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Old 05-20-2022, 09:28 AM   #1
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Allison: Calibration Maximum Gear Range Increase Policy

Noting the frequent threads here concerning 'opening' 6th gear on Allison transmissions I figured I'd post this. I don't think I've posted it before and I don't recall seeing it here before either, though there are so many threads about this subject I could easily have missed it being included. If this was an unnecessary posting just consider the source - I'm an old guy.

I don't recall when I found this letter but it was during a search for all things Allison on the web at which point I downloaded a bunch of stuff, this being among it. I promptly forgot about it till this morning when I was poking through my manuals on my laptop.
Attached Files
File Type: pdf Range Increase Letter.pdf (710.3 KB, 64 views)

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Old 05-20-2022, 11:47 AM   #2
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Originally Posted by Oldyeller View Post
Noting the frequent threads here concerning 'opening' 6th gear on Allison transmissions I figured I'd post this. I don't think I've posted it before and I don't recall seeing it here before either, though there are so many threads about this subject I could easily have missed it being included. If this was an unnecessary posting just consider the source - I'm an old guy.

I don't recall when I found this letter but it was during a search for all things Allison on the web at which point I downloaded a bunch of stuff, this being among it. I promptly forgot about it till this morning when I was poking through my manuals on my laptop.
Hey old guy, that was a really great share !

Thanks!
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Old 05-20-2022, 05:01 PM   #3
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I have always been very curious how much people who’ve gotten 6th unlocked actually get to use that gear. Having driven my bus to florida and back on the interstate, I’m pretty sure I would have only been cruising in 6th on a perfectly flat highway with no major headwind. In any other terrain the bus would surely have been jumping up to 5th. I’m guessing that has to do with my rather high rear end ratio (5.13).

I’m sure 6th could unlock some potential fuel savings, but I’m just not convinced it would work as well as we hope here. This letter definitely implies that there’s a real reason they lock out 6th in most buses.
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Old 05-20-2022, 05:30 PM   #4
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Originally Posted by dbsoundman View Post
I have always been very curious how much people whoíve gotten 6th unlocked actually get to use that gear. Having driven my bus to florida and back on the interstate, Iím pretty sure I would have only been cruising in 6th on a perfectly flat highway with no major headwind. In any other terrain the bus would surely have been jumping up to 5th. Iím guessing that has to do with my rather high rear end ratio (5.13).

Iím sure 6th could unlock some potential fuel savings, but Iím just not convinced it would work as well as we hope here. This letter definitely implies that thereís a real reason they lock out 6th in most buses.
There are a lot of factors that have to be taken into consideration. Transmission ratios, engine torque and HP, final drive ratio, bus weight, tire size, etc. In some cases I suspect changing the final drive ratio might make more sense than going through the frustration of unlocking the tranny - but again, it just depends . . . and I'm not talking about adult continence control.
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Old 05-20-2022, 05:31 PM   #5
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6th gear

My math indicates 6th gear is only usable on flat or slight down hill, like out of denver going east on I 70. Add more than a little load and a down shift would be in order, but that is also how you get fuel mileage. If I keep at 65 mph I will rarely use 6th.

William
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Old 05-20-2022, 05:33 PM   #6
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Originally Posted by magnakansas View Post
My math indicates 6th gear is only usable on flat or slight down hill, like out of denver going east on I 70. Add more than a little load and a down shift would be in order, but that is also how you get fuel mileage. If I keep at 65 mph I will rarely use 6th.

William
That is a good point, 6th may be more useful if you're trying to cruise at 70. I'm governed at 65 and find it pretty comfortable to be honest, so I don't have any plans to pay $$$ to CAT to have them adjust the governor for me.
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Old 05-20-2022, 06:08 PM   #7
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I have had mine unlocked since March. I have about 4 hours of interstate driving since. I haven't measured MPG yet since the change, but my front engine is noticibly quieter at 65 on the highway. I have a 5.29 and 36" low pro tires. I used to be RPM limited at 67mph or so. Even though I'm unlocked, 65-67 is still ideal for cruising for me. I now sit at about 2100rpm at 65 where before I was close to my redline and defueling at that speed. I did get 12mpg at 65 before, so I'm curious if the lower RPM will change that much. I do plan bigger tires and more weight in future, so lots a variables in play for me.
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Old 05-20-2022, 06:20 PM   #8
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I'll be happy to cruise at 60-62 and I think, during my very first drive last fall, I figured that'd put me at about 2100 but time will tell.

