4T65E with 2.79 final drive

[pontiacjeff]

I am trying to get this thing to not throw the Incorret Gear Ratio. Customer wanted less gear for highway driving, so I set up a 3.05 diff with the 37/33 sprockets to achieve the 2.79. I need to know how and where to input the correct values and what those values should be. Thanks.

I assumed the same sprocket ratio as the 2.93 and same input speed sensor count b/c it has the same parts on that end, but that did not work. Do I need to go in and actually change the gear ratios by ~8% (3.05 vs. 3.29)

[pontiacjeff]

anybody?

[Russ K]

Post your stock & latest tune. If the stock drive sprocket ratio is 1.121, then leave it stock & enter 3.05 as the gear ratio.

Russ Kemp

[pontiacjeff]

thanks, I have been tuning with another program for years and still not used to seeing things differently in HP.

[Russ K]

Which file is the stock one?

Russ Kemp

[pontiacjeff]

Which file is the stock one?

Russ Kemp

The "read" file is the stock 2.93 (3.29) I downloaded it and used it to attempt the gear ratio swap in the "2.79" (3.05) file.

[Russ K]

3.05/1.121=2.72. Stock is 3.29/1.121=2.93. Not sure what gear ratio your using.

Russ Kemp

[pontiacjeff]

I am using the 3.05 diff with the stock 37-tooth drive/33-tooth driven chain sprockets for a 2.72 final drive (sorry, the 2.79 was an erroneous statement)

Here's what bugs me about the GM tuning. The 1.12 value is actually the inverse of what is happening in the transmission. To calculate drive ratios, one must divide the driven gear by the drive gear. In actuality, the 2.93 final drive is derived from a .89 overdrive ratio provided by the chain sprockets (the 1.12 is the inverse, which value in reality produces an underdrive condition as in the 3.42 or 3.69 ratios used by GM and subsequently popularized in the aftermarket.)

Thus in the 4T65E, the 3.29 final drive ratio is direct drive with a 35-35 chain sprockets (3.29*1=3.29), underdriven by 12% with the 33-37 sprockets (3.29*1.12=3.69) and overdriven by 11% with the 37-33 sprockets (3.29*.89=2.93)

The calibration values have been erroneously calculated by at least two popular tuning packages. I seriously doubt GM intended the 1.12 value to be used in this way. But that's the way it is and I have learned to deal with it. That's why I left the 1.12 in the calibration, but didn't know how in HP to fix the change from 3.29 to 3.05.

I went in and put "3.05" in the gear ratio box in Speedo folder, the self-calculating function. I deselected "auto calculate" for shift values since I already scaled them as I know they needed to be for the gear change, I didn;t want to scale them again. I might go back to the stock file and let it auto calculate and see how close it is to what I did.