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Old 02-14-2013, 08:10 PM   #15
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Originally Posted by Sky_Boss View Post
This may sound like a silly question but here goes....

I would appreciate some opinions on that.
Hi Don,

Opinions are like, umm...how does that old saying go?

Rather than offering my opinion, here are a few links that may be helpful in your quest for knowledge. They all come from the Cummins website: Home - Cummins Engines

The first is a presentation given by Mark Conover at a FMCA rally last August: http://cumminsengines.com/uploads/do...mmins_0812.pdf

This one is the "Speaker Notes" for that presentation: http://cumminsengines.com/uploads/do...0812_Notes.pdf

Finally, this one is the Cummins brocure for the ISL400 engine used in motorhomes (I think that what you've got in your DSDP): http://cumminsengines.com/uploads/docs/4971107.pdf

In that one, you'll see (on page 2) a couple of charts showing torque vs. RPM and horsepower vs. RPM. You'll notice that max torque is at 1300 RPM. Max horsepower is at 2100 RPM. As others in this thread have suggested that you'll climb best with engine RPM just slightly higher than max torque; about 1400RPM. I think you'll achieve your best accelleration at your peak horsepower. Your engine is governed to not exceed 2200 RPM.

OK, most of the above is fact, to the best of my understanding. The rest is opinion. To the best of my ability, the opinion is based on my understanding of the facts.

Take care,
Stu
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Old 02-14-2013, 09:31 PM   #16
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Don,
As explained and stated several different ways above, let me try to simplify for you.
Unlike a gas engine the modern diesel will give you the best performance (power/torque not horsepower and fuel mileage) at low RPMs and this is where you want to operate the engine, in the high torque band.

Just remember diesels don't spool up fast and once you get use to that you will enjoy the longevity and economy of the diesel. In fact when you climb let the computer do the down shift for you, it will keep all your parameters in both a safe and economical range. If you have a need for exhilaration when climbing you can manually downshift but when you do you are going to sacrifice fuel consumption. As far as starting from a stop, you can stand on the pedal without any harm but if you learn to ease the pedal you will see much better fuel consumption.

A another thing to remember about the diesel, you will have some type of engine brake, all this does is turn the engine into a big air compressor causing resistance when going down hill and that is where you want to keep the RPM high--but don't let it red line, either hit the brake and slow down or up shift to prevent expensive damage.

Once you get accustomed to the diesel you are really going to like it over your old gas.

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Old 02-14-2013, 10:06 PM   #17
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but don't let it red line, either hit the brake and slow down or up shift to prevent expensive damage.
Not quite.
The Allison is programmed to upshift if the engine RPM gets too high. On our 2000 3126B I saw 2,600 rpm and panicked so I shifted up. I called CAT support and found out the 3126B is allowed to run up higher under braking than when pulling. Same with our ISC, max power rpm is 2,200, when the exhaust brake is engaged it can run up to 2,400 rpm.
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Old 02-14-2013, 10:40 PM   #18
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It's always fun to read all the posts of folks(who have driven a diesel for X number of miles a few times a year) that think they are smarter then all the design/operations engineers at Cummins/Cat and Allison and think they know how to get better performance then what they programmed into the control modules of the engine/transmission.
Set the cruise control and monitor temps.
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Old 02-14-2013, 10:57 PM   #19
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It's always fun to read all the posts of folks(who have driven a diesel for X number of miles a few times a year) that think they are smarter then all the design/operations engineers at Cummins/Cat and Allison and think they know how to get better performance then what they programmed into the control modules of the engine/transmission.
Set the cruise control and monitor temps.
I'm not going to suggest that folks don't know things - but I once forgot how to turn my wipers on while underway. Lol. Pretty sure I can't hold a graph in my head while driving. I'm with you. The computer, cruise control and temps are much smarter than I am. I let them do there job and I do mine - which is watch the road and whatever Annie tells me.
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Old 02-14-2013, 10:57 PM   #20
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I agree

I use my cruise control for up and down hills and flat roads. The computer keeps everything in check. The only exception for me is a real steep downgrade. I'll have to pump the brakes once in awhile. Once I slow it down. Back to the cruise control. It's much smarter than me and I know and appreciate that fact.
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Old 02-14-2013, 11:06 PM   #21
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When climbing grades at 55MPH the econ mode is ON along with cruise, and the coach is in 6th gear overdrive. If it is a really long 6% grade my boost goes up along with the EGT (PYRO) gauge. When my EGT reaches a little above 1200 degrees I apply the brake and exit cruise, and switch OFF econ mode. Engine is a Cummins ISB 340HP 6.7L chipped with an MP-8. deSanford how would you go up the grade?
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Old 02-15-2013, 08:21 AM   #22
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This brings up another question in my mind. I'm new to diesel's also (bought mine last April). I've put around 8,000 miles on it since then. When traveling, I've found the cruise control to be very effective. When climbing mountains, I usually turn Economy mode OFF because I like to climb the hills in a more sprightly fashion and am willing to forgo some economy.

But, in reading all of these posts, it sounds like I should really be DRIVING this thing, watching my RPMs, boost and Pyro. Wait, I don't have a Pyro gauge? Should I panic? If a pyrometer is so essential, why aren't those added to the array of indicators I already have?
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Old 02-15-2013, 08:21 AM   #23
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THANKS TO ALL!

