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Bigger Really IS Better

Bruce_Brown
Moderator
Moderator
Ever since we bought our current MH I've always thought it wasn't geared right. At a 70 MPH cruising speed it was turning over 2,000 RPMs, which is a bit too high for an ISL engine. Cummins gearing calculator agrees thats a bit too high, however a call to Spartan said it was running right where it needed to be. (Side note, Spartan was rather rude I'd even question their chassis configuration) Another odd item was our speedometer always read about 4-5 MPH faster than we were actually traveling, yet the odometer was 100% spot on, all GPS confirmed.

Last year it was time for tires, I decided bigger tires were a better option. After a lot of looking, we replaced the 275/70/22.5s with 11R22.5s. In our case they had the same load rating, they'd fit our rim width, maintained proper dual spacing, and lowered the tire revolutions per mile from 545 to 501 - plus we had clearance.

These larger tires ride sooo much nicer than the old ones, our RPMs were down closer to where they should be and our mileage picked up. How much I wasn't 100% sure as last year we only did one long trip and I didn't have enough data to make an intelligent conclusion of how much. Our speedo was now about spot on, the odometer is now off. More on that in a minute.


After the tires were installed I was having a discussion with LarryCad about air filters. We both had the situation of the filter restriction gauge always showing almost full restriction, even with a new filter. He changed his to a larger one and like his results, I figured I'd take a look.

According to the air flow requirement calculator I found our engine requires ~ 1000 CFM airflow at boost the RPMs it was turing with the smaller tires. The installed filter (gotta throw Spartan under the bus on this one again) had a max air flow of...1000 CFM. Enough - sure, in a perfect world. The reality is, as proven by the air restriction gauge, is this really wasn't enough. After some looking and measuring on just how much room we had, I went to a bigger filter. The new one has a max flow rating of 1,600 CFM.


We just returned form a 1,200 mile trip and I am happy to report the combination of these 2 upgrades work **very** well together. Prior to these changes we would run about 7-7.25 MPG. On this trip we got 8.73 MPG going down and 8.25 MPG on the return trip. On the return trip we were fighting some good winds as well as stop and go traffic on Rt 95. The miles were GPS calculated. To keep this as accurate as possible we stopped at the same fuel places both down and back and even filled up at the same pumps. I'm confident in the numbers and needless to say we're very, very happy.

The larger tires have resulted in slightly slower acceleration off the line, there are no other noticeable performance changes. The engine now runs ~1,750 - 1,800 RPM, give or take a few.

We are happy campers.
There are 24 hours in every day - it all depends on how you choose to use them.
Bruce & Jill Brown
2008 Kountry Star Pusher 3910
8 REPLIES 8

Bruce_Brown
Moderator
Moderator
Chris Bryant wrote:
Sounds like the chassis was set up for those tires originally.


I'd agree with that. I was surprised when I called Spartan on it, as mentioned, they said everything was right where it should be. I knew better. An ISL shouldn't be running over 2,000 RPMs at cruising speeds.

DSDP Don wrote:
Bruce....You have one last fix that will give you back that acceleration and will let you see 9mpg. Add the DIgiCR computer chip. You can buy it at Parleys Diesel or Brazells. It will add 52 HP and a 1/2mpg. It reverts back to stock at wide open throttle to protect your engine and trans and doesn't leave an imprint on your ECU.

I added one to my 2005 Diplomat with the ISL. I know others that use it as well. They're about $650.00 and only take a few minutes to install. Plug and play. It has multiple settings. Everyone with the 400 ISL set theirs at (9) with one exception. One coach didn't like (9) and was set at (8). It was a large heavy coach.


Hmmm...I've never been one to do much in the way of chipping anything I own. I'll have to do a little digging on this.

As to the slightly less performance off the line, believe me, it still has plenty. Before it was darn near a race car, well...as close to a race car as 30,000 # can be. :B

It was good enough I always said I'd give up some performance to gain some MPG. We gained the MPG, and other than the off the line stuff, gave up nothing.
There are 24 hours in every day - it all depends on how you choose to use them.
Bruce & Jill Brown
2008 Kountry Star Pusher 3910

Passin_Thru
Explorer
Explorer
Yup! And if you don't buy and install apyrometer you can melt a turbo above 1450 degrees. Be careful what you recommend to people .

DSDP_Don
Explorer
Explorer
Bruce....You have one last fix that will give you back that acceleration and will let you see 9mpg. Add the DIgiCR computer chip. You can buy it at Parleys Diesel or Brazells. It will add 52 HP and a 1/2mpg. It reverts back to stock at wide open throttle to protect your engine and trans and doesn't leave an imprint on your ECU.

I added one to my 2005 Diplomat with the ISL. I know others that use it as well. They're about $650.00 and only take a few minutes to install. Plug and play. It has multiple settings. Everyone with the 400 ISL set theirs at (9) with one exception. One coach didn't like (9) and was set at (8). It was a large heavy coach.
Don & Mary
2019 Newmar Dutch Star 4018 - All Electric
2019 Ford Raptor Crew Cab

ScottG
Nomad
Nomad
Very interesting info. It shows that the manufacturers dont always know best.
A word on Filter Minders; I had one go bad and always read 60% restriction. Even after changing filters and going to a freer breathing unit, it did the same thing. I finally change the FM and it works right now.
I don't understand exactly how it was failing but there were some tiny cracks around the seam of it. To me, that would make it show lower numbers because it wasn't sealed anymore but the results speak for themselves.

fyrflie
Explorer III
Explorer III
That's great information you have posted. And fairly simple upgrades.

I don't think you can ever give a diesel too much fresh air.

Chris_Bryant
Explorer
Explorer
Sounds like the chassis was set up for those tires originally.
-- Chris Bryant

Bruce_Brown
Moderator
Moderator
Its a Cummins ISL mated to an Allison 6 speed (MH3000).
There are 24 hours in every day - it all depends on how you choose to use them.
Bruce & Jill Brown
2008 Kountry Star Pusher 3910

Mr_Mark1
Explorer
Explorer
Bruce,

Do you have a 6 speed Allison or the 5 speed?

Glad that your improvements gave you good results!

Mark
Mr.Mark
2021.5 Pleasure Way Plateau FL Class-B on the Sprinter Chassis
2018 Mini Cooper Hardtop Coupe, 2 dr., 6-speed manual
(SOLD) 2015 Prevost Liberty Coach, 45 ft, 500 hp Volvo
(SOLD) 2008 Monaco Dynasty, 42 ft, 425 hp Cummins