Fuel Economy: ISC360 HP vs ISC400

[jbikesv]

I've been looking at a Dutch Star 3623 that has an ISC360HP engine. I'd rather have the larger 400 but that configuration seems difficult to come by. The issue of more torque  is relative to a decision. But, I might be persuaded if the fuel economy was markedly better. The simple question is; Given the same or similar configuration......what is the Fuel economy someone has gotten with  the 360 and what has someone else gotten with a 400?  Chime in with your engine and weight specs to make analysis easier.  To help with the torque question.... what speed you are going at the top of some famous mountain pass. I don't expect to do as well as my 2010F250 400HP/800lb torqe pulling my 5th wheel.... but I'd like t know what to       And yes, I know that If I have to ask about Fuel economy I shouldn't  buy. But the engine configuration is going to stay with me forever.. but I am asking anyway.

[wolfe10]

jbikesv,

There are a number of variables involved in fuel mileage including: combined weight, speed, flat/mountains, etc.  Enough that anecdotal information would likely be of little value and may even be misleading.

Consider the physics of the question:  If same weight, speed, terrain (i.e. all the variables are the same) MPG will be almost identical. Unlike gasoline engines, diesel engines are most efficient at low RPM, low throttle positions.  Unlike gasoline engines their fuel to air ratio can vary widely.

The only reason the more powerful engine would use more fuel is that you ask it to produce more power than the smaller engine is capable of doing.  In our coach with Cummins ISL 400, we get very close to the same MPG as with our previous coach with smaller 300 HP diesel.

[jbikesv]

Remarkably helpful answer, Thank you. I take it the 400 does much better on the hills. Noticeably better as in doing 35 at the top of the hill as opposed to say.... 50? 

[kingfr]

FWIW. Our former '08 40 Ft coach had an ISC 360. Our Current '15 42 Ft coach has a 450 HP ISL. The bigger '15 coach consistently gets .5 to 1 MPG better mileage than the '08, running 3 - 5 MPH faster on interstate HWYs. The GVW of the '08 was 32,000 lbs, the '15 (tag axle)  is 44,000 lbs. I attribute the mpg gain to the improvements in engine tuning that can be done with a DEF engine.

[wolfe10]

No, the speed delta on a grade will not be 35 vs 50.

Here is an excellent discussion of coach performance written by Caterpillar Corp.  Physics is the same for Cummins, or any other engine for that matter.  Go to the page showing HP demand at rear wheels on 6% grade: http://forum.dieselrvclub.org/index.php/topic,6594.0.html

BTW, there is a lot of other helpful information on the Diesel RV Club Technical Site: http://forum.dieselrvclub.org/index.php?action=forum

 

Agree with Frank, another variable is the EPA specs that the engine meets.  As happened with gasoline engines in 1975 when catalytic converters were introduced, HP and MPG both went up.  Same for diesels, with most improvement in engines built after 1/1/2007.  But, they newer ones are more complex and require DEF.

[jleamont]

 

5 minutes ago, jbikesv said:

Remarkably helpful answer, Thank you. I take it the 400 does much better on the hills. Noticeably better as in doing 35 at the top of the hill as opposed to say.... 50?

jbikesv, the weight of the coach and wind resistance will play a huge part in this. if you are looking at two coach's close in weight and size one with a smaller engine could be worse on fuel since it will be working harder. So a small DP with a 360HP engine just might out perform the 400HP coach if the unit is significantly lighter. 

We drove a few newer coaches with 330HP engines two years ago, I found myself with my foot heavy on the accelerator due to the coach size and HP/Torque rating just to drive along. Our coach is heavier but also has 70 more HP and 400lbft of torque over those, I press lightly on the accelerator to move along, RPMs are always low, my MPG's are great, for the most part. If I have a strong head wind or running mountains it drops 2 MPG on average. The heavy coach with the 400HP might crest the top of the hill at 35, the lower HP one might as well. Drive both or many and feel them out, take notes on how they perform.

[jbikesv]

10 minutes ago, wolfe10 said:

No, the speed delta on a grade will not be 35 vs 50.

