Caterpillar Optimum RPM's For Economy

[JERRYCCONRAD]

I have a Cat 3126 B turbocharged engine with a MD 3060 Allison and a 4.39 : 1 rear axle ratio in my 30,000 lb DP motorhome .. In reading the information on this engine from Caterpillar in one publication they state that the engine should be spec'd to run 60 MPH at 2000 RPM while the recommended engine RPM at curise should be 1800 RPM @ 55 MPH. Then another CAT publication states that for best economy the engine should use the lowest RPM between 1440 & 1800 to obtain the optimimum fuel economy. So what comments do others with this powertrain combination suggest or are experiencing?

Thanks

J.C Conrad

2000 Beaver Patriot 40'

2010 Jeep Rubicon toad

[wolfe10]

Jerry,

Welcome to the FMCA Forum.

Peak torque on your engine is 1,440 RPM. On flat ground, best MPG would be at or just above that figure.

If in rolling hills, 1,600- 1,700 would probably yield better MPG, as it would not downshift on every grade.

Use of economy mode will help in rolling hills. As would soft cruise which allows for more speed variation around the set speed before it takes the throttle to WOT.

And, with that tall a rear axle ratio, were you in danger of running out of fuel, better MPG would probably be at the 1,440- 1,500 RPM in 5th gear.

For more detailed information go to this page and download the Caterpillar document: http://forum.dieselrvclub.org/index.php/topic,6594.0.html

The publication for gearing for 2,000 RPM at 60 MPH is for trucks pulling a more weight, not motorhomes.

[CarlAda]

Brett, would you discuss soft cruise please. It's a term that I'm not too familiar with.

[JERRYCCONRAD]

Thanks Brett.. however I am still looking for a definition and explaination of the term "soft Cruise", prehaps an I am practicing it without knowing what the meaning is.? I usually cruise at 57 -59 mph with RPM's just below 1800 and with the Economy Mode ON except in the mountains then I manually take over the fuel management and gear selection. In the rolling hills of Oklahoma between Tulsa and Springfield, MO. I use the cruise and Economy Mode ON settings.Of course tire inflation is an important matter and wgt distribution are all considered.

Jerry Conrad

2000 Beaver Patriot

2010 Jeep Rubicon Toad

[wolfe10]

There are two "fuel saving" changes you can have programmed into your engine ECM:

Soft cruise: allows speed to vary by 2.5 MPH from set speed before major changes to throttle position occur. Regular/hard cruise is what is programmed in at the factory.

Control of the exhaust brake: LATCH mode is my favorite. This does require that your chassis maker provided a brake (service) signal to the Caterpillar ECM. In Latch mode, the exhaust brake switch can be left on all the time, yes allows COASTING when you want it.

With Latch mode, exhaust brake switch on:

When throttle released, you COAST-- no exhaust brake

When you step on service brake, exhaust brake is instantly engaged and stays on even if service brake released until throttle is applied.

Talk with your Caterpillar dealer about these two programming changes. If your engine diagnostic port is accessible, it only takes 10 minutes.

[JERRYCCONRAD]

Brett,

If reprogrammed into the "Latch Mode" on the exhaust brake and this allows coasting when you release the throttle, wouldn't this be counter productive when descending a grade in the mountains? If I understand what you said, then you would have to ride the service brake to gain the retardation of the exhaust brake under those circumstances..of course shifting down would not then help reduce the momentum going down grade if " coasting was to take place when you release the throttle.. Correct me here if I am not following this sequence correctly.

thanks

Jerry conrad

[wolfe10]

Jerry,

The important point here is "the rest of the sentence": ...and stays on even if service brake released until throttle is applied.

So, even a light application of the service brake for 1 second will turn on and keep on the exhaust brake until the throttle is applied.

Again, there are some coaches where programming latch mode will not work, since it requires a signal from the brake switch to the Caterpillar ECM.