allison question

[william phelps]

Hi Allison man; I was probably not very clear on what I wanted to do. I currently have a th400 behind a 454 chevy in a 28ft. motorhome with
4:10 gears in the differential. What I want to do is put in 3:21 gears in the differential and install the AT545 that I currently already have. When we compared the gear ratios between the first three gears in both transmissions the allison will still be lower in the first three gears with the 3:21s than the th400 is with the 4:10s. Thus giving me a 20% overdrive when I shift the allison in to 4th gear. See if this makes sence. When allison ran a scan it started falling in the acceptable range when we went with 3:50 gears. But the bottom line is if the first three gears are still going to be lower than what I have now then what would be wrong with using 4th gear as an overdrive.

 

[MichaelM]

allison question

The 400 gears are 2.48 for 1st, 1.48 for second and 1 to 1 for 3d.
The 545 has
First...................................3.45:1
Second..................................2.25:1
Third...................................1.41:1
Fourth..................................1.00:1
Reverse.................................5.02:1

The problem, that I feel you will run into, is that with the
3.21 gear you will not have enough torque to properly move the truck down the road in high gear. And you will most likely be at too low of a cruise rpm, causing the torque converter to be slipping all the time. The converter slipping (stalling) will multiply the gear ratio and may help the truck cruise ok. But with the converter stalling so much will cause an overheating condition for the transmission.
Gear ratios multiply torque. So if your engine produces 200 ft pounds of torque. A 3 to 1 first gear will multiply that 200 into 600. Then add a 4 to 1 rear end gear and you get 2400 ft pounds to the rear wheel.Minus frictonal loses off course. The torque converter also multipies torque when it is stalling. The amount varies to converter design. But anyways you can do the math with the ratios.
The 20 percent overdrive sounds real nice. The fact is, if it was practical the manufactors would have done it that way.
Allison does the research and devolpment. They know what the ratios got to be for everything to work correctly and for the truck to be drivable. SO I suggest to follow there advice.
My personal opinion would be that the 3.50s are not enough gear. If you cruise on strictly flat lands with little load, you may be fine. But get a load on and hit some inclines and I feel you will regret the 3.50 choice. Also, you may not save any fuel if you have to run the engine so hard that your in the four barrel just to cruise down the road. And this could very well happen with not enough gear.
So I guess bottom line, my suggestion, and that is all this is my opinions and suggestions, install the 545 if you want it and leave the rear end gears alone. MAybe 3.73s, but I wouldn't.