Here is an excellent discussion of coach performance written by Caterpillar Corp.  Physics is the same for Cummins, or any other engine for that matter.  Go to the page showing HP demand at rear wheels on 6% grade: http://forum.dieselrvclub.org/index.php/topic,6594.0.html

BTW, there is a lot of other helpful information on the Diesel RV Club Technical Site: http://forum.dieselrvclub.org/index.php?action=forum

 

Agree with Frank, another variable is the EPA specs that the engine meets.  As happened with gasoline engines in 1975 when catalytic converters were introduced, HP and MPG both went up.  Same for diesels, with most improvement in engines built after 1/1/2007.  But, they newer ones are more complex and require DEF.

 

[jbikesv]

It seems like everyone is buying the bigger engine (maybe a bigger coach) later on. I'd like to do it right the first time. asking you to go out on limb here,but is it YOUR opinion i'd likely be happier with that 400hp larger engine?

[jleamont]

What years are you looking at?

We landed the larger engine due to the size and weight of the coach (and an upgrade ordered by the previous owner), looking back I'm glad we did, the heavier coaches have more real woods inside vs laminates, ceramic tile floor vs vinyl. We chose to buy pre emissions years, with that being said its hard to locate nice ones up here in the Northeast, but it can be done. I'm a bit of a gear head, so for me more power is always a good thing. the only thing that would shy me away is if the engine is physically larger and it created a problem with space in the engine bay for repairs.

[wildebill308]

You have gotten some excellent answers so far. What year are you looking at? If you can I would get a new enough coach that it uses DEF. They will get better milage.

Someone will correct me but  An old rule of thumb was that you needed one horse power for every 100 lbs of gross vehicle weight. I.e. a DP with a GVW of 40,000 lbs, needs roughly 400 hp. Not all manufactuers adhere to that guideline.

I hear to many people complaining about not being able to do a certain speed over  hill X. There is a reason they give you 6 gears, learn how to use them. Look at the practical side how many miles do you spend climbing hills on a given trip? There isn't a diesel out there that can't climb any hill with pavement on it if driven correctly. The other thing is you will not be the only one on most hills. If you get caught behind slow moving truck traffic just slow down and look at the scenery. I was on several hills this summer and it would not be safe to pull around and try to pass the trucks because of short sight lines. The overtaking traffic was doing 70++ and you are trying to pass at 35-40 or slower because you had to slow behind a truck.

On one trip this summer I drove over 8000 miles over most of the big passes out west with a little 5.9 300 hp ISB towing my CR-V. I didn't feel handicapped.

Bill 

 

  

[five]

Maybe a minor point, but I don't think the ICS make a 400 hp engine...that's size is an ISL.  Also, the OP might not know that the ICS is 8.3L and the ISL is 8.9L.

[jbikesv]

The coach I am looking at is a 2010 Dutch Star.  I am discovering find the coach I want is like looking for a four leaf clover.... only harder to come by,  I feel like there are coaches I am missing but RVTRADER,,  MHSRV, RVT, yields nothing in the 36 - 38 foot arena. Suggestions on a search engine? 

[wolfe10]

Tell us what area of the country you are in and what kind of coaches/features/price are on your short list.

[jbikesv]

37-38 foot in length. An engine developing  at least 1050 pounds torque(more it now seems). able to tow 10,000 pounds. Less than 180,000$...   I really like the Dutch Star. Tiffin bus or to a lesser degree Tiffin Phaeton. I'm from Illinois but could find myself easily in the Texas to Florida area if something interesting comes along.

[wildebill308]

2 hours ago, jbikesv said:

The coach I am looking at is a 2010 Dutch Star.  I am discovering find the coach I want is like looking for a four leaf clover.... only harder to come by,  I feel like there are coaches I am missing but RVTRADER,,  MHSRV, RVT, yields nothing in the 36 - 38 foot arena. Suggestions on a search engine? 

Heare are a couple of sites to look at.

http://motorhomesoftexas.com/

http://www.mhsrv.com/

http://www.pplmotorhomes.com/

Bill

[manholt]

What size Fifth Wheel are you currently pulling?

I have 2 good friends, that pull FW trailers, only because they already have the trucks.  CH has a 40 foot that measures out at 44 foot and pulls with a 3500, true 1 ton Duramax and DH, has a 45 foot Custom trailer, that measures out at 51 foot and pulls with a F550.  They both run the flats at 70, I run at 64...only time we see each other on the road, is when we hit the 6%+ hills and I pass them, I run the gears!  They don't.  We all get around 6.5 mpg.  

