Going down steep grades -- Safe to have 3000 RPM's?

[Road-Chi]

Newby here who has been driving our rig for a couple of weeks now. It is a 40' Newmar MADP with Cummins 325hp (C8.3 325-RV) and Allison 6 speed. As we are fulltimers, we have the rig loaded (less than the recommended weigh limit) and are towing a car 4-down equipped with a break buddy. The MH has an exhaust break. On a normal grade or flat terrain, the tach stays fairly steady at about 1700 at highway speeds. When traveling down a long, steep grade on the highway, with the exhaust break on, the tach shows the rpms getting to 2800 to 3000. Too high? Safe? The tach meter max is 3000. The tranny does downshift to help reduce/control the speed. Here is how I deal with the steep grades: try to enter the grade at about 50 to 55 mph with the exhaust break engaged. My understanding is/was that the exhaust break and tranny would control the speed without me having to engage the breaks. Yet, with a downhill grade of 6%+ the speed slowly continues to increase as does the rpms. Then, since I do not want to keep going faster and faster, once we get to about 62 mph, with the rpms close to 3000, I hit the breaks (hard, fast, short term) to bring the rig back down to around 55 or so. Then the rig's speed again begins to slowly increase until I need to hit the breaks again. Does that sound right? Safe? Taxing the breaks too much? Ideas? Suggestions? What is too high of rpms for the Cummins C8.3 325-RV?

[palehorse89]

What year is the Coach? Sound excessive to me! I am going to venture to guess, 2200-2400 max rpm's using engine brake......... My ECM will not allow engine rpm's to get that high........ it will shift the transmission to keep the engine at a safe rpm.

[pagosajoe]

The "rule of thumb" is whatever gear you use to go UP a grade, use the same gear to go DOWN a grade. Speed is somewhat irrelevant. I've never had our 8.3 at 3K rpm on the tach going downhill or uphill. Cummins and Allison, in their great combined wisdom, have programmed the system to keep the engine safe.

My thought - gear down, slow down, and brake as you've described. We transit Wolf Creek Pass (wicked steep) frequently, and travel at around 35 MPH up and down.

You can go uphill fast any time, but you can only go downhill too fast once.

Hope that helps.

[dennis45]

Originally Posted by palehorse89

What year is the Coach? Sound excessive to me! I am going to venture to guess, 2200-2400 max rpm's using engine brake......... My ECM will not allow engine rpm's to get that high........ it will shift the transmission to keep the engine at a safe rpm.

I agree, the ECM should control the RPM. Mine will up shift at about 24-2500 at which time I apply the service brake to reduce speed and allow the transmission to shift back down to the lower gear.
Are you sure the exhaust brake is working?
My rule of thumb is to start my decent at no more than the speed I came up at. If you top the grade at 40 MPH, then that's your decent speed.

[onechaddude]

I will agree with others. Sounds like you maybe starting down the hill a little too fast. It's easy to pick up speed going down if you're going to slow. Conversely, it's hard to slow down going downhill if you're going too fast.

Chad

[Dr4Film]

The higher the speed the less effective your Exhaust Brake will be. 50 - 55 MPH heading down a grade is way too fast for me although I am a heck of lot heavier than your combo.

When driving grades I learned real quick where my Allison up-shift points were on the speedo. Cast those into memory.

When using an Exhaust Brake on a decent grade you will always increase in speed unless you keep it in check by trouncing on the service brakes to reduce the speed 8-10 mph under what you were doing. Then when it creeps up again, do it again.

On low % grades I never let my Allison shift into 5th gear at 54 mph. I will trounce on the brakes a few mph before it reaches 54. Once the Allison reaches 54 it will shift into 5th gear and away you go increasing speed quickly.

On really steep grades I will be in 3rd with my emergency flashers going and when it gets close to the shift point into 4th I will bring it back down.

I forgot what the up-shift point is from 3rd to 4th right this minute but once I get back on the road after sitting for almost 2 years I will remember right quick like.

Heading back to Alaska this April so once I get into the mountain ranges again it will all come back to me.

