Wanted: feedback from drivers late model 300-340 Cummins

eheading wrote:
We have had a number of motorhomes with Cummins and CAT engines with the 3000 series Allison transmissions. Our last Revolution LE had the 400 hp CAT. We now have the short Ventana LE with the 340 hp engine with the 2000 series transmission. Yes, it does shift down to 5th gear more than our 400 hp CAT did, but because it is governed at 2800 rpm, I have found that it does as well on hills as our larger engines did. In addition, the 2500 transmission is the smoothest shifting Allison we have ever had. The shift in and out of 4th gear was always a little rough, we can hardly feel this one shifting. In addition I like the 9,5 to 10 mpg compared to our 7.2 in our last motorhome, and I really like the flat floor all the way to the rear closet, a feature which I believe is only available with the 340 hp Cummins engine.

Thanks for that, you're in one of the models on our short list. I am presently of the mind to hold out for Ventana (non LE) to get the Allison 3000. Tiffin REDs but only 2016 onward should have a 3000.

I have seen others who share your perception the 2100 or 2500 shift more smoothly, and that should be the case, considering the use of torque management to pull power during the shift.

I could love the fuel economy of the detuned 6.7, however the GCWR is 33,000 and with our toad we would scale more than that.

gutfelt wrote:

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

My 2016 Yukon XL is by far much heaver than the 2015 Suburban LTZ we had. That said I love the 6.2L V8. I only tow the Yukon for service work. If my wife had the Explorer I can still drop the motorhome off and tow the Yukon. But at that point I am empty so weight isn't a problem.

We have had a number of motorhomes with Cummins and CAT engines with the 3000 series Allison transmissions. Our last Revolution LE had the 400 hp CAT. We now have the short Ventana LE with the 340 hp engine with the 2000 series transmission. Yes, it does shift down to 5th gear more than our 400 hp CAT did, but because it is governed at 2800 rpm, I have found that it does as well on hills as our larger engines did. In addition, the 2500 transmission is the smoothest shifting Allison we have ever had. The shift in and out of 4th gear was always a little rough, we can hardly feel this one shifting. In addition I like the 9,5 to 10 mpg compared to our 7.2 in our last motorhome, and I really like the flat floor all the way to the rear closet, a feature which I believe is only available with the 340 hp Cummins engine.

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.

^^^ 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.

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.

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

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.

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.

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.