings and thicker ring gear flange. According to the SVF catalog, the G31 side bearings have almost twice the load capacity of the 016/083 bearings.
The G31 side gears and pinion gears are also beefier. The G31 side gears are thicker and the gear teeth have a slightly larger cross section. These are the side gears.
Although The G31 and 016/083 transaxles share the same CV joints, the output flanges are considerably beefier on the G31. The shaft diameter is larger and the flange has a thicker cross section along with thicker bosses for the CV bolt threads.
The G31 uses 10 x 1.25 bolts to attach these and the 016/083 uses 7 x 0.75 bolts. Another advantage of the G31 is that the late 915 and 930 output flanges are a direct bolt on which allows for the use of 930 (208 mm) CV joints.
Other G31 Options
I am using the G31 in my V8 powered car because I believe it to be much stronger. However, based on my research, the 016/083 is not necessarily week. There are a few things that can be done to add significant strength to these. Unless you can’t keep from dropping the clutch, there may not be any reason to swap to the G31.
To change transaxles to the G31, you need the transaxle, the torque tube, the shift lever and two shafts, the front shaft mount insulator and either the correct suspension torsion tube or modifications to the torsion tube to mount the transaxle. (either lengthening the inner swing arm mounts or fabricating brackets to use the 016/083 top mounts)
As stated earlier, the 915 ring and pinions can probably be successfully used in a G31. This means that the 8:31 (3.875) can be used along with the 10:31 (3.10) available in the aftermarket. This could give you options other than the European 8:33 (4.125) which in the US is difficult to come by. I ended up buying a transaxle from someone in the UK and shipped it here. The 915 billet bearing retainers directly bolt on along with the billet differential case cover.
I am also considering using the 016Y input shaft so I can use the 2.7857:1 first gear ratio. This gives close to optimum ratios for the V8. (see below) But this would require a special coupling that has the 25mm (1” x 23) on one end and a 20mm (13/16 x 24) on the other. Another possibility would be having the G31 first gear modified like is common for the 915. First gear is machined off and a new gear is welded on. The first gear from an 016Y or Z could be used. Then you would also need 1st driven gear, the slider and the reverse idler from the 016Y or Z.
If this doesn’t work, there is the option of using the European 8:33 (4.125) ring and pinion in the 016Y. (see below) Other options would be to try to locate a used racing gear set, either the 9:33 (3.66) or the 9:31 (3.44), but this is not cost effective. I found a supplier for these that had NOS gear sets but wanted €3000 (approx. $4100 as of this writing) each. There is also a 10:31 (3.10) aftermarket ring and pinion available for the 915 from Albins Off Road Gear.
Another simple change allows you to use 930 108mm CV joints. All that is necessary is to swap the 930 output flanges with the stock G31 100mm output flanges. You need the fine spline output flanges used on the 78 and newer. And if you are using 924 or early 944 steel rear swing arms you can buy forged 930 108mm stub shafts from VW off road suppliers for about $150 pr. that bolt in. You would also need the 930 splined CV shafts which are available from the same VW suppliers as well. The 924/944 CV joints use shafts with 33 splines and the 930 uses CV joint shafts with 28 splines.
Using 930 108mm CV joints increases torque capacity dramatically. After doing some calculations based on the bolt size, bolt circle, bolt grade and contact surface area I came up with some torque capacities. (all done with bolts tightened to specified torque) The standard 100mm CV joints use M8 x 1.25 grade 12.9 bolts. With greasy flange surfaces it works out to about 950 ft. lbs. and with clean/dry surfaces 4700 ft. lbs. The 930 108mm CV joints use M10 x 1.5 grade 12.5 bolts on a larger circle. With greasy flange surfaces it works out to about 1600 ft. lbs. and with clean/dry surfaces 8000 ft. lbs. However, this does not take in consideration fatigue.
With this information you can see the benefit of using the larger CV joints, especially if you do drag race like starts. It can also be seen that keeping the flange surfaces clean and dry during assembly is very important.
The 968 01E 6 speed transaxle is based on the AUDI 01E design and previous 016 design with a few updates from the 083 version. However, the Porsche version did not receive any of the AUDI 01E updates.
