Porsche 928gts 5 4 G28

Porsche 993 Repair Manual

Modifications to Transmission Housing and Rear Cover

The transmission housing now features fittings for the connection lines to the radiator in the front end of the vehicle.

Transmission Breather

Modified Transmission Breather Location

The transmission breather in the upper transmission cover has been discontinued and is now replaced by a newly developed breather in the rear part of the transmission (arrow). Observe correct installation position:

When seen in the direction of travel, the vent hole must point to the right-hand front side at an angle of 45°.


Countershaft Gearings

New needle roller and cage assemblies adapted for higher loads have been filled to both ends ol the countershaft. The geometry of the needle roller and cage assembly remains unchanged, i.e they may also be fitted to earlier G28 transmissions.

Reinforced Drive Pinion Bearings in the Drive Shaft

Outside and inside of both needle roller and cage assemblies (1 and 2) have been increased to allow them to support higher loads. For axial support, a drilled thrust washer (3) is fitted and is held in place by a lock ring (4). To improve lubrication, the drive pinion has a bore along its axis.

All Transmission Spur Gears now made of Steel with Increased Load Hatios

The new l7CrNiMo6 gear material is suitable for higher continuous ioads. In addition, the surface of the tooth faces of the loose and fixed gearwheels has been compacted by wire shot blasting to in crease load capacity even further

Differential Gear Teeth Creased

Drive Pinion and Ring Gear

The tooth faces of the meshing teeth of drive pinion and ring gear have been ground after the hardening process. Grinding provides for a high degree of silence when the teeth mesh under load and deceleration. In addition, the reduced circum ferentia! backlash now specified helps to ensure that adverse noise in the load-change mode is avoided. Machining process: Gleason

Setting Dimension E and Circumferential Backlash F

Design dimension R0 = 70.70 mm remains unchanged. Setting dimension E (example: 70,97 mm) is to be adjusted to a tolerance of +0 03 mm

The precision-ground tooth faces allow the circumferential backlash to be reduced considerably. The circumferential backlash recommended (example: F = 0.10 mm) should never be exceeded when adjustments are made,

Since the adjusting washers only permit adjustment in stages of 0.05 mm, make sure the smaller value is within the specifications. When adjusting with drive sets that have a recommended backlash of less than 0.09 (F 0.09 or below), the backlash must never be selected |

below 0 05 mm.


Modified Countershaft Ratios of Transmission G28/57

New countershaft ratio: Zi = 24, Z2 = 35 (was 22:32)

The ratios of gears 1, 2, 3 and 4 thus are slightly longer

Transmission ratios - transmission G28/57 (worldwide)

Zi iz Z2:Zi

Icoim 35:24


IA 30:11

















3rd 4th


22 50










1.3577 1.0000
















Zi = Number of teeth of 1st gearwheel in power flow of corresponding gear

2.2 = Number of teeth of 2nd gearwheel in power flow of corresponding gear iZ = Transmission ratio in gear icoun = Countershaft ratio iSG = Total ratio in manual transmission

VlOOO = Calculated vehicle speed at 1,000 rpm engine speed

V6000 = Calculated vehicle speed at 6,000 rpm engine speed with tires 255/40 ZR17

Spray Diagrams For Change




260 2H0Km/h




260 2H0Km/h

20 40 60 80 100 120 140 1H0 180mph

Nole: The values in the transmission diagram are indicative and are based on a mean effective rolling radius. Minor deviations due to tolerance of tires, change of rolling radius, wear and slip of tires have not been taken into account.

Spray Oil Cooling and Spur Gear Lubrication

The heavily loaded spur gears of the G28/57 transmission are now cooled by spray oil and reccive additional lubrication by an oil pump (1) driven by the differential

The pump operates in accordance with vehicle speed and draws hot transmission oil from the rear transmission housing and feeds it across the ATF line known from the automatic version to the front end of the vehicle into the ATF cooler also known from the automatic version. The cooled transmission oii is then returned along the central tube to the transmission housing. A spray tube supplies spray oi! across special bores to the gears


1 Gear pump

2 - Pressure relief valve

The transmission housing is fitted with this spray tube (3) that projects spray jets onto the gears for countershaft (V), 4th (4), 3rd (3) and 2nd gear (2). At the end of the spray lube (A), a certain quantity of the supplied oil is fed back to the rear transmission section

Porsche 904 Limited Slip Differential

Differential Housing With Porsche Limited Slip Differential and New Friclion Discs

The Porsche limited slip differentials are fitted with new Valeo friction discs. The disc packs consist of: 8 inner discs, part no. 928.332.728 00

7 outer discs, part no, 928.332.726.05 (thickness 1.5)

928.332.726.06 (thickness 1.7)

928.332.726.07 (thickness 1.9)

928.332.726.08 (thickness 2.1)

Adjustment Instructions

• Oil disc to be installed with transmission oil.

• Line up bores of friction discs.

• Stack disc pack in an alternating seguence, starting with a Vaieo inner disc

« Prepare disc pack according to dimension 35.2-0.2 under a load of 50 N (= 5 kg)

Manual Valve Transmission


Only the fully assembled differential housing for the manual transmission is fitted with a drive gear (arrow) for the oil pump used for transmission oil cooling.

The only difference between the differential housings of ibe manual transmission G28/57 and the automatic transmission A28/18 is the fitting diameter of 120 5 mm for the tensioning nut used on the manual transmission. (The nut has an 83-tooth involute spline on its outside diameter to drive the oil pump).

Drilled Drive Pinion

To improve lubrication of the needle roller rings of the drive shaft, the drive pinion is hni low. Oil traps (1) at the head of the drive pinion allow lubrication oil to enter the pinion. The oil then exits at the front end of the drive pinion near the needle roller and cage assemblies.

The oil trap for the bearing assembly (?) is retained and is fitted next to the oil trap for the holfow drive pinion.



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