Wanted: feedback from drivers late model 300-340 Cummins

Backstory- we're in the market for a short coach, less than 36 feet. It's our preference (due to the types of parks we enjoy). Likely a Newmar Ventana or Ventana LE or Tiffen RED. We'll pull a large toad. Perhaps new, but we'll also consider going back a few years, but not before DEF engines.

I dislike a powertrain that's maxed out on torque and has to shift gears too often. Defeats the whole purpose of a diesel so to speak. And I've operated some trucks like that.

The real technical stuff: Most coaches in this range are built with the Allison 2000 series trans, a few have the 3000 Allison. The smaller transmission has limited torque handling capability and is used with the ISB 300 or 340, with peak torque ratings of 600 to 700. The Allison 3000 enables use of the ISB 360 which is 800 lb-ft torque. Which seems like not much difference, right?

But here's the thing; the 2000 series requires the use of torque managed engines, which limit the torque when shifting (not a bad thing) and also the torque in certain gears (and this is my concern). The 2000 series trans allows up to 700 lb-ft in gears 3-5. In other conditions it's limited to 575 lb-ft. Limitations on the Allison 3000 are not a concern with the ISB, as the trans can handle 1250 lb-ft and torque management is not required.

So... it would appear that the ISB360/Allison 3000 combo can draw on all 800 lb-ft of torque to maintain speed on a grade in any gear, whereas the ISB340/Allison 2500 by comparison can only use 575 lb-ft in 6th gear. That sounds like a recipe for easily dropping out of 6th with the smaller transmission. But does the real world experience match?

Thanks for your patience in reading my dissertation, and I appreciate your feedback.

Ductape wrote:
The short version: if you have a recent Cummins 300-340 hp range does your coach hold 6th gear crossing interstate overpasses and mild grades? Or does it downshift at any mild rise?

Thanks for your feedback, and if you want to read all the whys and wherefores, the next post I'll drone on. :B

neighbour has the 360 ISB in a 2017 tiffin pulls his ford F150 CC 4x4 and says severely lacks power in the hills(loaded pulling truck)
I guarantee you it will downshift when in the situations your asking about

I'm running a 2014 Thor Palazzo 33.3. It has a ISB 300 and Allison 2100 MH. I could go to ISB 340 / 700 with the same transmission. I have towed a 2015 Suburban, 2015 GMC Acadia Denali, 2016 Yukon Denali XL, and 2017 Ford Explorer. The I am at my max GCWR when I tow the Yukon and the motor home is empty. The transmission does downshift. I have traveled over the rocky mountains 4 times with this setup (pulling the Acadia) we had no problems. I probably will upgrade to ISB 340.

Thanks for sharing, my pet peeve is not the mountains, as I don't have any objection to gearing down to climb a long grade, that's expected. What I dislike is a gear change just to climb an overpass in flat country, or a moderate rolling hill.

A little work with a spreadsheet has given me my answer I believe. Taking as an example recent Newmars:
Ventana with the 360/800 will produce 2,486 lb-ft at the axle in 6th gear @ 65 mph. Disregarding accessories and drivetrain losses.

Ventana LE with the 340/700 will be only 1,759 lb-ft under the same conditions due to torque limiting in 6th gear.

Dropping the LE into 5th gear provides full torque and gets the axle up to 2,476.

So in essence 5th on the Allison 2500 equipped coaches will be equivalent of 6th in a coach with the Allison 3000 and the 360/800 engine.

That's enough to convince me to seek out a coach with the bigger trans. Not to mention running the weight numbers I'd be over GCWR on the smaller Allison with cargo in the coach plus a 6k toad.

We had a 2014 40ft Red with 340hp. I dont recall it shifting on overpasses but the shifting is pretty seamless from 6-5. We were pulling a crewcab GMC with a couple motorcycles in the truck. I wasnt happy with the power so,went to a Phaeton with ISL.

Does your Allison had a mode button? My 3000 on the 300 hp CAT switches into an economy mode that delays the downshift until RPMs drop below 1500.

msturtz wrote:
I'm running a 2014 Thor Palazzo 33.3. It has a ISB 300 and Allison 2100 MH. I could go to ISB 340 / 700 with the same transmission. I have towed a 2015 Suburban, 2015 GMC Acadia Denali, 2016 Yukon Denali XL, and 2017 Ford Explorer. The I am at my max GCWR when I tow the Yukon and the motor home is empty. The transmission does downshift. I have traveled over the rocky mountains 4 times with this setup (pulling the Acadia) we had no problems. I probably will upgrade to ISB 340.

A suburban weighs more than a 2015 yucon or explorer

Ductape wrote:
Thanks for sharing, my pet peeve is not the mountains, as I don't have any objection to gearing down to climb a long grade, that's expected. What I dislike is a gear change just to climb an overpass in flat country, or a moderate rolling hill.

A little work with a spreadsheet has given me my answer I believe. Taking as an example recent Newmars:
Ventana with the 360/800 will produce 2,486 lb-ft at the axle in 6th gear @ 65 mph. Disregarding accessories and drivetrain losses.

Ventana LE with the 340/700 will be only 1,759 lb-ft under the same conditions due to torque limiting in 6th gear.

Dropping the LE into 5th gear provides full torque and gets the axle up to 2,476.

So in essence 5th on the Allison 2500 equipped coaches will be equivalent of 6th in a coach with the Allison 3000 and the 360/800 engine.

That's enough to convince me to seek out a coach with the bigger trans. Not to mention running the weight numbers I'd be over GCWR on the smaller Allison with cargo in the coach plus a 6k toad.

No help here, sorry. But you're first post with all the technical info and your name had me pegging you an engineer. This post and the spreadsheets come out, now I'm convinced. Carry on.

^^^ Guilty as charged ;) For part of my career anyway.

JPlante, thanks for bringing up the mode button; I hadn’t remembered that. My understanding is the Allison is programmable and the function of the mode button depends on what the builder has chosen. If there’s an economy function that holds top gear down to a lower RPM that would resolve my concern fully. I don’t mind losing a couple of mph in a short grade. What I hate is a downshift climbing an overpass and an up shift 200 yards down the road. Hard on the drivetrain and bad for fuel mileage.

And I do like to use the cruise control.

To my knowledge, ALL RV chassis makers have the Allison ECU programmed to default to power mode each time the engine is started.

So, to get into economy mode, one must push the mode button each time the engine is started.

To verify you are in economy mode:

Next time it is safe to accelerate from a stop to 45 or so MPH, do so at WOT (wide open throttle) with mode button off (engine started, but mode button not pushed) and then again after pushing the mode button (red mode light on).

Yes, in economy mode, transmission upshifts at lower RPM and downshifts at lower RPM.