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Pelican Technical Article:

Transmission Upgrade

Mitchell Sam Rossi

Applicable Models:

Porsche 911 (1972-86)


This article was originally published in HPBooks' Porsche 911 Performance: Building the Ultimate 911 for High Performance Street or Road Racing, by Mitchell Sam Rossi. Rossi is a contributor to European Car, Road & Track, Sport Compact Car and the Los Angeles Times. As part of our selection of vintage 911 must-reads, this book provides a comprehensive look into the high-performance modification of 911's for the street and track. Covering topics ranging from engine building to chassis strengthening, this 144-page guide will certainly take your venerable 911 to the next level of performance. Click here to order this book.

Figure 1: The early 901 five-speed transmission.

Most 911 enthusiasts love their car's sensuous styling, their wind slipping form, the synthesis of steel and glass. Some enjoy the raw power, the speed, the handling. Others relish the harmony of the exhaust note or bask in the cocktail party value of announcing they drive one of the ultimate touring cars.

Personally, when it comes to my 1970 911S, I love the five-speed transmission. Admittedly, saying that you fancy a gearbox is not the most tantalizing of conversation starters, even amongst the purest of Porschephiles. My confession usually garners a smirk or a roll of the eyes. Only occasionally, maybe once or twice, have I detected a slight nod from a soul mate, someone else who shares my affection for the superb 901 gearbox.

Figure 2: The race-inspired 901 shift pattern.

Unlike Porsche's later five-speeds, the 901 transmission of the 1965-1971 911 utilized a race-inspired "I-H" paradigm for its gear selection. First and reverse gears are located through a spring gate with first gear engaged at the bottom of the "I" and reverse directly above it.

Figure 3: The 915 five-speed transmission case.

To enlist the higher gears, the shifting lever is moved through the gate and into the conventional "H" pattern. This places second gear in the upper left corner of the pattern with the remaining gears in their corresponding order.

Used in nearly identical form in Porsche's 904 and Carrera 6 race cars, the 901 transmission embodied the perfect configuration for competition. First gear, after all, is primarily for leading the procession out of pit lane. From that point on, the driver's access to second through fifth gears is forthright and effortless.

In 1972, when Porsche increased the 911's displacement from 2.2-liters to 2.4, the 901 gearbox was considered ill-equipped to handle the extra horsepower. A new gearbox, designated the 915, was derived from its competition sibling, the 916 five-speed trans-axle. This transmission proved itself more than capable, having managed the 350 horses of Stuttgart's incredibly successful 908 racers. In fact, except for the 911 Turbo, the 915 was mated to every 911 motor until 1987.

Figure 4: The commonplace shift pattern of the 915 five-speed transmission.

Unfortunately, when the 915 was designed, Porsche concluded that a majority of their clients were sitting in traffic jams more often than on a starting grid and the new 5-speed transmission was given the typical "H-I" pattern. In this layout, first through fourth gears are in the standard pattern. The spring-loaded gate separates the fifth and reverse plane to the right.

Figure 5: The 8:31 ring and pinon gears.

Figure 6: The eight-toothed pinion shaft modified to drive a mechanical speedometer.

Thus, when approaching the end of a long fast straightaway at triple digit speeds, the driver must mindfully draw his shift lever out of fifth gear, cross the spring-loaded gate, and pull downward into fourth gear. Go straight back and you will kiss reverse, pull too hard and you'll skip fourth and engage second gear. This classic miss-shift will render the rev limiter useless in saving your valves and rocker arms.

The idea of metallic shards whirling about the crankcase makes me cherish the original gearbox all the more. The 901 shift pattern was so right for the 911S. But if Porsche was concerned that the transmission could not sustain the torque of the 2.4-liter street motor, it could hardly be expected to shoulder the S's more potent 2.7-liter engine or anything larger that may be tucked under the car's rear deck in the future.

I had little choice then to learn to tolerate a shift pattern that was identical to a Honda Accord. The best remedy for grief, I decided, was to assemble the perfect 915 transmission.

Slipping my beloved 901 on the shelf, an early version of the 915 transmission was acquired. While later three-piece 915 cases were made of aluminum, the '72-'77 boxes were made from lightweight magnesium with a load bearing section made from cast iron. This robust segment bears the brunt of the backlash thrust as horsepower is transferred from the pinon shaft to the differential.

Like the 901, the early 915 incorporated a 7:31 ring and pinon gear combination. For the S, this was replaced with the later and much stronger 8:31 ring and pinion. The earlier 8:31 pinon shafts were equipped with a small gear that drove the car's mechanical speedometer. Our particular pinon, however, was from a later car, one that would have utilized an electronic speedometer. To make this pinon shaft work with the S it was modified to accept the speedometer gear at Aasco Performance.

Figure 7: The S's 915 gear chart.

By substituting the 8:31 ring and pinon for the 7:31, the transmission's original gear ratios were no longer viable and new ones had to be chosen for the high revving 2.7-liter motor. The way to determine proper gear ratios is a bit of mathematics, a lot of experience and often a closely guarded secret.

