400 ISL vs 450 ISM mpg

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400 ISL vs 450 ISM mpg

November 19, 2014, 09:33:52 am

Headed to factory tour next week & to check out used 36' coaches @ FOT & MOT. I have searched the archives & have read the various post r/e Foretravel fuel consumption. I know this is a very tired discussion topic & I sense eyes rolling back into heads all over Foretravel land....... but, I just have a very simple question: All things being equal will an ISM 450 consume more fuel than an ISL 400 in a 36' FT?

Jim

Re: 400 ISL vs 450 ISM mpg

Reply #1 – November 19, 2014, 09:56:32 am

Jim

I cannot speak for either since I have a 350 ISC in my 36 ft U270. I can tell you that fuel economy is based on 2 things mostly.

The first is your head- how fast do you want to get there? Especially on hills, in stop and go traffic, etc. The head controls the second thing, which is you foot. If you have a heavy foot, jack rabbit starts, etc, your fuel economy will be lower.
A diesel builds power and speed over time,not an immediate reaction like a gas engine.

We have been full timing for 7 years now. I have averaged 8-8.5mpg over this time period with my coach and good driving methods. I usually travel with water full or almost full- never know where I will be or if we have some trouble so better to be prepared. I usually drive around 60-62 on the highway, above that you lose fuel economy due to wind resistance. When I push it, fuel economy goes down.

Best suggestion I can give you is to get a coach that fits your needs, is in good mechanical shape, and then enjoy it.
The difference in fuel consumption vs your fun over the years will be nothing to consider.

Best of luck with your search.
Ted

Re: 400 ISL vs 450 ISM mpg

Reply #2 – November 19, 2014, 10:01:14 am

I agree with Ted's comments.

I personally would choose the ISM 450 for the better performance (if everything else were equal on the two coaches). However, both engines are good engines, and I believe you could be happy with either one.

We generally get about 8 MPG with our 40' U320 with a 400 hp ISM driving about 70 MPH and towing a car.

Re: 400 ISL vs 450 ISM mpg

Reply #3 – November 19, 2014, 10:01:43 am

Jim: I have the 400 and tow a GMC Terrain. I have not been sucessful in getting better than about 8.2 miles per gallon. I had a 350 36 ft towing HHR before this and it did a lot better on fuel usage.
Not sure you will see much difference in a 400 and a 450. The 400 is a great engine and I have been very pleased with its performance. It does well in mountain driving. In any event to milage is about how heavy your foot is and how hard you push. Just use the economy button as it is not automatic but must be pushed each time you desire to use it. DAN

Re: 400 ISL vs 450 ISM mpg

Reply #4 – November 19, 2014, 10:15:18 am

Thanks, Dan - good info.

Jim

Re: 400 ISL vs 450 ISM mpg

Reply #5 – November 19, 2014, 10:18:36 am

Quote from: dukejhp – November 19, 2014, 09:33:52 am

... All things being equal ...

This discussion may rage on and may even have a hypothetical answer but the issue from my perspective in real world scenarios is that it is impossible to get all things equal.

I've read about folks that can consistently fill their tanks to within a cupful every time but then how do they account for all the environmental variables and their driving inconsistencies? Just pick the one that has been maintained the best and drive the hell out of it. When you look back you probably won't be lamenting over a couple of mpg tenths.

Re: 400 ISL vs 450 ISM mpg

Reply #6 – November 19, 2014, 10:20:12 am

Thanks David & Ted

Jim

Re: 400 ISL vs 450 ISM mpg

Reply #7 – November 19, 2014, 10:29:54 am

not if you keep your foot out of it.

Figue 7 - 9 MPG - tow heavy toad, in mountains or go faster, use more fuel.

about .50 a mile for fuel overall with reasonable aqua hot and generator usage

Re: 400 ISL vs 450 ISM mpg

Reply #8 – November 19, 2014, 10:32:34 am

Buy the coach - not the engine - all things will not be that "equal" either engine will be great, and fuel is a small % of the overall expense of the experience unless you do 30,000 or more miles a year

Re: 400 ISL vs 450 ISM mpg

Reply #9 – November 19, 2014, 10:46:21 am

In the grand scheme of things, given the average RV is driven 7K miles/year, if there's even a 10% difference (unlikely - as Ted and Karen put it best) you're looking at a difference of $300-500 per year in operating cost.