My thought has always been, if you want great mpg and zip over the mountain roads?  Buy a car and stay in Hotels!  As Bill said, when you get behind a 18 wheeler, enjoy the scenery...when the truck hits the top, I still don't pass, because I don't want to do 80+ mph going down the other side..geez!   

I had a 2011 Allegro Bus 36 foot DP 450 with DEF...got 1 mpg more than my present coach!  Yes, it did go over mountains faster and I had to stop for fuel more often than now, 100 gal. vs 150 gal.,  Also, the 36 foot weighs less than my current coach...I think it has more to do with, how you drive!  Too me it's the journey, not the Destination...

FIVE.  I see no mention in this post, about a ICS 400...360, yes! 

Carl

[jleamont]

It just dawned on me.

The ISC was discontinued a few years back. Its either an ISB or ISL in the HP range listed above.

ISB is 200-360HP/ 520-800 TQ

ISL is 300-450HP/ 1000-1250 TQ

[wildebill308]

Well as the OP is looking at 2010+ the most likely engine is a ISL.

Bill 

[manholt]

At 2010 it's a 09 engine, 2011 it's DEF.  I replaced my Particulate filter last January...you can buy several Hundred's of gallons of DEF for that cost ! 

Carl

[jbikesv]

12 hours ago, manholt said:

What size Fifth Wheel are you currently pulling?

I ve got a 38 foot weighing about 12,000 pounds. Truck and 5th wheel combined weight 20,000 I can do 70 all day and get about 10.5 mpg. I've got 800 pounds of torque now so when I think of a Dp that will weight say 40K.....almost everything seems under powered.... perhaps I need to learn to enjoy the ride.... I'm newly retired and still haven't gotten use to the idea... " I don't care how long it takes to get anywhere."

[jbikesv]

2 hours ago, manholt said:

At 2010 it's a 09 engine, 2011 it's DEF.  I replaced my Particulate filter last January...you can buy several Hundred's of gallons of DEF for that cost ! 

Carl

How many miles would I expect to get before This needs to be replaced?

[jleamont]

Carl, what happened to your Particulate filter? You couldn't possibly have 300000 miles on it. That's when we swap them at work unless something fails and takes one out earlier I.e driver ignores several regen requests, but even then we pull them and have them cleaned and baked and put back in. Did have one with diesel in the def tank (full tank of diesel) it regened and looked like a flamethrower, that one was trash after that.

[jleamont]

51 minutes ago, jbikesv said:

How many miles would I expect to get before This needs to be replaced?

Typically....longer than the coach itself would last, same as a pickup truck with the same technology. Usually a mechanical failure will take it out, I.e. coolant could cause it to crack if introduced.

Heavy trucks should see one replacement in their lifetime as long as nothing damages it. I have heard of the pre DEF or SCR technology engines having some problems (2007-2010) (engine year not coach year). While the engines with SCR technology are more complex they are better on fuel and seem to perform better than the first generation (07-10).

[manholt]

Joe.  You can't compare OTR with a Motor Home, by using miles as a yardstick!  I had 59,000+/- on it...yes, I could have had it cleaned and baked and been without my coach for a 3 month back log, OTR is first in line!  Oh, did I mention that the process is also not cheap?  Couple of hundred less, than replace and go!  How many miles does a OTR put on in a year?  Clean and bake is normally good twice, some luck out and get 3, then you get a new one...by then you have paid for 11/2 or 2 new...but that's called tax write off, we don't have that luxury Joe!

Carl

[jleamont]

Carl, we don't have any OTR trucks. All are local delivery inner city to suburbs. 10-16 stops a day to local merchants traffic all that good stuff. I pulled an invoice and looked, the last ISL that was clogged was at 221,000 miles in a straight truck (1st time it was pulled) and the cost to correct it was $3500.00 Fixing the root cause and pulling the DPF sending it out for cleaning and reinstalling, it was performed by a Freightliner dealer also. That's probably a good $$ reference number for the OP. The truck is a 2011 with SCR.