Dr4Film ----- Richard

[Steve Ownby]

Originally Posted by Road-Chi

Newby here who has been driving our rig for a couple of weeks now. It is a 40' Newmar MADP with Cummins 325hp (C8.3 325-RV) and Allison 6 speed. As we are fulltimers, we have the rig loaded (less than the recommended weigh limit) and are towing a car 4-down equipped with a break buddy. The MH has an exhaust break. On a normal grade or flat terrain, the tach stays fairly steady at about 1700 at highway speeds. When traveling down a long, steep grade on the highway, with the exhaust break on, the tach shows the rpms getting to 2800 to 3000. Too high? Safe? The tach meter max is 3000. The tranny does downshift to help reduce/control the speed. Here is how I deal with the steep grades: try to enter the grade at about 50 to 55 mph with the exhaust break engaged. My understanding is/was that the exhaust break and tranny would control the speed without me having to engage the breaks. Yet, with a downhill grade of 6%+ the speed slowly continues to increase as does the rpms. Then, since I do not want to keep going faster and faster, once we get to about 62 mph, with the rpms close to 3000, I hit the breaks (hard, fast, short term) to bring the rig back down to around 55 or so. Then the rig's speed again begins to slowly increase until I need to hit the breaks again. Does that sound right? Safe? Taxing the breaks too much? Ideas? Suggestions? What is too high of rpms for the Cummins C8.3 325-RV?

You should ask this question of Cummins. 3000 rpm certainly sounds high for an engine governed at 2200. Cummins engines are designed to exceed their governed speed at closed throttle with the exhaust brake engaged. My ISM is governed at 2100 but will turn up to 2400 before shifting up. I suspect that 2600 might be ok, but I would check with Cummins. Your tach might not be accurate also.


Steve Ownby
Full time since '07

[stuhly]

Simple solution is to manually downshift to a gear that the exhaust brake will hold it at 2100 - 2200 RPM. It should never wind over 2500.

[Coma]

Quickserve says 237 HP available for braking at 2800RPM. We travel loaded as you are. We did The Eisenhower tunnels this summer. 33mph up hill in 3rd gear at 2000RPM. Down hill 2nd gear, I don't remember the speed. Because the exhaust brake equipped ISC put out ~100 HP less for braking you should start down in a gear one lower than you climb.

[Ramets]

Are you manually downshifting? Whenever my exhaust brake is engaged, and my foots off the accelerator, my Allison will shift into 4th gear. On steep descents like Monarch, or Wolf Creek, 4th isn't low enough, I manually shift it down to 3rd. On a few winding two lane decents I've even put it into 2nd to make it easier to take turns without having to ride the brakes. 3000 RPM's is really high. Just keep in mind that the Allison won't let you down shift if the RPM's are too high. You should down shift before you start your descent. Otherwise you'll have to brake really hard to get your speed down to be able to downshift.

[DAJO]

Originally Posted by Road-Chi

Newby here who has been driving our rig for a couple of weeks now. It is a 40' Newmar MADP with Cummins 325hp (C8.3 325-RV) and Allison 6 speed. As we are fulltimers, we have the rig loaded (less than the recommended weigh limit) and are towing a car 4-down equipped with a break buddy. The MH has an exhaust break. On a normal grade or flat terrain, the tach stays fairly steady at about 1700 at highway speeds. When traveling down a long, steep grade on the highway, with the exhaust break on, the tach shows the rpms getting to 2800 to 3000. Too high? Safe? The tach meter max is 3000. The tranny does downshift to help reduce/control the speed. Here is how I deal with the steep grades: try to enter the grade at about 50 to 55 mph with the exhaust break engaged. My understanding is/was that the exhaust break and tranny would control the speed without me having to engage the breaks. Yet, with a downhill grade of 6%+ the speed slowly continues to increase as does the rpms. Then, since I do not want to keep going faster and faster, once we get to about 62 mph, with the rpms close to 3000, I hit the breaks (hard, fast, short term) to bring the rig back down to around 55 or so. Then the rig's speed again begins to slowly increase until I need to hit the breaks again. Does that sound right? Safe? Taxing the breaks too much? Ideas? Suggestions? What is too high of rpms for the Cummins C8.3 325-RV?

