Engine HP - Is there a noticeable difference?

[RETLT]

Hi All,

I'm considering a few used DP's. All are 40 foot coaches with Allison 3000 transmissions. The three engines I keep coming across are the 400HP Cummins, the 350HP Cummins, and the 350HP CAT. For those of you who have driven MH's with at least a couple of these, did you find much of a noticeable difference from one to the other? I plan to drive all over North America and pull a toad.

Thanks for your advice!

[RV Wizard]

All of them will get you there and all of them have proven to be good engines. The question you will have to ask yourself is how fast do you want to pull steep grades? I don't like the feeling of having to get out and help push a coach up the mountain to stay out of other's way. You will also have to keep in mind as to how much the total load will be if you want to size a power plant reasonably.

[hogdriver]

Of course HP vs weight and speeds are factors. I first owned a 35 ft with a 230HP Cummins. Then a 40 ft with a 330 HP Cummins. Now a 42 ft with a 450 HP. All of them cruised at 65mph. All of them went 35-45 mph up the major passes. All got 6-9 mpg. I know someone will chime in about their 600 HP racecar motored RV but IMHO they all have similar performance.

[Alan24601]

Torque is the factor that gets you up the hill, hp is misleading as an ISB 6.7 l is rated at 350hp but only about 700 lb ft torque. ISC is the same hp but over 1000 lb ft of torque. Get a ISC 8.3 l minimum for a 40 ft mh is my opinion.

[Medico]

I was going to add that torque is more important than HP. 2 motors with the same HP can have much different torque ratings. The engine manufacturers will list what the torques are for each motor. Pick the one with the highest torque rating.

[ahicks]

I doubt seriously you'll never get to the point where you'll hold the speed limit going up all hills. So I think it depends on how many trucks you want to pass when going up a hill, and how many will pass you? Our 275hp rig stays in the middle. I pass some, some pass me. My ego is fine with that....

[336muffin]

Originally Posted by Alan24601

Torque is the factor that gets you up the hill, hp is misleading as an ISB 6.7 l is rated at 350hp but only about 700 lb ft torque. ISC is the same hp but over 1000 lb ft of torque. Get a ISC 8.3 l minimum for a 40 ft mh is my opinion.

I agree, horse power means nothing with a diesel. Torque is the crucil number you need to be concerned with.

[Steve Ownby]

Originally Posted by RETLT

Hi All,

I'm considering a few used DP's. All are 40 foot coaches with Allison 3000 transmissions. The three engines I keep coming across are the 400HP Cummins, the 350HP Cummins, and the 350HP CAT. For those of you who have driven MH's with at least a couple of these, did you find much of a noticeable difference from one to the other? I plan to drive all over North America and pull a toad.

Thanks for your advice!

If the the weight of all 3 coaches is in the same range then yes you will notice the difference.
1. The 400 Cummins is likely an ISL which is an 8.9 liter engine with about 1200 ft/ lbs of torque. This is atso the only engine of the 3 which can have a true Jake brake, which is a big deal. The Jake was optional so check for sure.
2. The 350 Cummins is likely an ISC with just over 1000 ft/lbs of torque. It could be an ISB which is smaller with considerably less torque at 660 lbs.
3. The 350 Cat is a C7 with 860 ft/lbs of torque.

All but the ISL will be equipped with some version of exhaust brake which is somewhat less effective than the true Jake.

All should be able to achieve close to the same range of mpg. If one is actually an ISB, it will achieve 1-2 mpg more than the others at a considerable performance penalty.

[Cbones]

I have a Discovery with an ISB 6.7XT which is 350hp/750lb-ft. I travel often with a buddy who has a Providence with an ISL which is 450/1250. I get 9 mpg avg, he gets 8.2 avg. we always are neck and neck in all but the steepest grades. Ultimately we both arrive at the same time. We usually travel at 62 mph, so my gut is don't worry so much about the engine. The floorplan is most important. None of these machines are rockets except maybe an ISX, but you will pay dearly for that level of power.

[Craigls2]

Originally Posted by Alan24601

Torque is the factor that gets you up the hill, hp is misleading as an ISB 6.7 l is rated at 350hp but only about 700 lb ft torque. ISC is the same hp but over 1000 lb ft of torque. Get a ISC 8.3 l minimum for a 40 ft mh is my opinion.

I'm going to agree with Alan on this one. If you don't drive in the mountains or don't care about going 15 mph on a long, steep pull then it doesn't matter. Last summer we left Estes Park for Moab and hit the long, steep climbs on I70 west of Denver. We were in a 2004 Excursion (27,000 lbs, 4000lb toad, 350hp ISC max torque 1050) and went up the mountain at 35mph. Our friends in a 1999 American Eagle (~34,000 lbs, 5,000 lb toad, 330hp ISC I think and max torque around 850 ft/lbs) could only maintain 20-25mph. Dividing the max torque by the max weight of the coach is about the fairest way to compare pulling power between different rigs, in my opinion. We can argue if it's HP or torque that gets you up the hill but my money is on torque.

[Craigls2]

Oops, reverse that. I meant divide the max weight of the coach by the max torque. Then you compare how many lbs of weight you push with each ft/lb of torque. Sorry.

[stuhly]

The difference between a 300 HP and a 450 HP will be about 30 minutes or less at the end of a hard day.

[RETLT]

Thank you all for your very informative replies. I'm new at this, so I want to make the least mistakes I can!

Steve Ownby, or anyone else who cares to chime in, can you tell me the difference between a Jake brake and an exhaust brake? What makes the Jake better? Thanks again.

[Steve Ownby]

Originally Posted by RETLT

Thank you all for your very informative replies. I'm new at this, so I want to make the least mistakes I can!

Steve Ownby, or anyone else who cares to chime in, can you tell me the difference between a Jake brake and an exhaust brake? What makes the Jake better? Thanks again.

A Jake Brake (so called because it was built by the Jacobs Co) is internal to the engine. It's also called a compression brake. When it is active, it changes the valve timing and turns the engine into a big air pump. The Jake will have a on/off switch and a high/low switch. A few have 3 positions. This switch controls how many cylinders are involved. A Jake is more effective and is generally in heavier coaches.

An exhaust brake is a valve placed in the exhaust just after the turbo which, when activated, blocks the exhaust flow and creates back pressure in the cylinders which slows the coach. The exhaust brake only has an on/off switch. Some Cummins engines utilize the VG turbo as an exhaust brake by using its electronic control to restrict the exhaust flow.

In the Cummins engine lineup, the ISL and above are equipped with a Jake. The ISL can be equipped either way. The ISB and ISC have an exhaust brake.
Cat C7 & C9 are exhaust brake equipped and C13 a Jake.
There are other engines but these cover about 95% of coaches.

Hope that helps.