This is a CAPS era ISL370 with engine build date of July 2003, in our 2004 coach.
I went ahead and paid the costs to have Cummins update our ECM to the ISL400.
That's an 8% HP increase. Max Torque remained the same at 1200lbs. (On edit, brain fart - can't recall if it's 1250 or 1200lbs
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Last year I had sat with a shop owner going over this change, and held off from doing so. The extra 30 HP is at the upper RPM end. However, the Torque and HP curves are slightly different between along the RPM band.
At the Cummins Cal Pacific shop, it was not inexpensive and also had about a $100 charge to register/record with Cummins. The tech took just under an hour for this full change... But in regards to costs, I look at this as over the next 10-15 years of planned usage of this coach. So that amortizes the costs out petty low over the years.
So an extra 30HP, what did it do? Too soon to know anything about MPG, that will take several tank fulls to determine a trend in changes, if any at all.
But how about the power?
From a stop, I do fell the coach has the ability to accelerate faster. Not quite as much at the get go, as the ECM working with the Allison Brain, keeps torque levels controlled to avoid driveline stress. But on merging speeds, when say going from 40MPH up to 55-60MPH in order to merge into a freeway. Yes, it is better, and I can notice the difference. (Not a major difference, but noticeable, and thus an improvement.)
On hill climbing. We have a stretch leaving San Diego that climbs Interstate 8 East up thru the towns of Alpine and on towards the Pine Valley area. I have pulled this stretch many times, both with just the coach when out for it's regular every 4-6 week exercise, and again fully loaded with toad at the start of trips that are heading East. (Vs North, where we usually take I15 North to avoid Los Angeles.). I manually downshift for this hill climb, keeping the RPM range where it will have the best chance to maintain speed while climbing this stretch. The extra 30HP, and changed Torque/HP curve, comfortably pulled this stretch in 5th gear most of the way. Only one time did it drop to 4th gear (I did not downshift as I normally would have, as I wanted to see how long it would pull the hill in 5th.) We maintained 55 MPH for about 3/4 of the climb, dropping to 47 MPH in 4th. This was fully loaded with toad for a trip to Prescott, AZ. Prior to the change to ISL400, we would need to downshift to 4th gear at three points (the long grades varies from 6% down to 3-4% at places). We would slow down to 40-43MPH in places.
The Turbo's Max Boos Pressure remained the same, where it very seldoms goes into the mid 22 psi range. (Again, CAPS era non VGT turbo.).
Overall I'm happy with the change. For sure would have been a non deal for me if it was back with the engine was under warranty and Cummins wanted several thousand to do this upgrade, due to warranty as I understand it. But for about $425 (I had discounts from the total bill, due to other items being done.), I'm glad I had them do the jump to ISL400 level.
I've posted on many threads, that with the weight of my coach (40' with Tag), that I felt the ISL370 covered me well the majority of the time. That I only wanted a bit more power for hill climbing and acceleration reserve ability. And stated many times, not to win races, but for safety reasons. Am I now totally satisfied? I'm in the school that reserve power capacity equates to safety options when power will help out in a situation. So I would of course loved to have had an ISM or BIG CAT or BIG DETROIT block in this rig
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I think I will run this as is for a good period of time. Then make a decision if I'll add the TST Performance MP-8 with cockpit control, turned off most of the time, but then having the ability to increase fuel if needed for hill climbs and or acceleration needs. If I do this addition in the future it would be expensive, as I'd also add a EFT Temp Sensor and dash Gauge to monitor things. As well as a FASS like fuel pump to aid the OEM setup.
IMO, for those with the old ISL370 in a heavier coach, for under $450 the pump to ISL400 is worth doing. Not a dramatic change, but an improvement for sure. Many have noted they saw none to little change by this bump to ISL400. Well 8% is not that much, and I could see it not being 'felt' the majority of the time. I had the baseline of that hill climb to go check my results, and feel confident at highway speeds, I'm seeing the difference noted above. I would think lighter weight coaches, would feel more of a change.
Best to all, be safe, have fun,
Smitty