The update that this transaxle got from the 083 version is the slightly larger ring gear (.3”), the larger pinion bearing, and the wider 3rd gear. (.15”)
The case has one major difference is the design, the differential cover is on the opposite side. This allows the use of the same basic design ring and pinion as used in the G50. In order to do this and keep 6 speeds forward was to flip the differential. And to ease assembly the cover was moved to the opposite side.
One update the 968 01E did not get is a modification to first gear that was done to strengthen the 01E for AUDI 5000 turbos. First gear was made wider to higher torque capacity.
There are various other small differences between the AUDI and Porsche versions, but none that would affect strength other than the first gear modification.
Based on what I have found this transaxle should be equal in strength to the 951 083 5 speed. So, other than different gear ratios, there is no strength advantage. As a side note, the differential in the 01E is the same as used in the G50.
I have been reading that some are saying that the G31 is not strong because of the ring and pinion ratio. Even the 7/35 (5.0:1) has significantly more tooth contact area and cross section than any of the 016 ring gears. Every one of these people that I have replied to only knew of the 4.7143 ratio. There is a 5.0 ratio, (from the 016Z) a 4.125 used in the ROW (rest of world) cars and the GTR and GTP used 2 special ratios: 3.66 & 3.44. None of them has actually seen one of these transaxles or made any comparisons with the AUDI based transaxle internals. All of the ring and pinions are stronger in the G31 because besides having a larger ring gear, they have wider and taller teeth and a thicker cross section than all of the AUDI 016 based transaxles. All of the diff housings are physically larger and all have thicker cross sections, larger and additional ring gear bolts, larger side and spider gears, and a larger diameter spider gear shaft.
The gear teeth in the G31 are also cut at a slightly lesser angle than all of the AUDI gears. This makes the teeth stronger by increasing the area of tooth contact as well as reducing the thrust loading between gears, surfaces and bearings.
By Peter Tinucci
about the author:
I have been a car fanatic since I was a little kid. I used to help (watch really) my dad do tune ups to the family car and his business van. At that time my parents had a large piece of land in central Wisconsin with a small house where I spent most of my summers with my mom, my sister and my brother. We had an old Willy's Utility Wagon there that I learned to drive when I was 11 years old. When the engine developed a rod knock, this became the first vehicle that I performed an engine swap on. This was 1974 (my Junior year in HS) and my dad bought the adapter and engine mount and I found a 307 Chevy v8. Around about 1981 I ended up doing major work to this Jeep. I swapped a 455 Pontiac and turbo 400 automatic trans into it along with a GM 8.5” 10 bolt posi disc brake rear axle. I used a Pacer front suspension, which gave it a modern wishbone front suspension with disc brakes and power rack and pinion steering.
Since then I can’t seem to leave any of my cars alone. I have owned 9 classic muscle cars 3 924’s a 1976 911S coupe and my current 1984 944. The first 924 I owned, I rebuilt the engine and swapped transaxles to a 5 speed from the original 4 spd. This car became the first Porsche that I performed an engine swap. At the time, I had a 231 CID Buick turbocharged v6 from a Regal. This was the original T-Type carbureted pre-fuel injection engine. I made up mounts and a bellhousing adapter and since I couldn’t use the original carburetor without cutting a big hole in the hood, I used a side draft 2 barrel Weber.
The car was fast but needed a lot of additional work and since I was loosing my place to work, I sold it. This was in the early 1980’s. My current car is a 1984 944 that I am building as a 924 Carrera GTS clone except it is powered with a 500+ HP 302 Chevy v8. This is what prompted me to research transaxles. As stated in my article, I have a 1980 G31 US transaxle with a European 8:33 (4.125:1) ring and pinion along with the European .706:1 5th gear. I replaced the US 7:33 (4.714:1) ring and pinion because I wanted a better first gear ratio and the .600:1 5th gear I felt would make the engine lug when driving on the highway.
I have loved the 924 Carrera GTS since I first saw one in 1982. Although like many people, these cars have been a little out of my budget. There was one on E-bay recently that didn’t sell for $110,000!
I guess my car interests and mechanical abilities helped me to make the decision to go into the engineering field. I have been a Mechanical Engineer for over 27 years.
and 944Hybrids http://944hybrids.forumotion.com/ forums using the screen name v8carreragts.
Revised January 4, 2012
© 2007, 2010, 2011, 2012 by Peter Tinucci All rights reserved.
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