I'm old, I drive slow . . . I'm becoming THAT old guy, oh well.
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Old 05-20-2022, 08:19 PM   #9
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I've always been a bit of an adrenaline junky...fast cars and motorcycles on mountain roads. I'm still that way at 68 but I don't feel the need to go fast in the bus. It's huge. I wouldn't mind saving fuel though if that were possible. Reduced engine wear is a plus too.
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Old 05-21-2022, 07:20 AM   #10
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this is a great read! and what ive been mentioning all along is driveshaft speed.. when I installed the 6 speed in my red bus i had my whole driveshaft balanced as a set and installed it as a set. but its very easy on some engines to over-speed the driveshaft if you have a low rear end gear and a fairly high revving diesel engine.. the duramaxx series are the most susceptible as they are high revving diesels and with a 0.65 or 0.61 6th gear and a low rear gear , driveline overspeed is easily attainable..



every rear gearset also has recommendations for RPM as well.. one of the propblems can be that the pinion gear spin so fast it slings too much of the oil off to maintain a good film, or causes the oil to foam and be less effective...



I have had people tell me that upping to a 4.10 in my red bus isnt enough of a ration change from 3.54 to be worth it, however i want to keep my driveline speed down and be able to use all my gears..



I program my 6th gear to come off pretty easily as to not lug the engine..



unlocking 6th gear and then still running your bus against its rev limiter is going to put more stress on the driveline no question..
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Old 05-21-2022, 09:34 AM   #11
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This seems like a good thread to repost a link I had posted in the past. I was surprised that nobody even commented in the other thread, guess it was just me that found them useful, even fun to work with.

I will state that this calculator requires effort to learn to use but I think it's worth that effort if you're serious about making changes to your driveline for whatever reason. Once you learn to use it you can make changes in seconds and rerun for results and even compare 2 tire diameters at once.

https://www.blocklayer.com/rpm-gear.aspx

Here's my thread from 2020.

https://www.skoolie.net/forums/f9/ca...ors-31682.html
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Old 05-21-2022, 07:01 PM   #12
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Originally Posted by cadillackid View Post
I have had people tell me that upping to a 4.10 in my red bus isnt enough of a ration change from 3.54 to be worth it, however i want to keep my driveline speed down and be able to use all my gears..
Going from 3.54 to 4.10 will make hill climbing easier, but will also increase the driveshaft rotational speed. With 3.54s the driveshaft spins 3.54 times for every rotation of the rear wheels, with 4.10s that changes the driveshaft speed to 4.1 times for every rotation of the rear wheels, or a 15 percent increase in roatational speed.

Personally, I shoot for whatever rear end ratio puts my engine into its best mileage range at cruise in top gear.
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Old 05-21-2022, 09:53 PM   #13
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If and when I'm retired and out and about I'll probably find a place to unlock 6th gear just because it's available. I'm not entirely sure my 12v 5.9 is going to pull it although the guy that drove it out to me said it would do 80mph the way it is.
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Old 05-22-2022, 01:55 PM   #14
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Originally Posted by Veloc View Post
Going from 3.54 to 4.10 will make hill climbing easier, but will also increase the driveshaft rotational speed. With 3.54s the driveshaft spins 3.54 times for every rotation of the rear wheels, with 4.10s that changes the driveshaft speed to 4.1 times for every rotation of the rear wheels, or a 15 percent increase in roatational speed.

Personally, I shoot for whatever rear end ratio puts my engine into its best mileage range at cruise in top gear.

it will indeed increase my driveshaft RPM.. but i'll still be well under recommended maximum by allison for my 1000.. going to a 4.30 or 4.88 gear would put me really close to attainable maximums if I were driving the bus at 75 MPH...



right now the 3.54 is too tall for the overdrive trans.. the 4.10 will put my engine in its best place for cruising 65-70... as it stands now my 6th gear rarely gets used..


my main point was that i hear many people talking about unlocking 6th gear on their bus as a way to increase its top speed.. ie they have a really low-gear driveline and bus has a top speed of 60.. they want to unlock 6th gear so they can travel 65-68.. meaning they are still running close to the engine top mark.. and at the same time they may reach or exceed the maximum RPM for the driveshaft...



even a small imbalance in a driveshaft can be samaging to bearings and seals at higher RPMs vs lower RPMs..



the driveline shop ive had all my sets balanced at (had my racecar driveshafts balanced at this same place over the years too).. mentioned that all of the school bus driveshafts i broiught them were pretty far out...


most school busses spend 90% of their life on city streets where imbalanced drivelines dont really cause any impact... as skoolies that role is flipped.. most people building skoolies are trekking across the country , up and down, back and forth.. some may take the slower back roads.. but others here (esp those unlocking 6th gear) plan to run lots of highway miles...


this letter printed, addresses that very concern.. my guess is from experience or feedback from repair shops that have seen issues opened after 6th gear was unlocked...
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