Professional teams and highly competitive club racers may carry two or more transmissions in their racing arsenal. For circuits that are dominated by short straight-aways and tight turns, there is the "short box" built with low gear ratios. Conversely, the "long box" transmission is assembled with taller ratios in order to propel the car down long straights and through broad, sweeping curves.

Those of us who need to keep food on the table must be satisfied with a single transmission that falls somewhere in between or, better yet, geared for the racing circuit we frequent the most.

Figure 8: Assemblage of the 915 transmission.

Another consideration when choosing gear ratios is the engine's torque and power bands. This is best charted on an engine dynamometer but certain factors can be taken for granted. While not every 2.7-liter is built to the same specifications, knowing the compression ratio, engine stoke, cam profile and the particulars of the heads can give a transmission builder a starting point from which to select gear sets. Tire diameter is another factor that must be added to the gear ratio equation.

Jeff Erickson estimated that for the S's motor, the maximum torque would be at 5,600 rpm with max horsepower around 7,100. The rule of thumb, he explained, is to run the engine up from peak torque to peak horsepower. Thus, the gears ratios were chosen so that each shift would drop between 1,000 and 1,500 rpm.

For the gear sets, we turned to Paul Guard of Guard Transmission. Guard carries over 40 different 915 ratios along with gears for all the later 911-based race cars. Used by many professional teams, Guard's gears are designed with thick tooth bases and a special tip relief that places the wear pattern directly in the center of the tooth. After heat treatment, all gear sets are shot-peened for further strengthening.

Figure 9: Guard Transmission's billet steel retainer plate.

From Guard, we also acquired their one-piece bearing retainer plate. Under extreme loads, the pinon and mainshaft have a tendency to migrate away from the ring, hastening wear on the bearings and gears. Guard's billet steel plate secures the two shaft bearings in place alleviating this problem.

Figure 10: The inner workings of the 915.

All new bearings and sliders were installed throughout the transmission. New synchromesh hubs, rings, thrust blocks, guide sleeves and dog teeth were also added along with their asymmetrical update kit.

Figure 11: The 915's gear cluster.

One problem that often plagues the early 915 gearbox was the design of the retaining tabs on the mainshaft. These tabs brace against the second gear, but under the extreme forces of a high horsepower motor they can crack and break free, resulting in a total transmission failure. Porsche rectified this flaw in 1978 replacing the tabs with a complete sleeve.

Aware of this weakness, Erickson installed a late mainshaft in the S's gearbox. Erickson also followed the factory's wisdom by using the improved locking clips for third, fourth and fifth gears. Like the mainshaft, these heftier clips were revamped in 1978.

Figure 12: The WEVO gate held in its internal location.

To soothe my lingering anxiety over the gear pattern, or, more honestly, my fear of mis-shifting the box and sending the 2.7-liter motor into oblivion, I turned to Hayden Burvill of Windrush Evolutions in San Carlos, California. Burvill's company produces several components designed to increase gear selection accuracy of the 915, whether it is nestled in a racer or a street 911.

Windrush's WEVO Gateshift kit replaces the original internal lever arm that operates off the shift rod to actuate the shifting forks. The new lever works in association with a sturdy, internal gate located inside the 915's center housing. Together, the gate/lever assembly provides a guide to the shift pattern. While not able to prevent a mistaken gear change, the geometry of the gate's double H pattern provides strong tactile feedback to the driver thus helping verify each shift.

Figure 14: The WEVO PSJ coupler (upper) vs. the stock coupler (lower).

One thing that needs mentioning is that the 901 shifting rod must be replaced by a 915 rod as it is 16 mm shorter.

Inside the cockpit, WEVO's shifting hand piece offers the driver an even greater degree of control. As mentioned, the original tower uses a spring-loaded gate to separate the fifth and reverse plane from those of the lower gears. But there is no distinction between the planes of first/second and third/fourth.

Figure 15: The stock 915 shift tower.

Figure 16: WEVO's precise shift lever.

To address this, WEVO offers a short throw gearshift lever that is specifically designed to create pressure between these two planes creating a pattern that is, in effect, a "I-I-I" layout. Here, the gearshift lever is designed to favor the neutral position between third and fourth gears. Thus, to change from fifth to second requires far more effort than a correct downshift from fifth to fourth.

While each of Windrush's components can be installed separately, installing the complete WEVO system seemed the best way to subdue my shifting anxieties.

Figure 17: Porsche's ZF limited slip differential.

In 1970, the 911 was offered with a new performance option, the ZF limited-slip differential. Unlike conventional differentials, where torque moves to the wheel with the least amount of resistance, the ZF differential uses preloaded friction disks to transfer torque, in varying degrees, from the spinning wheel to the wheel with the greatest adhesion. Depending on how the friction disks, or clutch plates, are arranged during their installation, the locking force the ZF differential employs can be set at either 40 or 80 percent of total torque.

While my particular 911S did not receive this option, factory ZF differentials are not hard to come by. Purchasing a rebuilt unit, we set the torque to 80 percent and slipped it into the case.

Figure 18: The S's quick spooling billeted aluminum flywheel.