I'm the kind of guy that has to understand the basic mechanics and clearly, adjusting from a Camry Hybrid, to a Workhorse 8.L to a Cummins ISL400 in the span of 12 months has challenged my knowledge. LOL

The root of my question was the different feeling I was getting accelerating from a dead stop to any speed. It felt different and I wanted to verify that it was a normal transitional learning curve. The comments about the turbo lag and fly-by-wire all make sense and explain a great deal of the difference.

The lugging thing is something I will just have to get a LOT more miles and experience to appreciate the concept. For now it appears the overwhelming sentiment is just let the computer do its job and enjoy the ride!

The cautionary tail of big climbs and higher altitudes suggests that I need to keep an eye on engine temp and if it rises to force the down shift and use higher RPMs. It would also seem appropriate that once the exhaust brake maxes out it's ability to slow down the bus that would also be the point to brake to slower speed then force the down shift.
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Old 02-15-2013, 09:07 AM   #24
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With my set up higher RPMs means more friction in cylinders, therefore higher EGT. That is why I shut it down and drive up the hill S L O W L Y. deSanford
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Old 02-15-2013, 09:32 AM   #25
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Main thing is to watch the water temp, oil pressure, and anything else you can monitor. I've said it before, the gauges on motorhomes are not the best to keep accurate tabs on what is really going on. I have a American Coach and do not trust my gauges with the important temps and pressures. As said previously, the computers are smart and when trusted they do a great job of managing the engine and trans. I drove over 5 million miles in trucks for a living with 13 and 18 speed trans in which I had to make the choices. I also had at my disposal 18 gauges to give me the information I needed to operate the drive train! I suggest to sit back and enjoy the ride and trust the computers to help make the trip as enjoyable as possible. It's a just a big car.
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Old 02-15-2013, 06:30 PM   #26
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THANKS TO ALL!

I'm the kind of guy that has to understand the basic mechanics and clearly, adjusting from a Camry Hybrid, to a Workhorse 8.L to a Cummins ISL400 in the span of 12 months has challenged my knowledge. LOL

The root of my question was the different feeling I was getting accelerating from a dead stop to any speed. It felt different and I wanted to verify that it was a normal transitional learning curve. The comments about the turbo lag and fly-by-wire all make sense and explain a great deal of the difference.

The lugging thing is something I will just have to get a LOT more miles and experience to appreciate the concept. For now it appears the overwhelming sentiment is just let the computer do its job and enjoy the ride!

The cautionary tail of big climbs and higher altitudes suggests that I need to keep an eye on engine temp and if it rises to force the down shift and use higher RPMs. It would also seem appropriate that once the exhaust brake maxes out it's ability to slow down the bus that would also be the point to brake to slower speed then force the down shift.
Sky_Boss, I think you've got it figured out.
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Old 02-15-2013, 06:37 PM   #27
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An added comment for you to chew on since you are new to diesels and this will only apply if you have a rear mounted radiator, doesn't apply to side mounts. You should spay and wash both the radiator and CAC (Charged Air Cooler-looks like a radiator in front of the coolant radiator). If you don't and if the engine blow-by hasn't been extended you will eventually collect enough gunk to restrict airflow through both and this will increase your inlet temps and you will notice and increase in coolant temps.

What I and many others do at least once a year is to take a degreaser (I use simple green in a lawn and garden sprayer) and spray the CAC from under the MH as well as lifting the bed so you can get the top. Also, spray through the radiator fins. Let it soak for a little bit and then spray it off (I use a pressure sprayer that attaches to my hose (used hot water from water hear once but now just use it from the outside tap) and you will be surprised how much dirt and grim comes off.

To help reduce this buildup just measure your blow-by tube and get a good quality piece of hose and extend it to within a couple of inches from the ground. This will allow most of the blow-by gases to exit out from under the rear rather than getting pulled up by the engine fan.

Not trying to confuse you just trying to help, it happened to me. After driving big trucks and retiring it never entered my mind the the rear radiator would pick up so much gunk because with the radiator in the front on trucks and gas MHs one doesn't have that problem.

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Old 02-15-2013, 06:53 PM   #28
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Sky Boss
An added comment for you to chew on since you are new to diesels and this will only apply if you have a rear mounted radiator, doesn't apply to side mounts. You should spay and wash both the radiator and CAC (Charged Air Cooler-looks like a radiator in front of the coolant radiator). If you don't and if the engine blow-by hasn't been extended you will eventually collect enough gunk to restrict airflow through both and this will increase your inlet temps and you will notice and increase in coolant temps....
Thanks Tom...I have a side radiator but am thinking about this because of a recent "scare". I took my MH to the shop to have some cooling system fittings replaced. In the process the "green" tech diagnosed a leaking rear main seal. Can you say $3-3K?

Well, long story short, I had them finish some work and asked for them to show me this problem. One of the veteran techs greeted me at the bus and said...OOPPSS. Nothing wrong. What was happening was the blow by tube was pointed into the general direction of where the engine and tranny connect. What was thought to be oil from the seal leak was only the gunk you speak of. The new guy got educated too. WHEW! So...when the weather warms up, I'm going to look into an extension.
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