Various Diesel Engine HP

[Bayrat]

For you folks that tow with your 34-36 foot coaches, what do you recommend for the engine/transmission? Most of the rigs I see have the 5.9 Cummins with a 4 speed Allison which varies from the older models at less than 250HP to the newer ones at 275-300. Some do have the 8.3 with a 6 speed transmission but are difficult to find other than the very high end stuff. The beefier engines don't appear to be offered even as an option on most rigs when new.

I want the capability to tow at least 5000 pounds. Obviously torque rules and certainly the 5.9 is more than adequate for my Dodge pickup, however, not so sure when it comes to a motor home which weighs in at ten ton once a boat or car is hooked to it!

 

[WILDEBILL308]

Most of the class A that I looked at in the 1999-2005 range had the 300 Cummins and a 6-speed traney. Some had the Cat motor but were comparable. I just got back a week ago from a run up to Shawnee Oklahoma and had no problem towing my 2012 Civic with a tow dolly.
I have a 2003 Bounder 38N it will tow 5000# with no problem.
Bill

 

[Jim Godward]

A rule of thumb that some use is 1 hp for 100 Lbs.  Thus if your MH weighs 30,000 lbs. and the tow is 5,000 lbs., you need at least 350 hp.  If you expect to tow greater than the 5,000, increase the HP accordingly. 

I work out to about 113 lbs. per HP so I am a little slow up hills and use a little more fuel but still get an overall average of about 8 mpg in the 68,000+ miles and that does not allow a reduction for the generator.  Whatever the generator has used is in that ~8mpg that I quoted.  Not sure what it would be if I deduced a 1/2 gallon per hour for the generator but obviously it would be better as we do use the generator for bood docking and while driving on hot days.

 

[garyb1st]

It sold before we got a chance to check it out but a 1994 Country Coach Magna in the Fresno CA area was listed and sold for $24,000 about two months ago.  It had an 8.3 cummins mechanical C with a six speed.  As I recall the engine had at least 300 hp.  Don't know what the tow rating was but it would certainly pull 5,000.   

 

[Marsha/CA]

Bayrat,

We have a 36' diesel pusher with a 330 cat engine and a 6 speed Allison transmission.  We are rated to tow 10,000 lbs.  We weight just around 29,000 lbs and tow a 3,000 lb car.  We also towed my 3 horse steel horse trailer that weighted close to 8,000 lbs fully loaded with horses, gear and feed.  I would not have wanted anything less than my 330 cat with that load.

IMO, I think the 300 hp 5.9 cummins with a 4 speed would struggle a bit with the coach weight and the 5,000 lbs.

The beefier engines don't appear to be offered even as an option on most rigs when new.

 

I'm not sure where you are getting that info, but when we were looking for diesel pushers we saw very few, if any, 300 hp with 4 speed allisons.  Most were bigger than that with 6 speed transmission and not an option.

I think you will find the bigger engine will do better with that load and climbing hills.  We have over 90,000 miles on this coach and have been up and down lots of steep grades....it does great!!

Marsha~

 

[Gary RV_Wizard]

Only the older 5.9L Cummins would have a 4 speed automatic and those are typically rated at 190-220 hp. Anything new/larger than that will have a six speed.  I would avoid the 4 speeds if possible - the six speeds do a much better job.

 

[DailyDriver]

Look at the torque being produced, as that tells you a lot about the hill climbing ability of a diesel.  If you get the I.D.# of the mfgr, their technical staff can tell you anything about the engine you want to know. 

For instance, mine is a 230 HP cummins with a 6 speed that churns out a little over 600 ft/lbs. of tqe at 1500-1700 rpm, dropping down to 593 ft/lbs. at 1800 rpm.  At the same time, the brake specific fuel consumption (BSFC = rate of fuel consumption/rate of power production) goes from 0.326 @ 1500 to 0.333 @1800 rpm where the HP is 203.  Max HP isn't reached until 2300 rpm, where the BSFC has increased to 0.365.  If you're really curious about the units of the BSFC, wikipedia has a good explanation but I won't go into it here.  Anyway, as you suspect, as you go past your peak torque rpm, torque falls off while fuel consumption and HP increase.  After maximum HP is reached, everything starts to decrease except fuel consumption.

Back to HP....  It's simply a calculated number derived from torque X RPM divided by a constant of 5250.  Don't ask me where the 5250 came from, I just remember it from my old hot rodding days, and it works.  So, torque is what turns the wheels, and horsepower is simply a calculated number indicating work (torque) being done at a given RPM. 

My GVWR is only 23k, with a net carrying capacity (NCC) of over 3800 lbs. and rated to tow 5k.  I tow a 3000 lb. Tacoma and am not nearly maxed out on GVWR or GCWR so total weight being pulled around is 24.5-25k lbs.  That 230 HP number doesn't mean squat to me unless I'm pulling Wolf Creek Pass in 3rd or 4th gear at 2300 rpm and need to keep the revs up to get rid of the heat..!  I cruise at around 1800 rpm (about 60 mph) where I have close to maximum torque and have economical BSFC numbers.  We traveled across many big hills in AZ, CO, WY, and the Black Hills of SD this summer, and yes we did climb Wolf Creek Pass.  My little mouse Cummins motor has plenty of power for all but the big hills that will slow down almost any larger rig and it averaged 9.4 mpg on the entire trip while towing.  If my weight were higher, I'd want more power but would prefer to approach it with an 8.3 liter block instead of a higher HP mouse block.  Just as in the 60's, "There is no substitution for cubic inches".

So, my recommendation is to focus more on the torque of these diesel stump pullers in your operating RPM range, and quit chasing the HP number.  8)

 

[Bayrat]

I have been attempting to find information relative to torque on the 1999-2001 rigs on my hit list, however, the archive manufacturer brochures list only 5.9 or 8.3 along with Allison transmissions, omitting the number of gears and engine torque.

 

[Gary RV_Wizard]

If you know the engine size (liters and horsepower, you can usually find the matching engine specs online. For example, Wikipedia lists the basics for some of the Cummins 5.9L (B series engine) variants
http://en.wikipedia.org/wiki/Cummins_B_Series_engine

Cummins lists many of them as well:
http://www.cumminsdieselspecs.com/