How To Install 350z Final Drive

So, you're thinking about swapping out your 350Z's final drive. Maybe you're craving quicker acceleration, or perhaps you're looking to optimize your gearing for a specific type of driving. Whatever your motivation, changing the final drive ratio is a surefire way to alter your car's performance characteristics. But before you dive in, let's break down what a final drive is, why you might want to change it, and the process involved in making the swap on a 350Z.

The Final Drive: More Than Just a Gear

Imagine a bicycle. You have the pedals (representing your engine), a chain (the driveshaft), and the gears on the rear wheel. The rear gears determine how many times the wheel rotates for each revolution of the pedals. This is essentially what the final drive does in your car. It's the last gear reduction step before the power reaches the wheels. It takes the rotational speed from the transmission and further reduces it, increasing torque in the process.

Think of it like this: you're trying to move a heavy box. Do you want to take small, quick steps, or fewer, more powerful strides? A lower final drive ratio (numerically higher, like 4.08 compared to 3.54) is like taking those quick steps – more torque to the wheels for faster acceleration, but potentially at the cost of top-end speed and fuel economy. A higher final drive ratio (numerically lower, like 3.54) is like taking longer strides – less initial torque, but better suited for high-speed cruising and fuel efficiency.

How it Works Mechanically

The final drive is housed within the rear differential. The differential itself allows each wheel to rotate at different speeds, which is crucial when cornering. Without it, the inside wheel would have to spin at the same speed as the outside wheel, causing tire slippage and making turning difficult, if not impossible. The final drive gears, usually a ring and pinion setup, are the primary components responsible for the gear reduction. The pinion gear, connected to the driveshaft, meshes with the larger ring gear, which is connected to the differential case and ultimately the axles.

When the pinion gear spins, it turns the ring gear. The ratio of the number of teeth on the ring gear to the number of teeth on the pinion gear determines the final drive ratio. For example, if the ring gear has 41 teeth and the pinion gear has 10 teeth, the final drive ratio is 4.10 (41/10 = 4.1). This means the pinion gear needs to rotate 4.1 times for every single rotation of the ring gear, and subsequently, the wheels.

Why Change the Final Drive? Scenarios and Considerations

The most common reason for swapping a final drive is to improve acceleration. A lower (numerically higher) ratio provides more torque at the wheels, making the car feel quicker off the line and out of corners. This is particularly beneficial for autocross, drifting, and spirited street driving. However, it's crucial to understand the tradeoffs.

Pros of a Lower Final Drive Ratio (e.g., 4.08, 4.36): Faster acceleration, improved throttle response, better suited for shorter tracks.
Cons of a Lower Final Drive Ratio: Lower top speed, reduced fuel economy, higher engine RPM at cruising speeds, potentially more tire wear.
Pros of a Higher Final Drive Ratio (e.g., 3.54, 3.36): Higher top speed, improved fuel economy, lower engine RPM at cruising speeds, potentially less tire wear.
Cons of a Higher Final Drive Ratio: Slower acceleration, reduced throttle response, less suited for short, technical tracks.

Use Cases:

Drifting: Many drifters prefer a lower final drive (4.08 or even higher) to help initiate and maintain slides. The increased torque makes it easier to break traction and control the car at extreme angles.
Autocross/Track Days: Depending on the track layout, a lower final drive can significantly improve lap times by allowing the car to accelerate faster out of corners.
Daily Driving: If you primarily use your 350Z for commuting or long highway trips, sticking with the stock ratio or even going slightly higher might be more suitable for fuel efficiency and comfort.
Forced Induction (Turbo/Supercharger): When adding forced induction, a taller gear can help tame wheel spin and put the power down more effectively.

Manufacturer Examples: Nissan offered different final drive ratios in various 350Z models and trim levels. The base models typically came with a 3.54 ratio, while some performance-oriented models, like the Track edition, featured a slightly shorter ratio (3.50). Aftermarket manufacturers like Tomei, OS Giken, and Cusco offer a wide range of final drive ratios to suit different driving styles and applications.

The Installation Process (Overview - Not a Step-by-Step Guide)

Installing a new final drive is not a simple bolt-on procedure. It requires specialized tools, a good understanding of differential mechanics, and meticulous attention to detail. This is a job best left to experienced mechanics. Attempting this yourself without the proper knowledge and tools could result in serious damage to your differential and potentially render your car undrivable.

Here's a general overview of the process:

Removal of the Differential: The rear differential needs to be removed from the car. This involves disconnecting the driveshaft, axles, and any associated brackets and hardware.
Disassembly of the Differential: The differential case is opened, and the existing ring and pinion gears are removed. This requires specialized tools to overcome preload and properly disassemble the components.
Inspection and Cleaning: All components, including bearings, seals, and the differential case, are thoroughly inspected for wear or damage. The case is cleaned to remove any debris.
Installation of New Gears: The new ring gear is installed onto the differential carrier, and the new pinion gear is installed into the differential housing. This requires precise measurements and adjustments to achieve the correct gear mesh and backlash.
Setting Backlash and Preload: Backlash is the amount of play between the ring and pinion gears, while preload refers to the amount of force applied to the bearings. These settings are critical for proper gear engagement and longevity. They are typically adjusted using shims and spacers.
Reassembly of the Differential: The differential case is reassembled, and new seals are installed.
Installation of the Differential: The differential is reinstalled into the car, and all components are reconnected.
Final Checks: After installation, the differential is thoroughly inspected for leaks and proper operation. The car is test-driven to ensure the new final drive is performing as expected.

Tools Required (Illustrative List):

Socket set
Wrench set
Torque wrench
Differential carrier spreader (essential for removing and installing the carrier)
Dial indicator with magnetic base (for measuring backlash)
Inch-pound torque wrench (for measuring pinion preload)
Bearing press (for installing and removing bearings)
Shims of varying thicknesses (for adjusting backlash and preload)
Gear marking compound (for checking gear mesh pattern)

Real-World Insights and Recommendations

Before making any changes, consider your driving style and the intended use of your 350Z. If you're primarily a daily driver, a drastic change in final drive ratio might not be the best choice. However, if you're a serious enthusiast who frequents the track or enjoys spirited driving, a lower final drive can significantly enhance your driving experience.

Start with Small Changes: If you're unsure, consider starting with a slightly lower ratio (e.g., going from 3.54 to 3.70 or 3.90) to see how it affects your car's performance. You can always go lower later if you desire more acceleration.

Proper Gear Setup is Crucial: As mentioned earlier, proper gear setup is absolutely critical. Incorrect backlash or preload can lead to premature wear, noise, and even catastrophic failure of the differential. Investing in professional installation is highly recommended. Even seasoned mechanics may choose to bring the differential to a specialist for the gear setup.

Consider the Impact on Other Components: A lower final drive can put more stress on your drivetrain, including the axles and transmission. If you're significantly increasing the torque output, you might want to consider upgrading these components as well.

Tire Size: Changing tire sizes effectively alters the final drive ratio. A smaller diameter tire gives similar acceleration to a lower gear, but with a corresponding increase in RPM. Remember to take your tire sizes into consideration.

Ultimately, changing your 350Z's final drive is a balancing act. It's about finding the right compromise between acceleration, top speed, fuel economy, and overall driving experience. By carefully considering your needs and understanding the implications of different ratios, you can make an informed decision and transform your car's performance to suit your specific preferences.