Cost differential/value of the ISL9 vs. the ISX15?

[rgkroeber]

I'm considering two Renegade coaches, one has an ISL9 with 350HP and 1,000 ft./lbs torque and the other has an ISX15 with 550HP and 1,850 ft./lbs. torque. Each coach is the same length (37 ft.) with identical floor plans and interiors that I like; however, the ISL9 coach has more options/features. The asking prices on both rigs are nearly the same, with the ISL9 dealer currently asking $2k less than the ISX15 dealer.

My dilemma: I'm concerned whether 350HP is sufficient if I plan to regularly tow a Chevy Silverado throughout the western states and PNW, but is the 550HP alone worth it and at what premium ($) over the 350HP? I could be happy with a more powerful ISX15 that has less options/features, knowing that my power plant was rock solid and wouldn't have me regretting the choice as I tackle those long road grades of the Rocky Mountains. But the ISL9 coach with additional options/features would likely make my full-timing experience more enjoyable (when parked/boon-docking).

[DD788Snipe]

You didn't say what year. My coach came with the ISL9 @ 350hp & 1000 ft/lbs of torque. I paid a Cummins shop $400 to reflash the ECM and bumped it to 400hp and 1200 lb. I tow a 2017 Canyon @ 4500 lbs my coach is 32,500 lbs so I'm 37,000 GCW and have all the power I need. Let's put it this way, I went from decent power to a fire breathing dragon for $400. Does it run a little warmer? Yes but even on a 7% grade I've never seen over 205° and at 1900 rpm I'm running 65 mph and never drop below 50 climbing passes at 6 to 7%. The ISL is a workhorse. Very dependable and the ISX has had its problems. Do some research check maintenance records and ask for oil analysis. Hope this info helps and good luck. Any more questions PM me.

Note: The ISL9 ECM can be reflashed to 425 HP but no increase in torque.

Sent from my SAMSUNG-SM-T337A using iRV2 - RV Forum mobile app

[Chowboy]

Does the gearing reflect the difference in HP/Torque? Not sure of the engines and +/- but I have traveled in a VW bus in the 60's unable to beat head winds and elevation in the Rockies and still enjoyed the destination even after enduring 35 mph highway travels. My guess is either will approach or exceed the speed limit on most interstates and where it won't there will often be slow lanes. If it is the destination that is important and both engines are reliable why not enjoy the coach that makes you happiest. If you are wanting to set records for arrival compromise.
Just a reasonably happy trailer tugger.

[guardrail53]

"You didn't say what year. " Very important!

X-2, My thoughts exactly! Rail!

[rgkroeber]

Both rigs are new 2018 models.

[tom chelbana]

Tiffin has a coach that comes with a 450 ISL engine and if you want the ISX 600 with Allision 4000 trans. It is a $60k option for the ISX. HP equal extra cost.

[Fiesta48]

If you drive 10k miles a year, thats double most, it is only 2.5% of the time. 97.5% sitting. 5k is 98.5% sitting.
Amenities will mean more.
If driving is more important to you, then get the larger engine especially if you will be driving in hills, Mtns etc.
I've opted for a compromise. 380 ISL 1150 ft lbs and a coach with many options available. And smaller engine probably gets better MPG. I get 9.

[Smitty77]

Assume these two 2018's have different rear end's, as well as stronger driveshaft!

A few one-liner's to consider:

'Having more power, means never having to say sorry while climbing a hill.'

'Having more power, also brings more Braking Horse Power via the Jake Brake, which provides enhanced safety in mountain driving.'
(Does the ISL on the Renegade have a Jake Compression Brake, or Exhaust Brake?)

'Having more power, usually means the engine is loafing along. Many times a Bigger Block engine, can yield the same - or better, MPG then a smaller engine.'
(That goes directly back to the gearing...)

'Much harder to safely add more power if you find a smaller engine is not quite enough for you.'

'If all things are almost equal, but you have an option to get a bigger engine, do so.'

=====

Now. Those comments aside. The ISL is a good engine, and if it can be reprogrammed to produce more power, that is an option. But it still will never be an ISX. As mentioned, the ISX had a few years of problems with Valve Trains. Due diligence to validate if the 2018 ISX engine in the Renegade is a problem ISX or not. (And note, the ISL also had some problem engines too! But earlier then 2018!)

