Transmissions, Flywheels and Clutches:

    901-915-930-G50

-901-

901-series transmissions were used in the 911 from 65 until 71. The last two years' 901's were known as 911 gearboxes since they employed a slightly different case that permitted the new 225mm "Pull-type" clutch. 901's were also used in the 914/6.

These gearboxes work quite well for engines in the 2.0 litres to 2.4 litre sizes without significant modifications. We do recommend installing the aluminum intermediate plate in any 901 that will see any competition. This adds a good deal of strength to help support the shafts. For 2.7 litre engines and larger, we also recommend installing a cooler and pump. Race cars should also get a pressurized lubrication system that locates an oil jet at each gearset and the ring & pinion.

These transmissions can be regeared quite successfully for racing and street performance. Since 2nd gear is machined as part of the mainshaft, this entire part must be changed when replacing 2nd gear unless you use a 904 mainshaft. This special, racing part permits swapping any 904 2nd gear ratio without changing the shaft.

Pictured below is a modified and strengthened 901 transmission. Notice the aluminum intermediate plate that provides much better support to the main and countershafts under high torque loading

-915-

The 915 appeared in 1972 with a major upgrade in strength, compared to its predessor. The first ones didn't shift as well as the later ones due to some improvements in the shifter and syncro's. 915's were made with magnesium cases from 72 until 77. Some of them had aluminum final-drive cases in 77 before the entire gearcase was made from aluminum in 1978. 915's used 7:31 ring & pinions until 1975 when they switched to the stronger 8:31. These later ring & pinions are the preferred item for racing or high-horsepower applications and the later 915's had stronger differentials and mainshafts.

All Porsche transmissions use a very steep hypoid angle in the differential due to the necessity of close shaft spacing. This is a very highly stressed item and requires the use of a GL-5 Specification gear Lube. Since these gearboxes all use the Porsche-design balk ring syncromesh, they depend upon some friction for these brake-band type syncro's to work properly. The best gear lube for a 901-915-930 transmission is the superb Swepco 201 gear oil. Using a synthetic gear oil in these transmissions will result in higher syncro wear and stiffer shift quality.

There are currently more gear ratio choices for the 915 transmission than any other Porsche gearbox. Call us for a custom gearstack recommendation for your car based upon usage and rear tire sizes. A close-ratio transmission will add 50 HP to the performance potential of your Porsche.

As an example, here is an excellent and economical gear stack for a street-driven 911SC or Carrera 3.2 with an 8:31 and 25"-26" tire.

1st Gear-  11:35 (stock)

2nd Gear- 15:30

3rd Gear- 21:31

4th Gear- 24:27

5th Gear- 28:24 or 27:24

This is a very streetable gear stack that will make a big difference in acceleration!

-930-

The 930 transmissions used in the 3.0 litre Turbo, 3.3 litre Turbo and various 934's and 935's is a big, beefy 4-speed gearbox that was built withstand heavy torque loading and high power. Street versions can handle 326 lb-ft of torque and the racing version of this transmission, equipped with an oil pump and cooler, withstood the 625-750 HP of the 935 Group 5 racecars.

Stock Turbos' are geared with top speed and fuel mileage in mind, not for optimal acceleration so there are a few things that one can do to improve performance.

Changing the stock ring & pinion to a lower geared set will enhance acceleration and reduce top speed. Lowering 3rd and 4th gear will also really help since this also closes the large RPM gaps at each shift. Some Turbo owners opt to install a special, shortened version of the G-50/50 5-speed for a much better usable power curve. Porsche, starting in 1989, equipped the Turbo and the new C2 Turbo with this ultra-strong 5-speed transmission.

-G50: 5-speed & 6-speed-

Porsche began installing this improved transmission in the 3.2 Carrera beginning in 1987. This gearbox was designed to handle the increased torque of the 3.2 litre engine and to address complaints about difficult shifting. These transmissions, made by Getrag, use the Borg-Warner style syncromesh and have an input torque rating of 221 lb-ft. Variants of the G-50 5 speed were used until the introduction of the 993 which featured the 6-speed G-50.

Due to fact that Porsche has actively raced 911's since 1990 with the Carrera Cup cars and the RSR's, there is a good selection of racing ratios for these transmissions that allows a custom gearstack to be installed.

Since most of the OEM gearing was selected with fuel mileage in mind, there is lots of opportunity for performance improvements by regearing these cars. The strongest variant of this gearbox is the G-50/50, used in the 89-on 3.3 Turbo and C2 Turbo. The G-50/52 used in the C2 Turbo also featured an asymmetric Limited Slip Differential that had a 20% lockup on acceleration and 80% in braking to help limit trailing-throttle oversteer.

There are some gearing differences between the USA-spec G-50 and the ROW-versions. Installing the lower 2nd through 6th ratios from the Euro-spec gearbox is somewhat expensive but worthwhile when you are limited in engine enhancements due to emissions regulations. For example, in 5th gear, a USA-spec 993 was timed from 45 MPH to 75 MPH in 8.6 seconds while a Euro-spec 993 did this in 7.0 seconds. The engines are both rated at 282 HP. The G50/21 3rd-4th-5th-6th gearing is an excellent upgrade for any 993.

For racing, the G-50 has experienced some difficulties with ring & pinion failures. Rennsport Systems recommends installing a  transmission cooler and pump when power levels reach the 350 HP level and above. Pressurized lubrication to the ring & pinion as well as the gearsets is also worthwhile under these stresses.     

 Here is a Six-speed G-50 Transmission from a 97 993 3.8 RSR modified with additional cooling and pressure lubrication

Rennsport Systems has special, racing gears ratios for 901, 915, 930 and G-50 transmissions for street use or track. We can design a special gear stack for your specific application using your Ring & Pinion ratio and tire size.

- Flywheels & Clutches -

Here is the new Aluminum flywheel for the 2.4-2.7-2.8-3.0 litre engines with 6-bolt or   9-bolt crankshafts. These weigh 4.7 lbs and are compatible with any Sachs OEM clutch. These work very well in street or racing applications and will enhance engine acceleration and ease shifting for 915-equipped Porsches.