What year is this rv? If it is the c8.3 I don't think they have a computer control engine so RPM may be up the driver to control.

[deandec]

Originally Posted by Road-Chi

. Here is how I deal with the steep grades: try to enter the grade at about 50 to 55 mph with the exhaust break engaged. My understanding is/was that the exhaust break and tranny would control the speed without me having to engage the breaks. Yet, with a downhill grade of 6%+ the speed slowly continues to increase as does the rpms. Then, since I do not want to keep going faster and faster, once we get to about 62 mph, with the rpms close to 3000, I hit the breaks (hard, fast, short term) to bring the rig back down to around 55 or so. Then the rig's speed again begins to slowly increase until I need to hit the breaks again. Does that sound right? Safe? Taxing the breaks too much? Ideas? Suggestions? What is too high of rpms for the Cummins C8.3 325-RV?

If you have a working exhaust brake, it will assist in maintaining a downhill speed.

It will not manage the downhill speed without your help. Especially if you enter the down slope at a high rate of speed like 50 mph.

Manually shift to gear 3 or 4 on the Allison at the top of the down hill run.

5th and 6th gears are just forms of fuel economy selections on the Allison.

(I am almost always in 4th gear on the uphill side if not 3rd gear.)

Enter the 6% downhill slope at @ 40 mph, not 55 mph.

Use the service brakes to slow the rig when gets to @ 2200 rpm or too fast for driving control. 3000 rpm is too high!

Repeat as necessary.


If 40 mph is too slow, you can adjust the speed upwards once into the slope but starting into the slope too fast will heat up your brake components when trying to regain control by reducing speed.

Your goal is to be able to stop your rig in several hundred feet......... on a 6% slope your service brakes will be severely challenged if at 55mph!


If you are hurtling down the slope and unable to stop the rig, there are usually runaway truck ramps to use to stop the rig.

It will be an expensive alternative but you will stop.

[Dutch Star Don]

A ton of good info. I had a 2005 Monaco Diplomat with the 400ISL and exhaust brake. It was a rocket, so the old adage of descend the hill at the same speed as you climbed it, doesn't necessarily hold true for many modern day engines.

Since I was towing a 4 door Chevy Silverado, my exhaust brake didn't provide much braking on 6% - 8% grades, especially at higher speeds. One poster said that you should make a mental note of the speed at which your engine downshifts into different gears while using the exhaust brake. My recommendation would be to actually write it down on a card and keep it near the dash.

The exhaust brake on my Diplomat would hold the coach back on slight grades if I kept the speed around 50 mph. When the grades got steeper, I knew I would have to get the coach down under 44mph where the coach would downshift into third gear. Typically, in third gear, with the exhaust brake on, I could hold a steady speed. The downside was that I was getting passed by everyone.

No matter what you do, you have to be willing to accept that an exhaust brake is limited and if you really want to stay off the brakes on a steep grade, you'll have to keep the speed down to under 45 mph (3rd gear). Don't let other vehicles, that are going faster, intimidate you. Practice with your coach and get a feel for what speed the exhaust brake will hold it back, usually you'll need to get down to third gear to hold on a steep grade.

Lastly, I watch my temps when climbing and descending. The Cummins engine computer does not look at exhaust temperatures. It looks only at intake manifold temps. I use the SilverLeaf computer program and monitor the intake manifold temp. When using the exhaust brake, I saw that intake manifold temperatures rose dramatically. Keep this in mind once you turn off the exhaust brake. Give the engine a chance to cool down before stopping.

One of my must haves on my new coach was a two speed engine brake.

[Mr_D]

Originally Posted by Dutch Star Don

One of my must haves on my new coach was a two speed engine brake.

The Magna we're getting tomorrow has a three position switch as well as the on/off. On flat ground the low works better than our ISC 350 exhaust brake (which is only off/on). Going to take some getting used to!