8L90 experts please read - can 7th and 8th gear be locked out?

[lucas287]

In a big, heavy truck that has 3.23 gears and 33" tires I find myself disliking the way 7th and 8th gear feels like it's lugging.

I enjoy driving around in 6th and 7th gear and I've done several highway tests and there's really not much difference in fuel economy.

Is there any long term harm in simply raising the mph to an unattainable level for 7th and 8th? Effectively this would limit it to 6th. Since this is a very adaptive transmission is there some adaptive learning that wouldn't occur by not having those two extra gears?

Thanks in advance!

[lucas287]

To anyone reading this...I decided against doing it. I had actually forgotten that I was on the stock tune (for a month lol) because I had to take it in for warranty work at the dealer. Flashed my latest calibration and part throttle is torque/response is so much better that 7th and 8th feel fantastic.

[cmitchell17]

What about trying how it feels with the torque converter unlocked? Go to the vehicle controls and special functions button in the scanner, and from there you can force the TCC to never lock so you can drive it around and see how you like it.

[lucas287]

The EGR cam retard is what makes 8th gear useless to me. I zeroed it out and all is well now. Still don't like 8th much, though not because of power, but because I get vibrations between 1600-1800 RPM worsening at 1700. idk what actually causes it but it's annoying. I feel it driving up through every gear too. 8th puts me right in that zone at highway speeds. 7th gets me 2100 @ 70 - perfectly smooth.

[blindsquirrel]

Check the angle at the front u-joint. They need to be at around 2* or more, if it's less than that you get that weird rumble vibration at a certain speed. If you find the angle is too small simply stick a 3/16"-1/4" stack of washers under the rear trans mount.