bobbolotune wrote:
SoCalDesertRider wrote:
There's only a small rpm difference between those 2 ratios at equal road speeds, with equal tire diameters and transmissions. I would have to do the math to know for sure, but going by memory I think it's only like 300 rpm.
The greater torque multiplication of the 4.10 is worth more than the slightly lower rpm of the 3.73
This finally helps me understand. But I have a question. I might guess that torque would be in direct proportion to rpm. That the purpose of lower axle ratio is to increase rpm.
If it is only slightly more rpm wouldn't that imply only slightly more torque? Or are there other factors other than engine rpm which contribute to torque?
The increase in torque comes from 2 factors. One is increased rpm and greater horsepower and torque output from the engine at the increased rpm. The other reason is torque multiplication through gearing.
Gears are a simple machine that multiply torque by using the same force (torque output from the engine) and applying it over a longer distance. Gears are like a lever and fulcrum, but they are round, rather than straight. When you want to lift something really heavy and you don't have another guy to help you, find a block and pipe and use the length of the pipe to help increase the force you can apply to the heavy thing you're lifting.
So let's say, for sake of example, that you have 400 ft-lbs of torque available from the engine, and you're running in direct gear in the transmission and have 3.73 gears in the axle. You have 400 x 3.73 = 1492 ft-lbs of torque available at the rear wheels.
Now lets say you have 4.10 gears and that the engine is also putting out 25 more ft-lbs of torque at the slightly higher rpm it's spinning with the 4.10's at the same road speed. So now you have 425 x 4.10 = 1743 ft-lbs of torque available at the rear wheels, an increase of 251 ft-lbs.
So lower ratio gearing is a double-whammy effect on towing power. Using the higher power output range of the engine, as well as multiplying it's output through leverage of the gears.
On my gas engine F350, I went from 3.55 to 4.56 gears, which is a huge jump. My little 351 engine puts out about 325 ft-lbs of torque at about 2800 rpm.
With the 3.55's I wasn't able to reach 2800+ rpm at speeds I could tow at, without going down to 2nd gear. The 4.56's put me right in the engine's sweet spot, at reasonable towing speeds, in 3rd gear, while also multiplying the engine's higher output.
I don't remember the exact numbers, but let's say my engine was putting out 250 ft-lbs of torque at the lower rpm. Multiplied by 3.55, I had 888 ft-lbs at the wheels. With the 4.56's and the full 325 ft-lb output of the engine available, I got 1482 ft-lbs of torque available. That's a HUGE increase (594 ft-lbs)!
Now, at those times that I need to shift down to 2nd to climb a hill that's just too much for 3rd, that 1482 ft-lbs is further multiplied by 1.52, which is the 2nd gear ratio of my transmission, while I adjust the road speed so the engine stays in it's max torque rpm operating range.
The gears made all the difference in the world in the performance of the truck. Before I did the change, I was going to have to sell the truck and buy another one with a bigger engine because the truck couldn't do what I wanted it to do, anywhere near my satisfaction. It was really depressing. I didn't want to sell the truck though. It was in beautiful condition, with very low miles. So changing the gears was the next best solution.
