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Post Info TOPIC: Why do motorhomes turn so high rpm... somebody stop me from changing my differential


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Why do motorhomes turn so high rpm... somebody stop me from changing my differential


I have a problem with 1800 rpm at 65mph.   However, at 62 mph and 1680 rpm I do get 8 mpg.

My coach has a Cummins 370 ISL and 33,0000 gross weight. 1200 ft pds torque at 1300 rpm is my max torque. At 65mph and 1800 rpm I have lost about 200 ft pds torque. At 70 mph i am under 1000 pounds of torque and that means the slightest grade is going to drag my speed down until i hit the higher torque in my curve.  I see this as a problem. RAMBLING THOUGHTS FOLLOW. 

200 hp will push a 70,000 semi down a level road. So hp is not the issue here in motorcoach world.  Hill climbing and pulling power is the issue and that is more dependent on torque and less on hp

rpm over 1500 rpm is wasting fuel i have been told by people more knowledgeable than i regarding efficiency of a diesel truck.

A sweet spot is usually no more than 200 rpm over max torque so i have been told.  For my coach that means a 1500 rpm cruise speed is best.

A 250 to 300 rpm window is optimum cruising sweet spot and torque advantage.

Currently i have a 488:1 differential ratio and it gives me 1800 rpm at 65mph. I am thinking about a 372:1 ratio or close  that will give me over a 20% drop in rpm and keep me less than 1500 rpm and within my cruise sweet spot. Which I feel is about 200 rpm over my max torque.

Folks, i have been around heavy trucking for about 30 years and have had a coach for about 5 years.  We have loaded semis that gross over 70,000 pounds and most get 5.5 to 7 mpg loaded.  Good drivers can get 8 mpg providing the rigs are set up properly.  Most rigs have around a 370:1 differential or taller.  Most rigs cruise at 65mph and turn in the 1400 to 1500 rpm range.  Why are motorcoaches set up by the builders to turn 300 rpm more at 1800 and 65 mph. That is 500 rpm out of the max torque point and wasting fuel to boot.  Someone please tell me why i shouldn't go with a 370:1 ratio.  I realize motorcoach engines are medium duty and semis are mainly heavy duty and have higher torque.  Motorcoaches weigh half the max semi load.  So, i don't see this as a problem.  Perhaps an engineer from one of the coach builders can weighin on this subject and enlighten me.  I have called a couple of builders and no one will address the matter with me.  They say the engineers tell them what to do. Most big pusher coaches are set up in this high rpm manner.  Why?



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most highway rigs have extremely larger engines as in 1800-2000 lbs of torque and 15L (cid)
yours is 8.9L(small block) I have the 425HP ISL 1200lb torque and it does about 1650 ish at 63 MPH
I get about 7.5 west coast driving(lots if hills and mountain passes)
also large OTH rigs have 18 speed trans so have extreme low gears to start off with
you switch to 3;70 gears from 4;88 and you will have no power on the hills or starting off

the higher gear ratio will make it tougher(slower) to get up to the 1300-1400 rpm hi torgue range

I can keep above 50  MPH on long steep climbs but in  4 gear and 2200RPM



-- Edited by ticat900 on Tuesday 10th of November 2015 05:03:29 PM



-- Edited by ticat900 on Tuesday 10th of November 2015 05:20:14 PM

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A Gear Vendors over drive might work as well. Is your coach a factory spec coach or was it spec'd by a previous owner?

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That is just the way the smaller CID engine operates. They are meant to rev a lot higher than the larger 15L engines. The really annoying part is the smaller engines equipped with exhaust brakes. In order for an exhaust brake to work efficiently and correctly the engine must run at a higher RPM. The exhaust brake has very little effect at lower RPM. The tranny on most will downshift from 6th to 4th when activate the exhaust brake....................The engine screams like a banshee.

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TRAILERKING wrote:

That is just the way the smaller CID engine operates. They are meant to rev a lot higher than the larger 15L engines. The really annoying part is the smaller engines equipped with exhaust brakes. In order for an exhaust brake to work efficiently and correctly the engine must run at a higher RPM. The exhaust brake has very little effect at lower RPM. The tranny on most will downshift from 6th to 4th when activate the exhaust brake....................The engine screams like a banshee.


lots of them(mine included) use a 2 stage engine brake which is two- three times as effective as a exhaust brake but they(engine brake) still downshift as low as 4 th gear also  and if u call 2200-2400rpm screaming like a banshee indian then mines guilty as charged although I dont hear it as its at the rear LOL

 



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Alie and Jims Carrilite wrote:

A Gear Vendors over drive might work as well. Is your coach a factory spec coach or was it spec'd by a previous owner?


 4;88 are used in most all 8.9 L diesel pushers and for good reason its what is needed to make the system work correctly

Personally I have never seen nor heard of anyone using a over drive system on a  Diesel pusher nor do I believe there would be any benefits what so ever the way I understand the systems

My 8.9L  425HP ISL works just perfectly fine reving 1650 all day long at 63MPH



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