Hey Folks,
We've been selling the RS400 and 500 in the previous packaging T45RSxxx since 2007, and have hundreds of them in the market. With the new case, and close ratio high torque gearing, along with more than 15 PerfectFit versions came the RS last month. Our run rate on the RS now far exceeds that of the TKO, so we'll start seeing a lot more of these units in the field soon. There are T45RS owners here on Lat-g.
Regarding the gearing, there are a couple of considerations. First, the Overall Ratio in first we like to see in the range of 10.x-11.x so that you will have good acceleration from a launch. When the Overall Ratio gets up to 13.x, you may likely feel the car is overgeared - especially if you have a big block.
To understand the difference in gearing a little better, put it in terms of torque multiplication. Let's say you have an engine putting out 400HP and 400LBFT over the power band. Multiply the torque by the gear ratio, and that is the output torque generated out of the transmission (example: 400 LBFT x 3.37 = 1348 LBFT torque in first gear with the RS400/500; or 400 LBFT x 2.80 = 1120 LBFT with the RS600). That's torque gain is great from the point that you can bring your car into motion quick. The downside is that your wheel rotational speed for any given engine rpm will be slower with the numerically higher gear (ex, 3.37) than with the numerically lower (ex 2.80) first gear. Also, because of the law of physics that energy is neither lost nor gained, the HP at the output of the transmission is inverse multiplication of the HP at the input ... meaning that HP in first gear is 400 HP / 3.37 = 119 HP for the RS400/500, and 400 HP / 2.80 = 143 HP for the RS600. Now, understand that the RS400/500 does not use more power than the RS600 in first gear. But simply it makes more torque, but equally less horsepower.
Gear multiplication occurs once again at the differential, with the main difference being that the diff affects ALL gears. The 3.73 is perhaps the most popular GM performance gear because it offers considerably more torque multiplication across all gears than say 3.36. So the 3.73 happens to be what I have found is on the end of the scale for pairing with the RS400/500 and a big block or hot small block. So, 3.9 and numerically higher we generally pair with the RS600 to prevent over-gearing in first.
Now, you have to put this into context as to your engine's output at a given rpm. For the case of the Duntov cam'd small block, that engine doesn't make a lot of bottom end torque, but likes to rev and make HP on the top end. So, that engine can benefit from the additional torque multiplication to get the car motivating without having to slip the clutch excessively. Combine that with the higher rpm redline to make a really fun blast from first gear. For the case of a low end (rpm) torque monster big block, the numerically high first gear of the RS400/500 coupled with lots of engine torque means there will be a whole lot of torque at low rpms, and the consequence may be excessive wheel spin right from the start.
Looking at real world examples is another good point of reference. All Mustang GT w/5-speed cars since the 90s through 2010 run 3.3 first gear ratios. And there are a lot of fast (10.xx sec 1/4 mile) Mustangs running the 3.3 first gear ratio.
Rolling all of the above into consideration, my general guideline for muscle cars running 25-27inch tires is this:
Engines up to 500 torque and 500 HP:
diff ratio of 3.08-3.73 => use RS400/500
diff ratio of 3.9 & up => use RS600
Engines up to 600 torque and 600 HP:
diff ratio of 3.36-4.1 => use RS600
We always discuss these points with the owner/builder during the quote, and any questions or special considerations (large tires, special purpose apps, etc) can be reviewed.
Hope this helps you all to understand the impact of gear multiplication and selection.
Shafi Keisler
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