I have been asked about whether changing to one of my Final-Drive Gearset Upgrades will increase or decrease torque with an 820 transaxle. Well, let's see... For purposes of computation, this Post will use a 20hp engine driving the transaxle with a 1:1 pulley setup (the engine pulley and transaxle input pulley are the same size) at the engine’s top governed no-load speed of 3600 RPM. For simplicity, we will also use the 1:1 (25T:25T) gear-selector ratio, which is normally 3rd or 4th gear in most 5- and 6-speed 820s.
The stock input ratio is 2.54:1. The stock final-drive ratio is 8.02:1. With the 1:1 gear-selector ratio, this gives a 20.37:1 reduction ratio, meaning that at 3600 Input RPM, the axles are turning 20.37 times slower, or 177 RPM.
Gearset Upgrade 1 changes the 8.02:1 stock final-drive ratio to 5.61:1, meaning that the axles are turning at 253 RPM.
Gearset Upgrade 2 changes the 8.02:1 stock final-drive ratio to 3.44:1, meaning that the axles are turning at 412 RPM.
To convert these RPM figures to Torque, we will need to use the standard scientific formula Torque = (HP x 5252) ÷ RPM.
With the stock final drive, this gives us 20 x 5252, or 105040, ÷ 177, which equals 593 units of axle torque.
With Gearset Upgrade 1, we have 105040 (the same 20hp and 5252 formula constant, multiplied) ÷ 253. This gives us 415 units of axle torque, a 30% decrease, or 70% of stock axle torque.
With Gearset Upgrade 2, 105040 ÷ 412 gives us 255 units of axle torque, a 57% decrease, or 43% of the stock axle torque.
So, even though the Gearset Upgrades give us increased axle speeds, we have less torque available.
If you know your engine horsepower and RPM, you can use your pulley sizes and the above formula to compute torque for any of your gear ratios.
And, for those fortunate enough to be running the "017 Kit" with a 2.31:1 Final Drive Ratio, your axle is turning at 613 RPM, with a corresponding torque value of 171 units, 29% of stock torque (a 71% decrease).
I hope this helps.
Respectfully; Rodney Rom; Rom's Reworks; Butler, MO