Practically in the noise versus the signal of maintenance/repair and well worth owning the coach you prefer especially when you consider the enjoyment factor.

Re: 400 ISL vs 450 ISM mpg

Reply #10 – November 19, 2014, 10:47:43 am

I have a 400 ISL and have pulled a 24' enclosed trailer for two and one half years now. The trailer weighs loaded 9,500 lbs. Everything has been weighed at Escapees. By the way the people who weighed my rig stated as far as weight distrubition, Foretravels seem to be the best ballanced overall. Getting back to milage. I average 6.5 to 7 mpg. In the Arkansas Ozarks I average 6 to 6.5 mpg. On flat land I will get 7 to 7.5. Thats driving at 60-62 mph. This is a general mpg estimate as I havent measured down to the tea cup. The coach pulls fine with this load.

Re: 400 ISL vs 450 ISM mpg

Reply #11 – November 19, 2014, 11:28:10 am

I agree with Michelle. Lots of potential buyers fret over fuel use. +- 1/2 mpg around 8 mpg over 60K miles at current fuel prices is about 6¢ a mile. You can save more than that with the right credit card.

Our ISM 450 2001, 36' U320 towing a Jeep Wrangler has gotten 8.0 mpg over 63,000 miles. I have seen 6.5 to well over 10 depending on roads, terrain and especially wind. Best performance at 62-63 mph. Best mileage at about 57 mph, flatland, long way between small town you have to slow down in.

As suggested, find a coach in good condition with a floor plan you like and go for it. Colors and fabrics cn be redone. Not so easy with floor plans. Dash and ceiling changed in 2001 U320s. What ever you find ill be good.

Roger

Re: 400 ISL vs 450 ISM mpg

Reply #12 – November 19, 2014, 12:22:08 pm

agree on the $.50 per mile, think diesel fuel is about the cheapest part of enjoying a nice coach, worry more about tax load.

Re: 400 ISL vs 450 ISM mpg

Reply #13 – November 19, 2014, 12:32:26 pm

Do any of the new flat nosed coaches get over 8 mpg at 60 mph regardless of the engine?

Re: 400 ISL vs 450 ISM mpg

Reply #14 – November 19, 2014, 01:34:34 pm

After we bought our coach in CA we drove ours back to MN through AZ and NM (Raton Pass) with no toad, pretty light at 63-66 mph, mostly freeway and got 9.3 for the trip. 8.5 + is not uncommon on some legs but so is 7.5. Best so far was a westbound leg in IA where VMSpc reported 14.6 mpg. I think we had a 35 or 40 mph tailwind. Next day, northbound, same wind, upper 6s. Sort of averages out. VMspc reports engine computer miles and fuel used since day one at 8.0 mpg. Fine with me. At our pace, fuel is not the biggest expense on the road.

It is not really flat nosed. It has a bit of a curve to it.

Re: 400 ISL vs 450 ISM mpg

Reply #15 – November 19, 2014, 01:45:06 pm

I own a couple of corvettes and people always ask what kind of milage do you get. And my response is if you want to drive a corvette then don't worry about milage. Same holds true on your motorhome. Get the best onethat fits your style.

Re: 400 ISL vs 450 ISM mpg

Reply #16 – November 19, 2014, 02:06:33 pm

A fascinating thing about diesel engines is that there is a direct link from fuel used to power produced. If you send more fuel to the combustion process you get more power.

Two things about this. One is that sending more fuel than it can use just sends it out the tailpipe. Compute controlled engines should not do this (much), mechanicals like my 1987 Mercedes 300D can be set to over-deliver fuel and leave a black cloud. So the ideal is to send just enough but not too much fuel.

The second thing is that the way to make an engine that can use more fuel is to make it bigger, more displacement, or to turbocharge it, which makes it act bigger. Other than weight, the main difference between the ISL and ISM engine is size.

So what it comes down to is this, at the same horsepower produced, both will consume practically the same amount of fuel. At full throttle, the ISM will consume more. So with weight, wind, grade, and speed all the same, the difference is in the driver's foot and how fast that driver wants to get up to speed or up the hill.