Yes the costs for yearly maintenance will be higher with the ISX vs the ISL, but SWAG Less Then $500 more a year on higher oil volumes, as one example.

I have and ISL, it gets the job done. But, I'd have preferred a bigger block engine for the reasons above... (DW saw the galley in this coach, and it was game over - it could have had a Briggs & Stanton engine for all she cared!).

Best of luck on your decision,
Smitty

[DD788Snipe]

Well being that they're 2018s that's a game changer. They're under warranty so if you bought the ISL and you want to reflash the ECM Cummins will nail you for around $3,000 (that's software and install give or take a few bucks) for the software to keep it under warranty. Also more than likely it has an exhaust brake and the ISX has a compression brake which is better. That's the one thing I wish I could change on my coach.

[petrel]

Cummins considers one a heavy duty engine and one a medium duty engine. How much you travel may be the best way to determine which one is right for you. I've driven OTR rigs with the ISX and it seems more powerful than it's specs indicate. It is a solid engine, built for continuous duty.

You see few to no threads started by folks who regret purchasing a motorhome with plenty of power. However, there are quite a few threads discussing how to get more power out of existing engines.

[38Chevy454]

Just as Petrel said, nobody ever said they wished they had a lower power engine. Or back to the 60's saying "no replacement for displacement". The extra torque of the bigger ISX really does make for a nicer driving experience.

I have the ISX, with a three stage engine brake. I like not having to continually downshift and going slower when I encounter a hill. Just give more throttle, increase boost and burn a few more dinosaurs to stay at speed or near full speed going up the hill. The three stage engine brake is great going down, select the range that provides the desired resistance.

Also tend to agree that an engine running at less of total power percentage will be running easier. That should translate to better reliability and engine life. Although almost none of the RV applications ever come close to Cummins engine design life, no matter which engine. The gearing of the larger engine will be different, and the bigger engine will have a lower cruise rpm. Also lower redline, for example my ISX at full throttle upshifts will not exceed 2000 rpm.

The ISL is a good proven engine, so is the ISX. I think either can work for you, just the ISX will work better.

[hypoxia]

Originally Posted by tom chelbana

Tiffin has a coach that comes with a 450 ISL engine and if you want the ISX 600 with Allision 4000 trans. It is a $60k option for the ISX. HP equal extra cost.

That's a lotta dough! Window sticker on mine says the boost from 525HP to 600HP was $10,000.

I'll beat you to the top of the hill with my ISX 600 and then you'll whiz past me at the fuel stop with your little ISL.

Climbing hills is over rated, IMHO. Drive a few thousand miles through the mountains with a big engine and keep track of the time saved and then calculate how much time was spent getting fuel.

No matter what engine I want a 150 gallon fuel tank, minimum.

[38Chevy454]

I don't think the mpg difference is as much as some think it is. You have basic physics of weight and aerodynamics. To overcome those going down the highway at 65 mph takes a given amount of power. Let's just call it 100 hp for discussion. So your engine has to make 100 hp, which takes a given amount of fuel. The smaller engine is working at a higher throttle setting vs the larger engine. Both need to make the 100 hp to maintain the cruise. The smaller engine may have slightly less friction due to smaller parts rotating around, and being smaller pumps less air through the engine. But in the end, you still need to make the 100 hp available to move the motorhome, regardless of what size your engine. At max power, such as climbing a hill, of course the larger engine making more power will use more fuel.

I just do not think the mpg difference is as large as many think it is. For a motorhome with 35K of weight, and a big box shape for less than good aerodynamics, it takes a lot of energy to overcome. That is physics, and physics always wins. Speed has far more to do with aerodynamics and required power to maintain a cruise speed than engine size, just pointing out for argumentative purposes.

I am not saying either engine is unacceptable for the OP's purpose. Either can work. Both have trade-offs in the decision, especially the significant cost difference.

[Mr_D]

The new ISX does NOT have valvetrain problems like the 2007 EPA DOHC engine. The new SOHC ISX seems to be problem free.
My ISX 650 after the last rebuild is great in the hills and the truckers aren't used to and some don't like being passed.