Rear Diff Gear Size

[WPS 73 Bronco]

I’ve got an interesting question. What is my “net” rear gear?

On my Bronco I have the Terminator X Engine (LS3) and Transmission (6L80) control system. For the tranny, it wants to know my tire size and rear gears in order to program the shift points on each gear. I input 34” and 4:10 gears. I’m still not fully satisfied with the shift points, which made me ask the question: Since the Terminator is expecting a 2 wheel drive setup and no transfer case, do I need to give it a “net” rear gear?

So I’m asking if anyone knows the answer to my question. I installed the Atlas II transfer case, and when I asked the forum what rear gears I should use it was recommended that I go with 4:10.

 

[Broncobowsher]

It would only matter if you were in low range.

Even then, it still doesn't really matter as it doesn't see wheel speed. It sees transmission output shaft speed and put that through some math and figures the rest out.

As for the shift points, it will probably take some manual tuning. Going through the back of the head guessing of what an LS3 would be like, the drag of a Bronco, tire size, and I think you don't have enough gearing in the axles. It's not as horrible as if you had 3.50 gears, but I could see 4.56s being a better match. Without actually changing hard parts, you will probably have to manually move the shift maps around. I really hope Terminator X lets you do that, have not had the chance to play with that system. Probably won't get much time in that last overdrive.

 

[WPS 73 Bronco]

Thanks for the response. The Holley software gives me plenty of options to adjust the shift points to be whatever you want. I ran with custom shift points for the last 6 months, and then lately I adjusted back to "stock" shift points. In my custom settings, I had 2,3, & 4 coming in earlier than stock. This was an attempt to smooth out the shifting at modest acceleration. The issue I ran into was when I went to "kick down" the pedal and do some aggressive acceleration, it would often drop down two gears and jump! The engine has 500 hp, and a lot of torque. When I set it to stock, it seems to do better on the kick down. The only issue I have is I'm shift at higher RPM's than previously.

It's not bad at all. I'm just looking for optimum.

 

[Bruners4]

I would first start with the actual tire diameter and not the advertised size molded into the sidewall of the tire. For instance, a BFG KO2 33x12.50R17 shows a spec of being 32.5" tall. Performing a rollout measurement would get you the exact diameter but using the manufacture's specification is a good start.

 

[DirtDonk]

Sounds exactly like what my stock LS and 6R80 does. Although I don’t actually know which LS version, and which six speed transmission I really have.
Stock 2019 Silverado 5.3, but it’s been a big frustration with me too.
Regular acceleration, whether light or heavy, feels perfectly tuned up.

Say I want to accelerate a little more, from a low speed, so I push the pedal. Not much happens because it’s still in a high gear at a slow speed. Almost bugs down
Then instead of just popping into the next lower gear, it jumps two or three like I’m in an old “passing gear” mode and jumps away up! Because by that time the throttle is pushed much farther than I should have had to.

Unfortunately, I have to push farther on the pedal than I should have to, in order to get there so it really thinks that I want that big jump.
When all I wanted was a little light acceleration, instead of a bog.

Sometimes it feels like a bad throttle pedal sensor. Most of the time it feels like laboratory programming, instead of real-world programming.

 

[.94 OR]

Likely programming. When I am in that type situation, I can select "Tow/Haul" and it creates a better shift curve. "Eco" makes it super sluggish.

 

[DirtDonk]

Same here, but when I select that on my truck, it raises the shift RPM points higher than I like for my normal driving.
It also makes the shifts much harsher than I expected. Or like…
I realize that harsh usually means lower temperatures, but it’s also kind of annoying.

 

[ntsqd]

I'm in the habit of manually pulling it down a gear when needing to do something like that with an auto trans. When so equipped (FSB, CTD), just turning off the OD switch is usually enough to get me what I need.

 

[DirtDonk]

One of the reasons I even added a bad sensor to that list, was because of some of the circumstances under which it gets worse.
For example, if I’m trailing throttle, it’s worse.
If I’m trailing throttle, and even letting off a little bit around the corner it’s even worse still.
If I’m trailing throttle on a left turn and try to accelerate, I practically have to floor it to get it to downshift!
Couldn’t figure out what might change during a left turn other than the length of time that I’m under trailing throttle, but it was certainly a notable difference.

 

[DirtDonk]

I like driving around in a vehicle with paddle shifters that automatically take over when you use them. No need to change shift position or anything, just start using the paddles.
On mine, I have to pull the shift lever down one notch below Drive and leave it there if I want to use the manual up down button on the shift lever itself.
Not a real pain, but awkward enough I don’t use it as often as I’d like.

 

[Broncobowsher]

A lot of that is tuning. Needs to force a single gear downshift at a lighter throttle. But you could end up with a transmission that hunts between gears a lot more.
I think the fix for that would be more gear ratio in the axle.

I have a newer van with a 7-speed automatic. It likes to do double (maybe triple?) gear downshifts when you get into it pretty good. Don't really need to go from 2000 RPM to 5000 RPM, but that is how a lot of new stuff is programmed. I just don't have a massive hit of power, it just revs and is ready for me to give it more beans. And it will pull if I dig deeper. Or it realizes it went too far and goes back up a gear and we are all happy. That brings me to the 500HP that your engine was modified into. It's probably making power in excess of the transmission tuning. So modifying the engine powerband is likely messing with the transmission tuning.

You are likely going to need to get deep into some Holley groups and see what they can do to work on the drivability.

 

[WPS 73 Bronco]

I’m curious to know what the shift points are (speed and RPM) for others that have the 6R80 in their Bronco. My engine performs best between 2,000 to 2,500 rpms. When I’m running 4th gear at 2300 and 47 mph, it feels like I should be shifting to 5th. If I do, I’m now down to 1800 to 2000.

Maybe I need to just run at that rpm and delay shifts even further.

Broncobowsher, I did hire a Holley expert to help me with my overall tuning. The software and user (me, not him) determine shift points. One thing I think he could help improve is the DBW pedal position to throttle position. It’s very sensitive and I tend to have a heavy foot.