ISM Performance

[Venturer]

We just completed our first long trip in our new to us and first Monaco.  2200 miles back to WI for the summer in 5 days. Our previous coach was a 2013 Allegro Bus with the 450 ISL Cummins engine. The difference in performance is significant even though the Monaco is 1000# heavier. Total weight with my Avalanche tagging along was 48,000#. I drove at 63 mph and averaged 7.2 mpg for the trip. I am very happy with that especially fighting a strong cross wind through NM, TX, and OK. At that speed, the 1450 rpm was just above peak torque. It didn't downshift on many of the hills where the 450 ISL would have. The boost would peak out @ 42 psi and just hang in there to the top. It did downshift on some of the steeper long hills in Missouri. The engine and transmission never got to 200 degrees even on the longest hill in 85 degree OAT. Of the seven motorhomes we have owned in the last 25 years, this is the first Monaco and first ISM. We couldn't be happier. Well,,,,,,,,, maybe the ISX or the Detroit engines might have been better, but I'll just make do with this one.  

[JDCrow]

I’ve heard the ISM was a great engine. Sounds like you agree! Congrats on your new coach. 

[Gary Cole]

Congratulations Doug.

NM crosswinds. Know those well. 

I've never been satisfied with the performance of my ISL 400. Trucks with similar horsepower go by me like I'm parked. I understand that they are larger displacement engines with more torque. I don't know why Cummins phased out the very reliable ISM. Seems like the perfect displacement for a midrange engine. 

[Paul J A]

Doug; You have a perfect Weight/Horsepower ratio Coach. The ISM or Detroit DD60 are 2 of the best.

A few simple changes to the intake and exhaust components will make a significant improvement to the performance and fuel consumption. 

[JDCrow]

2 hours ago, Gary Cole said:

Congratulations Doug.

NM crosswinds. Know those well. 

I've never been satisfied with the performance of my ISL 400. Trucks with similar horsepower go by me like I'm parked. I understand that they are larger displacement engines with more torque. I don't know why Cummins phased out the very reliable ISM. Seems like the perfect displacement for a midrange engine. 

Probably was a victim of emissions. 

[Venturer]

4 hours ago, Paul J A said:

Doug; You have a perfect Weight/Horsepower ratio Coach. The ISM or Detroit DD60 are 2 of the best.

A few simple changes to the intake and exhaust components will make a significant improvement to the performance and fuel consumption. 

The big muffler has been removed and replaced with a short straight through resonator.  I always run with the jake on low, so I need to be a bit careful slowing when coming into towns. It does bark a bit when climbing long grades. I haven't done anything to the intake, however, I will address that if and when a problem occurs with it. The EGR function has been eliminated with the Ugly Fix. Overall, the engine is responsive and seems to like to run.

[Paul J A]

Doug

Replace all the intake pleated duct with aluminum tubing and use rubber fittings like truckers have.

Gets rid of air friction . Great simple improvement. 

Buy stuff from https://www.intakehoses.com/

Now the engine can breath. The engine air cleaner should be addressed too.

 

 

[Just Jim]

I thought it was just me...  I've loved the performance of my coach.  It just seems effortless running down the road.  Of course I had half the horsepower but a third less length in my old coach.  I drove several Dynasty and Imperial coaches with the 400 horsepower engine before buying the Executive.  They seemed adequate, but not sure I could go back now. 

[Dr4Film]

Here is my latest trip results. 2006 Monaco Dynasty 42 foot Countess III - 49,000 lbs combined towing a 2006 Saturn Vue with a ISL-400-2 Chipped to 425 HP. Our trip was 1495 miles from Florida to New Hampshire. We averaged 8 mpg driving 55 mph normally which keeps the Allison in 6th gear at about 1450 rpm's. Last year with the same setup but a slightly different trip path with fewer miles gave me 8.4 mpg.

My previous Windsor which was more than 10K lighter towing the same car over a similar trip path (fewer miles) gave me 9 mpg with its ISC-350. However, I would drive at 62 mph as that's where the Allison would stay in 6th gear although I can't remember what the rpm's were back then for the ISC-350.

Edited May 4, 2024 by Dr4Film

[Venturer]

12 hours ago, Paul J A said:

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. Richard, that's great mileage. I suspect I would be close to that as well if I slowed to 55. I don't think it would have stayed in 6th gear unless in economy mode and then might downshift on slight inclines. The hardest part for me would be another day on the road with 2200 miles to travel. 🙂 I wonder where your extra weight comes from when I have a heavier engine and heavier Toad, but weigh 1000# less? 

FWIW, My son's Newell weighs 60,000#, plus he tows a 15,000# enclosed trailer. He smiles when a strong tail wind that gets him up to 6 mpg, otherwise he's 5. He could probably do better but runs 70 most of the time.

[Joint Venture]

We have driven about 11k miles in the past year with our new to us ISM powered coach in my signature towing the toad in our signature as well. Generally fuel mileage has been 6.7 to 7.6 towing toad and cruising at 63-66 mph. Power is awesome and on cruise hardly ever see speed drop off in rolling hills. Very pleased with performance, fuel economy and quietness of the ISM engine.