Final Drive Gear Ratio
One way that manufacturers achieve fuel efficency is by gearing the cars longer to provide low rpm at highway speeds for better economy. A plausible explanation for the stock 4.1 ratio. Theoretical top speed at 7400 rpm is 270km/h or 168mph, which is unachievable with the stock engine and gearing.
Reducing the final drive ratio increases the torque multiplication to the road for every gear. The cost to this is top speed in each gear. Given that this car was never about the numbers, sacrificing 0-100 km/h (0-60mph times) fits in our objective on improving driving feeling.
The two main options for manual cars are to move to the 4.56 or 4.8 ratios. In keeping with trying not to deviate too far from the original choice by Toyota, the 5.1 ratio will not be covered.
Stock 4.1 Final Drive:
2nd Gear Top Speed: 61 MPH
Crusing @ 60 mph: 2600 RPM
Top Speed: : 174 MPH
Cusco 4.56 Final Drive:
2nd Gear Top Speed: 55 MPH
Crusing @ 60 mph: 2900 RPM
Top Speed: : 157 MPH
MFactory 4.8 Final Drive:
2nd Gear Top Speed: 52 MPH
Crusing @ 60 mph: 3100 RPM
Top Speed: : 149 MPH
Two videos can illustrate the difference in shift points on Tskuba track in Japan. This is a stock car with the 4.1 ratio, and here is the TRD Griffon with a 4.8 ratio. This is not a great comparison since the latter is much lighter, however the drivetrains in both cars are exactly the same.
The final decision was to go with the 4.8 rear end given the car's usage, an individual with a more highway biased commute might want to go with the 4.56. It is to be noted that in 2017 the FRS / GT86 / BRZ will move to a 4.3 rear end, This is a small clue to suggest how the engineers are developing the car. In addition, the Toyota 86 TRD Griffon also uses a 4.8 final drive, which shows it's more extreme application
O.S. Giken 1.5 Way LSD
The stock Torsen LSD is flawless except under a very specific set of conditions. Under high cornering load, the inside wheel may lift high enough that all power gets transferred to it, which effectively makes the differential open.
During aggressive cornering over chicanes, corner exit or drifting this causes an interruption of power which will resolve as contact returns. Based on my style of driving and objective to make this a fun car to drive, I chose a 1.5 way lockup. Here are more details on the O.S. Giken Superlock LSD .
The following graphic from Cusco explain the 1, 1.5 and 2 way options available. Given my tendancy to hit curbs hard, this will help lap times.
Update!
Since installing the 4.8 final drive, my fingers got a little itchy and I went with a 5.1 final drive to push the limits of the gearing possibilities. Since then, there has been a 5.26 version that appeared as well. Like everything, there is a law of diminishing returns and the 4.8 might have been the best compromise with the stock transmission ratios. Again, this is heavily dependent on individual application to find the right ratio for you!
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