Because the 915 gearbox is larger than the original 901, before final installation could be completed the chassis had to be modified by slightly stretching the transmission tunnel. This can be done with a trusty sledge hammer, something I have employed in the past, but this time, I chose to let the body repair specialists do the alteration since the S was already in their shop for additional work.

At this point in the project, we were not planning to attach an external transmission oil cooler and pump to the 915, but on Erickson's suggestion, this was the time to prepare the gearbox for this possible modification.

Figure 19: Two squirters in the transmission case are ready to spray the ring and pinon and fifth gears.

The case was plumbed with one scavenger line and two return squirters which were positioned above fifth gear and over the ring and pinion. Cooling the oil and spraying it across these critical gears will help ensure longevity when the system is fully installed.

Figure 20: Mated to the 2.7-liter motor, the 915 is ready for installation.

Finally, the 915 was ready giving the S some of the most advanced transmission components employed in club racing Porsches. Only time behind the wheel would tell if I could make use of the robust equipment.

Figure 21: GPE's raised shift tower puts the gear lever right at hand.


While this is not directly related to enhancing the transmission, it is an upgrade that I have found benefits the speed with which I shift gears. Regardless of what gear box is tucked into your 911 you must still reach down for shift knob and then, in the case of the 915 trans, stretch your arm toward the passenger side footwell to engage fifth gear.

If only the shift knob were closer. Maybe not as close as that in a international rally car, although I have seen that too, but just a few inches higher to make gear selection faster and more certain.

Tinsmith and fabricator Urs Gretener of Gretener Prototype Engineering has done just that. Using his experience with early and late 911, Gretener has designed a shift tower that lifts Porsche's shifter mechanism four inches up while also moving it back toward the driver by four inches. This places the shift knob within easy reach regardless of seat position. Although the center tunnel must be cut for the shift rod, the tower itself bolts to the chassis.

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Figure 1

The early 901 five-speed transmission.

Figure 2

The race-inspired 901 shift pattern.

Figure 3

The 915 five-speed transmission case.

Figure 4

The commonplace shift pattern of the 915 five-speed transmission.

Figure 5

The 8:31 ring and pinon gears.

Figure 6

The eight-toothed pinion shaft modified to drive a mechanical speedometer.

Figure 7

The S's 915 gear chart.

Figure 8

Assemblage of the 915 transmission.

Figure 9

Guard Transmission's billet steel retainer plate.

Figure 10

The inner workings of the 915.

Figure 11

The 915's gear cluster.

Figure 12

The WEVO gate held in its internal location.

Figure 13

The WEVO gate's double H pattern

Figure 14

The WEVO PSJ coupler (upper) vs. the stock coupler (lower).

Figure 15

The stock 915 shift tower.

Figure 16

WEVO's precise shift lever.

Figure 17

Porsche's ZF limited slip differential.

Figure 18

The S's quick spooling billeted aluminum flywheel.

Figure 19

Two squirters in the transmission case are ready to spray the ring and pinon and fifth gears.

Figure 20

Mated to the 2.7-liter motor, the 915 is ready for installation.

Figure 21

GPE's raised shift tower puts the gear lever right at hand.

Comments and Suggestions:
antonini Comments: hello
i am looking for a gearbox sportronic for a 911 L 1967.
louis Antonini
February 10, 2017
  Followup from the Pelican Staff: I have posted a thread in our forums, hopefully another member can assist. - Casey at Pelican Parts  
antonini Comments: hello
i am looking for a gearbox sportronic for a 911 L 1967.
loui Antonini
February 10, 2017
  Followup from the Pelican Staff: Posted a note in our forums hopefully another member can offer assistance! Good luck and I like that you're keeping it Sportomatic! - Casey at Pelican Parts  
Claudio Comments: I am considering to buy a 1983 911 sc, gorgeously restored. It has a 4 speed manual transmission. I am confused because every way I search I find that it should have a 5 speed transmission. The current owner says it always has had a 4 speed. Any comments?
June 19, 2015
  Followup from the Pelican Staff: It could have a 901 or 930 gearbox. That is the only way I can think of an SC having a 4 speed. Could also have the incorrect shift knob... If reverse is all the way to the right and down, chances are it was a fifth gear. If it is to the left and up, it could be a 901 4 speed or 930 4 speed. - Casey at Pelican Parts  
Sergei Comments: i want convesion my 911 turbo look 1986 transmission 915 change to tiptronic or G50 transmission??? es posible... tanks for respond.
September 12, 2014
  Followup from the Pelican Staff: I'mnot sure.

I opened a post in our forums. A Pelican community member may be able to answer your question.
- Nick at Pelican Parts
Porschke Comments: I wish to do all the same mods to my 915. Mating it to a hopped up 3.6l. Where can I source all the parts?
June 16, 2013
  Followup from the Pelican Staff: Give our parts specialists a call at 1-888-280-7799. They can help you put together a parts kit.
- Nick at Pelican Parts
larrym Comments: very illuminating - Thank You
February 21, 2013
  Followup from the Pelican Staff: Thanks for the feedback. Glad we could help.
- Nick at Pelican Parts

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Page last updated: Tue 2/20/2018 02:17:40 AM