HP to GCWR of Class A motorhome

Pretty new to this all but have a Diesel HP question.  I have heard that a good rule of thumb is you need 10 HP for each 1000 lbs of weight you want to move, including your tow vehicle. An example I was shows was that for a motorhome with a GCWR of 38,000 lbs you should have a motor with at least 380 horsepower.  (38000/1000 = 38 x 10 = 380).  Anybody have any thoughts on this "rule of thumb".  Thanks for any thoughts or idea about the size of a diesel engine you should have in a class A motor home.

It is a general guide that has been talked about on this forum (1hp per 100lbs). But my first 2 coaches did not pass this "test" and they were fine, even in the mountains. Maybe I was 5mph slower up 6% grades, but that's a small percentage of your driving, even in the west. My 1st coach had a Cummins 8.3 250hp and the coach and towed weight 28,500lbs. The 2nd was a Cummins ISC at 330hp and the coach&towed weighted 36,500. Current rig just passes with a Cummins ISL 425 hauling 41,500lbs. Even my current coach is no rocket in the mountains, just a little faster.

ken

Thanks Ken,

Appreciate you taking the time to reply.  We do live in the Denver area so which ever way we go we hit mountains some as high as 8 to 10,000 feet.  Unless we go East toward Nebraska or Kansas.  So we want to make sure we get the HP to deal with the mountain passes of our area of the West.  Thanks for your input.

Chuck

Chuck we have covered all of CO with a 330HP Cat hauling 31000 pounds and now have a 400 ISL hauling 38,000. The only real differences I notice is fuel consumption and how quickly the ISL will heat up in the summer on a long grade.


If there is one option that makes mountain driving easier it is having a 2 stage Jake Brake to give you more speed options coming down the grades.

Thanks Jeff,

I appreciate your time to respond and the info on Colorado travel.  Does is a Jake brake something you can add or should we look for a coach with that brake?

Chuck

howellad said:

Thanks Jeff,

I appreciate your time to respond and the info on Colorado travel.  Does is a Jake brake something you can add or should we look for a coach with that brake?

Chuck

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Most coaches will have an exhaust brake whereas a Jake Brake is actually an engine compression brake.  These will be designed into the engine prior to the chassis manufacturer choosing the engine.  The difference is that an exhaust brake is literally a butterfly valve that closes off in the exhaust system to create backpressure and hold the engine RPM down thus slowing the weight of the coach. 

An engine compression brake, aka Jake Brake, is a mechanical linkage which is added to the overhead of an engine which will allow the compression of the engine to reduce RPM/speed of the engine/coach and just before the injectors are to inject the fuel into the cylinder the exhaust valves are opened off of their seats to release the compression.  In short it add resistance to the pistons on every stroke thus reducing the RPM or slowing the crankshaft revolutions.  This is the exhaust "fluttering" or releasing sound that is heard with compression brakes.  Compression brakes are more effective and are rated is braking horsepower.

Exhaust brakes work well for the majority of mountain driving but a two or three stage Jake brake is hard to beat.  The Cummins ISL is the smallest engine in the Cummins family to offer an engine compression brake.  I beleive the CAT C12 is also the smallest of the CAT engine family to offer compression braking but I am not 100% sure on that.

Mike.

Mike

>>  I beleive the CAT C12 is also the smallest of the CAT engine family to offer compression braking but I am not 100% sure on that.<<

The Cat C-10 has a 3 stage Jake.

My brake is labeled "retarder".  Would that be specific for either of these brakes?  Just trying to figure out what I have.

"Retarder" could be most any form of auxiliary braking, including a "transmission retarder", which hasn't been mentioned yet. If I were a betting man, I would bet it's an exhaust brake on an Escaper, though.

Terry A. Brewer said:

Mike

>>  I beleive the CAT C12 is also the smallest of the CAT engine family to offer compression braking but I am not 100% sure on that.<<

The Cat C-10 has a 3 stage Jake.

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Thanks Terry for clarifying.  If memory serves the C-10 was available in the 385 hp and 1150 ft/lbs of torque version correct.  My aunt/uncle had one in their Beaver Marquis but I don't remember it having a compression brake. 

Mike.

Gary RV Roamer said:

"Retarder" could be most any form of auxiliary braking, including a "transmission retarder", which hasn't been mentioned yet. If I were a betting man, I would bet it's an exhaust brake on an Escaper, though.

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I agree Gary, I don't think the Escaper would have a retarder either.  The only coaches I have seen those in are the Foretravel, Prevost and I believe they were options in the Newells as well.  Great way of descelerating a rig although the tradeoff is some extra heat in the transmission.

Mike.

Thanks Gary.  Zmotorsports- I was just going by how the switch is labeled but I guess that isn't very informative ???.  Is it possible to tell by how it behaves when you put it on, ie- downshifts when applied or do they all do that?  Please excuse the newbiness :-[

The Allison Retarder is a very expensive option, so usually found only on high end rigs. Plus, I think the Escaper has a Cat 3126 engine and there is no engine (Jake) brake available for it. That only leaves an exhaust brake, e.g. a PAC, BD, Banks,  or similar device in the exhaust system.

They all downshift to work, though sometimes the driver has to do it manually. The downshift is done to increase the braking and is not inherent in the type of retarder device installed.

Ok, I think I got it, thanks for your patience!  What I do know is it works great and am really glad I have it.  Coming down mountains don't even need to put on the brakes,  just touch them for the curves, love it!

vetmom said:

Thanks Gary.  Zmotorsports- I was just going by how the switch is labeled but I guess that isn't very informative ???.  Is it possible to tell by how it behaves when you put it on, ie- downshifts when applied or do they all do that?  Please excuse the newbiness :-[

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No need to apologize.  We were all newbies at some point.  I was just trying to explain the differences.

Mike.

Mike

>>If memory serves the C-10 was available in the 385 hp and 1150 ft/lbs of torque version correct.<<


385 hp, 1350 ft/lbs torque & Allison 4000 trans...

Terry A. Brewer said:

Mike

>>If memory serves the C-10 was available in the 385 hp and 1150 ft/lbs of torque version correct.<<


385 hp, 1350 ft/lbs torque & Allison 4000 trans...

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Great, thanks Terry.  I couldn't remember.

Mike.

vetmom said:

Thanks Gary.  Zmotorsports- I was just going by how the switch is labeled but I guess that isn't very informative ???.  Is it possible to tell by how it behaves when you put it on, ie- downshifts when applied or do they all do that?  Please excuse the newbiness :-[

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If you have a diesel pusher application, it's very easy to see your actual exhaust brake system if that is what you are wanting to confirm you have. Lift up the bed and look at the engine from the top, find your turbo and there will be your exhaust brake. I've attached a picture pointing what one looks like. True Jakes (which most RVers call every exhaust brake a "Jake" and its simply not true or accurate in anyway) you can't see because they're within the engine. Take a look at the picture and see if you can find yours.

Thanks, pictures are really helpful!  I'll be looking under the bed tomorrow ;D

Thanks to all for the great information on braking systems.........Not related but any view points on side radiator vs. back engine radiator on diesel pushers.  I understand and maybe not correctly that radiators in the back take up to 25 HP away from the engine plus less air flow.  Fact or fiction?

Chuck.... still a newbie but getting better.