Oh, and the transmission. The ISM brings with it different transmission shift points, preventing you from loafing along at 55 in sixth gear like you can do in an ISL.

So what I am getting to in a round about way is that the ISM in itself doesn't cost much in fuel economy, a little more comes from the driver, and a little more from the transmission either keeping you in a lower gear or pushing you to a higher speed, which will cost more in fuel economy.

By the way, I believe that ISM450 can be tweaked to convert more fuel into more horsepower.

Re: 400 ISL vs 450 ISM mpg

Reply #17 – November 19, 2014, 02:30:22 pm

To add to Tom's description, unlike a gasoline engine which requires a specific mixture of fuel to air, a diesel engine can operate over a very wide range of fuel to air (read that-- it can run VERY LEAN). That is one of the primary reasons a diesel is more efficient than a gasoline engine at partial throttle (other reasons include more BTU's per gallon, no throttle on the intake that creates a vacuum that the pistons have to pull against, etc).

Almost certainly, in a gasoline engine, larger displacement= more fuel consumption. NOT SO in a diesel. Driven at the same speed/producing the same HP, a large displacement diesel does not use more fuel than its smaller cousin.

Re: 400 ISL vs 450 ISM mpg

Reply #18 – November 19, 2014, 02:37:29 pm

I found a post on another forum asking where are all the motor coaches that supposedly get 10 mpg. My U300 does with the cruise control set at 60 mph, so I replied. I read through a gazillion other replies and everyone was getting about 4-8 mpg with the new flat nosed coaches, regardless of the engine. I got 8 mpg all summer when driving in town back and forth to the grocery store. I got 9 mpg at 60 mph towing a trailer for the first time ever, probably because my cruise control was malfunctioning. I've since got the cruise control working again and expect to probably be back to 10 mpg, even towing the trailer... Don't you think the flat nose has something to do with fuel efficiency at highway speed? ...just saying

Re: 400 ISL vs 450 ISM mpg

Reply #19 – November 19, 2014, 02:45:53 pm

Quote from: Tom Lang – November 19, 2014, 02:06:33 pm

So what I am getting to in a round about way is that the ISM in itself doesn't cost much in fuel economy, a little more comes from the driver, and a little more from the transmission either keeping you in a lower gear or pushing you to a higher speed, which will cost more in fuel economy.

That's not quite accurate. My U320 with a 400 hp M11 will shift into 6th gear at just over 55 MPH. Combine with the fact that it has a higher (lower numerically) rear axle ratio than would a U270 or U295 with an 8.3 liter engine and the result is that for a given highway speed the larger engine will run at a lower RPM and thus have about the same fuel economy as would a smaller engine spinning faster.

Re: 400 ISL vs 450 ISM mpg

Reply #20 – November 19, 2014, 03:45:02 pm

Cummins states that the ISM-- is the smallest engine they make that is rated as "heavy duty " all other engines smaller than that are rated medium duty . All things other than that being equal , it would have to be ISM for the long haul for me . Hard to beat power when you pull anything and much so in the mountains . Brad Metzger

Re: 400 ISL vs 450 ISM mpg

Reply #21 – November 19, 2014, 03:52:53 pm

Okay you experts...is there any easy way to visually identify which engine, ISL or ISM I'm looking at? Do both of them have the red top valve cover?

Re: 400 ISL vs 450 ISM mpg

Reply #22 – November 19, 2014, 04:14:16 pm

Quote from: Chuck Pearson – November 19, 2014, 03:52:53 pm

Okay you experts...is there any easy way to visually identify which engine, ISL or ISM I'm looking at? Do both of them have the red top valve cover?

Here is a picture of an ISL: Formula For Fun Fmcas 87Th Show Cummins 9L Isl Engine Photo 8

Here is a picture of an ISM: Cummins ISM

The accessory drive covers are different (among other things).

Re: 400 ISL vs 450 ISM mpg

Reply #23 – November 19, 2014, 04:16:34 pm

Thanks, DJ

Re: 400 ISL vs 450 ISM mpg

Reply #24 – November 19, 2014, 05:44:23 pm

Quote from: RRadio – November 19, 2014, 12:32:26 pm

Do any of the new flat nosed coaches get over 8 mpg at 60 mph regardless of the engine?

Ours does except when we're going up Killington

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