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Old 02-04-2022, 06:43 PM   #1
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Exhaust braking, Allison, and steep grades

The exhaust brake on our 37M (ISB 6.7) is either ON or OFF. In the ON position, apparently it is full force, the trans will grab the lowest gear possible, and will allow the unloaded RPM (no throttle) to go as high as almost 3,000. I know Cummins says the governed speed (loaded) is less than that, but with grade braking and no throttle, a higher RPM is OK and will not harm the engine.

Problem is... I don't always need that much exhaust braking, nor do I want the engine turning that fast, or the transmission shifting that much. I'd prefer to let the exhaust brake grind in a pre-selected gear (5, or maybe 4) without shifting down. Is this possible with the Allison?

I have this thing...I don't want to have to use the brakes....will if I have to, but with that much braking HP available with the 6.7, it seems like it could be more...selectable? ...manageable? Are their exhaust brake mods to make it more usable?
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Old 02-05-2022, 07:42 AM   #2
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Not according to the Cummins folks at the FMCA Convention in July,
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Old 02-05-2022, 08:25 PM   #3
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Thanks. OK... another question(s) maybe you or someone else can answer:

We did lots of mountain pass driving today (Siskiyous). Help settle the argument with my other half:

He says use "Mode" exclusively and let the transmission do the work:
a) Putting the transmission in "Mode" makes it more like a traditional automatic. Press the throttle, grab a gear. However, this causes more hunting than I like, lots of up/down shifts. He says: Shouldn't have to manually select gears; it's an automatic!

I say:
b) When I'm getting ready to climb a freeway grade, I build turbo pressure in 6th and keep the throttle almost to the floor until RPM's start to grind as low as 1800, or pre-select 5th in anticipation. On a steep grade, it will need 4th, so I wait until RPM's drop to about 1800 again, and then select 4. RPM's are somewhere around 2300 when it grabs 4th, and this lets me let off the throttle slightly and maintain a steady climb, or apply more throttle if I need it. I would assume this allows a lower EGT. More spin, less throttle, better climb.

At the top of the hill, I may select 5 if its a gradual downslope, or leave it in 4 for the descent. I noticed if I leave it in 4, the Exhaust Brake is very happy. It really likes 4, above about 50mph. I can usually ride it all the way to the bottom without touching the brakes, and it stays in 4 unless I use the brakes some more. This is the "hang" I was looking for. No aggressive downshifting, just more or less constant speed with the turbo doing most of the braking...it is running anywhere from 2300-2700 when doing this however.

Is climbing at 2300-2500rpm too high?
Is letting the RPM drop to 1800 under full throttle too low? (No pyro on these, so I assume the computer is minding safe EGTs).
I assume the up/down arrows are meant to be used for grades like big truck automatics?

Sorry for the novel...had a lot of windshield time learning how this runs today and want to make sure we're driving it optimally.
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Old 02-05-2022, 08:32 PM   #4
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I had Allison shop reprogram mine so it pre-selects 4th gear instead of 2nd. It does cost me some engine braking at times, but makes it much more useful for the times you don’t want the aggressive downshifting.
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Old 02-05-2022, 08:35 PM   #5
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Our 15 Seneca does very little hunting for gears on steep grates (at 32,900# Weight). I rarely ever manually shift the trans to a lower gear unless we are going down a grade steeper than 8% with lots of hairpin turns and need to keep our speed lower due to the turns.

Our Cummins is supposed to be in the sweet spot climbing between 1500 to 2200 to keep it in the best torque range. Our Trans keeps it right in that range.
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