DP; downshift or let the turbo boost go to max? also Neutral for waiting at a light?

[oldryder]

2005 Vectra with 400hp Cummins. I have read many times that the Cummins/Allison coordination is excellent and that the driver can leave everything to the computers. However, at 55-60mph the engine rpm is 1500 or below and it just seems to lack power even with the turbo boost maxing out. Downshifting to 5th seems to work better, there is more response to driver input and no pegged turbo boost.

2nd question: at idle the coach wants to move out and it takes firm pressure on the brake to hold it back. Any reason not to put it in neutral when having to wait at a light or in stop n go traffic.

thx to anyone offering advice.

mark in MN

 

[Laura & Charles]

Mine won’t even think about shifting up to 6th until I get up to 55 mph. If I’m running 55-60 and turbo is maxed out, mine would’ve already have downshifted to 5th or even 4th. Does your tranny have an ‘economy’ setting option? (Mine does and I’ve never used it.). Seems like something that would retard the downshift points.. maybe you have such a feature turned on?

If you’re asking about manually downshifting before the tranny wants to on its own, I wouldn’t think there would be any harm in that… but somebody who knows more than I should be who you should listen to. (As to your question about shifting to neutral at stoplights, same answer.)

 

[Henry J Fate]

Neutral at stops is something I do with all my vehicles. My gas coach is always in neutral prior to a full stop. The only concern is making sure the transmission is fully engaged after placing it into a drive gear and taking off. That will take a second or so.

Manually shifting the transmission during certain times is helpful. The auto transmission is not a perfect device and can use a human touch here and there. Throttling the engine will also produce a transmission change or lack of, which is something to consider. I use both techniques to keep or change the auto transmission gear selection. I am in a gas vehicle so it is a little different but the basic control is the same.

 

[donn]

Look online for yoir motors torque curve chart. On my Ram 6.7L 1500 is right at the bottom of the torque curve. Ideally you want to be in the lower third for best combination of economy and power.

 

[Larry N.]

I'll occasionally downshift near the start of a long uphill run, which reduces the amount of slowdown a little, compared to the computer's choice, but I don't usually do that for lesser hills. It doesn't hurt anything, but adds a touch to your workload, including figuring out when to go back to auto.

You idle speed might be a tad high, but with caution it won't hurt the coach to select neutral at a stop light, though I rarely find that necessary on my coach (same on my previous Beaver, not needed).

 

[SargeW]

Modern diesels are designed to achieve their peak torque power band at a low rpm. When approaching a significant grade I will often downshift the trans to 5th gear at the bottom of the grade. This will kick the rpm's up about 200 and spin up the turbo a bit sooner. My rig has a Cummins ISL9 450 HP motor. Here is an excerpt from my chassis manual on "Driving Tips".
When ascending a grade, maintain engine speed to within 400-500 RPM of governed engine speed. Governed speed will be 2200 RPM on the Cummins ISL engine model. Road speed may decrease, but the engine will be at its peak in the power curve.

On a minor grade I let the Allison take care of the shifting, but by anticipating a steeper grade and downshifting, by keeping the rpms up the rig doesn't loose as much speed before the Allison downshifts. This will also keep the temps lower as the engine isn't laboring as hard to maintain your speed up the grade.
It is especially pertinent to monitor your water temperature gauge when climbing steep grades. Keep in mind that it is not uncommon for the temperature to increase, especially in hot weather. If the gauge reaches the end zone or if the temperature warning light on the gauge panel should come on, reduce your road speed, shift to the next lower gear and keep your tachometer within 500 RPM of engine governed speed. In many cases this will stabilize the water temperature.

And since we have talked about how to properly go up a grade, this is how to go down the other side-
A good “rule of thumb” for descending grades is to never use a higher gear than was used to climb the same or similar grade. Try to keep the engine within 500 RPM of governed speed. This will give the best engine braking and reduce the need to use the service brakes. Select a gear that will keep you at a safe speed with minimal brake application. Never ride your brakes when descending a grade since excessive brake heat will build up and your brakes could fade, leaving you with little or no braking power.

 

[ChasA]

The driving instructor at Lazy Days in Tampa told us diesel pushers all idle at 12mph. So yes, mine will go without any braking and if idling at a stop in heavy traffic, I'll put in neutral and set the parking brake. I don't experience that very much though because I try to avoid interstate highways.

 

[Gary RV_Wizard]

No reason not to use neutral if you like. Yes, at idle most class A diesels will move off from a standing stop and accelerate to around 12 mph or even a bit more. If it managed to work its way all to high gear, it would probably hit 25. If 1500 rpms in high gear is 55-60, then 600-700 rpms ought to be close to half that speed.

There is no cause to avoid max turbo boost either. The amount of boost merely reflects how much air the engine needs for efficient fuel combustion. If you demand most of the available horsepower in acceleration, then the turbo is going to max capability to help deliver it.

Both 5th & 6th are overdrive gears on an Allison 3000, but 5th is a bit lower and therefore does give quicker response (at the expense of optimal fuel economy).

 

[Lordismyshepherd]

I find that 5th gear and around 1700 rpm's works perfect for my 500 hp ISM. I rarely select 6th gear unless the road appears flat to the horizon.

 

[Ray-IN]

The Allison transmission has a TCM that interacts with the engine ECM to prevent damage to either component. The TCM will not allow you to downshift if the engine RPM will exceed the high RPM power-mode limit.
My SIL is the maintenance manager for a larger N.A. trucking company. I asked him about using the Allison Economy mode. He said use it all the time in my light MH (32K lbs.) because the Allison/ Cummins combo was originally designed for extreme-duty use in garbage and dump trucks, my light RVing use was not comparable.
I do de-select economy mode when experiencing mountain driving to allow the Allison to shift sooner and at higher RPM's.

 

[A Traveler]

2005 Vectra with 400hp Cummins. I have read many times that the Cummins/Allison coordination is excellent and that the driver can leave everything to the computers.…

Don’t believe everything you read.

The “Fatal Flaw” in that driving philosophy is that the computer cannot anticipate anything. It can only react to something that has already happened. By anticipating a steep grade and shifting down to fifth just as you get into the bottom of the grade, you set your coach up for a much better climb. You get the engine RPM up to 1,800 to 2,000 RPM, which is where the horsepower peak is. The engine will also cool better at the higher RPM. If you wait for the computer to figure this out, you’re already behind the power curve.

Thes big coaches are not cars. You cannot just leave them in “D” and expect to roll up and down mountain grades without problems. You have to drive them.

 

[oldryder]

Thx everyone. Advice has been consistent with what feels right to me as the driver. Also good to know the brake pressure needed at idle rpm is normal.

mark in MN

 

[catblaster]

I realize that your engine and tanny are much newer models than mine but this may still pertain. All the helpful comments above are true I'm not trying to discredit anyone. So to add to the pot, mine was running fine but while reading a service manual I was shown an adjustment on the throttle/trans interface. This set the idle and affected all the shift points. A very finicky adjustment but once made I idles lower and shifts better. Hard to say what the real improvements were since it has been so long ago.