Where do we go from here?

Making more power has been put on hold. We need to get the rest of the car up to the task of getting down the strip. We’ve done a bit of research and the weakest links in the driveline appear to be the front differential and the transmission. As for the front differential, Cusco seems to offer both a MZ (P/N LSD 137A) and a RS version (P/N LSD 137F). The pricing is 100,000 yen for the MZ and 108,000 yen for the RS version. From what we can tell from the pictures and limited English in the catalogue, the RS version is spring preloaded to have a slight amount of slip limiting at all times. The MZ version depends purely on acceleration to activate the slip limiting. The RS version also has a total of 10 friction discs as opposed to 8 friction discs in the MZ version. Both designs are of the one-way type for this application, meaning that they limit the slip in acceleration mode only. Based on this information, we are planning to opt for the RS version. We have a few sources to check for U.S. pricing and we’ll pass that along next month.

As for the transmission, OS Giken, Route 6, Phoenix’s Power and R&D all offer close-ratio gear sets. The majority of the sets replace First, Second and Third. Some sets offer and optional Fifth and some others come with the heavy duty input shaft. OS Giken also offers a “heavy-duty” version that is designed to handle additional power. This will be the unit that we investigate most heavily. With the aftermarket transmission gear sets, a variety of different gear ratio sets are available. Sharing one thing in common, all of the sets have a much longer first gear ratio. Whereas the stock box features a 3.214-to-1 ratio, the close-ratio gear sets have first gear ratios between 2.695-to-1 and 2.516-to-1. This provides both a performance advantage and disadvantage. On the plus side, having a long first gear (lower numeric number) allows the inter- gear ratio to be much lower. Whereas the factory transmission went from a 3.214 ratio in First to a 1.925 ratio in Second, an aftermarket set may go from a 2.516 ratio in First to a 1.669 ratio in Second. In the car, you see the difference when you shift at 8500 rpm out of First and the rpm drops to 5000 rpm in Second with the stock gearing or only to 5600 rpm in Second with the example aftermarket gearing. This “closer ratio” keeps the engine near its power peak for more of the run down the quarter mile. Now that’s the plus of the gearing aspect of the aftermarket close-ratio gear set.
On the minus side, changing first gear’s gear ratio to a longer ratio results in less torque multiplication to get the car moving. Using our above example, the 2.516 First would only deliver 78 percent of the torque normally transferred by the factory 3.214 First. Less torque means that it is harder to accelerate our GT-R’s 3500 pounds from a dead stop. Fortunately, there is a way to regain a good portion of this “launch torque” back if we are willing to sacrifice the vehicle’s ultimate top speed. By changing the final drive ratio from the original 4.11 ratio to a shorter 4.60, 4.70 or 4.88 ratio, we can deliver up to 92.6 percent of the original launch torque (with the 4.88 gear) while still having the performance advantage of the close-ratio transmission. As already mentioned, the tradeoff would be in potential top speed. If the maximum redline of the engine was 8000rpm, peak theoretical top speed (if enough horsepower was made) would be 194.9mph with the 4.11 gearing or only 164.1mph with the 4.88 final-drive ratio. How will we decide what gearing will work best? The answer is computer modeling.