The 915 Porsche Transmission, a legend in its own right, has powered countless Porsches on both road and track. With its magnesium casing and unique design, it offers a distinct driving experience. This article delves into the intricacies of the 915 transmission, specifically focusing on its application in a racing environment, drawing from two decades of firsthand experience.
Building a Bulletproof 915 for Racing
Racing demands significantly more from a transmission than street driving. High horsepower, aggressive shifting, and the constant stress of competition push components to their limits. Over 20 years of racing with a magnesium-cased 915 in Porsches ranging from 3.6 to 3.8 liters, paired with high-grip slicks (16-inch diameter, 13-14 inch wide) and generating 325-370 bhp, has yielded valuable insights into building a robust and reliable 915 transmission.
Bearing Retainer: Necessary or Not?
A common point of discussion revolves around the bearing retainer. While the WEVO one-piece bearing retainer is a popular choice, opinions differ on its necessity for magnesium-cased 915s. Some seasoned experts, with decades of experience in IMSA and other racing series, argue that the steel inserts in the magnesium case provide sufficient support. They view the retainer as a beneficial addition but not crucial.
Gear Selection for Endurance and Performance
Gear selection significantly impacts performance and longevity. Stock gears have their place, but in a racing environment, specialized gears often prove more durable. Beagle Engineering and Guard Transmission are reputable providers of high-performance gears tailored for racing applications. Guard Transmission, in particular, consistently delivers exceptional quality and durability, essential for withstanding the rigors of competition. Certain gears, especially under racing stress, can wear out faster.
Differential Choices: LSD vs. TBD
The differential plays a critical role in handling and power delivery. While a factory-style ZF limited-slip differential (LSD) is adequate for street use, a racing setup often benefits from a more robust solution. The Guard Transmission TBD (Torque Biasing Differential) and their LSD are excellent choices. The Guard LSD, in particular, offers superior strength and performance compared to the factory ZF unit.
Side Cover Upgrade for Added Strength
Upgrading the factory aluminum side cover to a reinforced unit, like the one offered by WEVO, enhances the transmission’s structural integrity. WEVO, known for its high-quality components designed by experienced racing engineers, provides a side cover that can withstand the extreme forces encountered in racing.
Ring and Pinion: Often Overlooked
The ring and pinion gears are crucial for transferring power to the wheels. Utilizing a factory-style 8:31 ratio offers a balance of performance and durability. However, proper maintenance and driving technique are essential for maximizing their lifespan. Avoiding excessive stress, such as curb impacts, significantly extends their service life.
Conclusion: Building a Winning 915
Building a reliable and high-performing 915 Porsche transmission for racing requires careful consideration of each component. From gear selection to differential choice and even seemingly minor parts like the ring and pinion, every element contributes to the overall performance and longevity of the transmission. Choosing quality components from reputable manufacturers like Guard Transmission and WEVO, paired with proper maintenance and driving techniques, ensures the 915 transmission can withstand the demands of competitive racing.
