Why not swap out the 4-spd and install a 5?
Remember the old days when automatics equip with four forward gears, and when manual has the 5th gear? This extra gear in that era had the same resemblance of hearing the word hybrid to the present buyers. From day one, it is a universal understanding that more gears is the recipe for maximizing engine efficiency such as better fuel economy, and quieter engine at freeway speed. Some gear box even offered reduction in gear ratios between each gear to provide performance enhancement.
The current trend in the automobile industry fully supports this up-the-number claim as well. Most econobox now comes with 6-spd, the luxury models have up to 8-spd like BMW’s new 3 series, and Fiat is currently working on a 9-spd automatic. So… had we considered replacing the 4-spd manual transmission in our LeMons car with a 5-spd also found in 1989 Sentras? Yes, we did but our analysis determined that the 4-spd was sufficient for the job. Here is why:
The Basis – Gear Ratios
The above table indicated that the stock unit (RN4F31A) has the same gear ratios as the first four gears as the 5-spd (RS5F31A) unit. The only difference to the RS5 is the extra 5th gear and its shorter final gear.
By eyeballing the specs, the shorter final gear suggested quicker acceleration in the first four gears and lower engine rpm in top gear with the RS5. It looked good so far since we wanted squeeze each and every single ounce of horsepower out from the engine by the shorter final gear, and at the same time reduce wear-and-tear in the high speed section from the help of the 5th gear. However, in depth analysis suggested the opposite.
Let’s put the calculator to work, first we’ll look at how the difference in final gear ratios (3.65 vs. 3.79) affect vehicle speed at engine redline (6,000 RPM).
From the above table, with the stock RN4 the car will be going at 34.1 mph in 1st gear at 6,000 rpm while RS5 the car will be 1.3 mph slower…etc.
The 5-spd’s short final gear would help on accelerating out of a corner but with only 70hp at the wheels the benefit of “slightly” shorter gearing is insignificant. Considering the engine makes most torque at 3,600rpm and the 12-valve SOHC head would run out of breath pass 5,000 rpm. There’s nothing to gain in high rpm = insignificant gain from the 5-spd.
Experience told us that our Sentra would max out at 105 mph in Buttonwillow. Therefore, the critical gears are 3rd and 4th. There is no benefit for the 5th gear as the top speed can be reached in 4th. The longer gear in 5th might cause the car to drop below 105 mph = there’s no need for a 5th gear.
Therefore in performance perspectives, 5-spd isn’t as beneficial as it may seem by eyeballing.
Second, let’s look at the differences in engine rpm at preset vehicle speed in fourth gears for wear-and-tear analysis.
This table suggested that when traveling at 80 mph, the engine revs at 3,916 rpm in RN4, while the RS5 would actually make the engine spin 150 rpm faster which suggested that there is NO benefit of using the 5-spd tranny in terms of reducing tear and wear on the engine as the taller final gear in the 4-spd happen to make the engine rev lower than the 5-spd. Therefore in wear-and-tear perspective – keep the 4-spd.
Unless we can get a 5-spd tranny off of a Nissan Pulsar NX with even shorter final gear then the benefit of shorter gear ratio to help acceleration would be higher. But that tranny is as hard to find as the Pulsar NX. So we retained the stock 4-spd and it served us well. End