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Old 10-24-2008, 11:55 AM   #1
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Hi,

I have been looking for a DP motor coach for a while and have been researching a number of issues including engine power. I have just learned that Cummins has a 350 hp ISL engine and a 400 hp ISL engine. I am looking at approximately a 39' coach and wonder if the 400 hp ISL is over kill compared to the 350 ISL. This size coach has a GVWR of 32,000 lbs.

Does anyone know if the 400 is a larger block engine than the 350 and can you offer any feedback on power adequacy/over kill?
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Old 10-24-2008, 11:55 AM   #2
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Hi,

I have been looking for a DP motor coach for a while and have been researching a number of issues including engine power. I have just learned that Cummins has a 350 hp ISL engine and a 400 hp ISL engine. I am looking at approximately a 39' coach and wonder if the 400 hp ISL is over kill compared to the 350 ISL. This size coach has a GVWR of 32,000 lbs.

Does anyone know if the 400 is a larger block engine than the 350 and can you offer any feedback on power adequacy/over kill?
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Old 10-24-2008, 01:33 PM   #3
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Same block, just de-tuned. Probably also will not have the same turbo boost rating.
Also lacks 150 Ft.Lbs of torque
50 HP would probably not be noticeable, but in any hilly area, the torque loss would be. But only if you had been used to driving one or the other.
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Old 10-24-2008, 01:40 PM   #4
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what year coach makes a difference in the answer to the question.350ISC was restroked and called a 400ISL.some where around 2003/04 new block was introduced with 2 stage jake option..basic rule of thumb for horsepower rating for every 100# 1 hp is required.
Depending on the model year of the 400 they had rod bearing lockup problem.
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Old 10-25-2008, 06:18 AM   #5
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Actually the ISL engine (which replaced the Cummins L10 in 1999) has been available is several different horsepower ratings over the years. It is an 8.9L block and was initially offered starting at 310 hp and currently comes in 400 and 425 hp models. All recent versions have the same 1200 ft lbs of torque, regardless of horsepower. I don't know if that was the case for the 350 hp version or not, but my 2003 370 hp ISL is a 1200 ft lb engine, same as the 400 and 425. The torque curve (torque vs RPM) is different, though, even though the peak torque is the same.
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Old 10-25-2008, 07:20 AM   #6
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The 2009 Phaetons/Freightliner chassis use exclusively the 360 HP Cummins ISC running ULSD. Emissions is not a problem and the engine is definitely not dropped. Freightliner did drop Cat and MBE engines though. They are strictly Cummins ISB, ISC, and ISL in all of their RV chassis as of this year.

The ISL "generally" had a two stage engine compression brake although some of the 350 and 370 HP version did have exhaust brakes. It all depeneded on how much the RV builder wanted to spend. The 350 HP ISL as originally used in the 2007 Phaeton 42QRH tag axle was an exhaust brake and the torque was detuned from 1,200 ft-lbs to 1,050. Now they are all getting the 360 ISC with 1,050 ft-lbs.

All of the ISC engines are 8.3 liter while all of the ISLs are 8.9 liter. Horsepower isn't as important as torque and 1,050 ft-lbs will get the job done. However 1,200 ft-lbs is even better. The ISL engines are definitely not overkill. Drive them both and you'll see that there is a difference but don't worry - you won't be smoking your tires off the line with the 400.
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Old 10-25-2008, 07:43 AM   #7
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Torque will be noticed in the seat of your pants for acceleration from the stop light.

But more horsepower will always get you up the hill sooner.

Torque vs. horsepower, a commonly misunderstood issue.
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Old 10-25-2008, 04:33 PM   #8
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<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Eaglesview:
Torque will be noticed in the seat of your pants for acceleration from the stop light.

But more horsepower will always get you up the hill sooner.

Torque vs. horsepower, a commonly misunderstood issue. </div></BLOCKQUOTE>
I had a 300 hp Cummins ISC with 950 lb/ft torque and my brother-in-law had a 300 hp Cummins ISB with 600 lb/ft torque. My motorhome was larger and much heavier and when we crossed the western mountains many times I always beat him to the top.

That proves torque rules and horsepower comes along for the ride. My present Cummins ISL has 1200 lb/ft torque and I love this engine.
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Old 10-25-2008, 04:37 PM   #9
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<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">But more horsepower will always get you up the hill sooner. </div></BLOCKQUOTE>

I have to strongly disagree with this statement. Infinite horsepower and zero torque will leave you stuck at the bottom of the hill forever.

But as a practical matter, the thing you care about is how much torque and horsepower can be delivered at the RPMS at which you actually drive. Large hp or torque ratings at sky-high rpms is power you can seldom make use of. Look for good hp and torque as low as possible in the engines rpm range. That's power you can use.
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Old 10-25-2008, 04:40 PM   #10
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Cruzer is right on. A buddy of mine just got a 2009 Phaeton 40Q last month and it has the 360 hp ISC with an exhaust brake. Another pal with an 07 has the Mercedes engine and an exhaust brake.
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Old 10-25-2008, 06:38 PM   #11
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I give up. I guess the laws of physics have been rewritten since I learned the relationship between torque and horsepower.
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Old 10-26-2008, 04:41 AM   #12
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The disagreement about horsepower and torque is on every RV forum and has resurfaced from time to time for as long as I can remember.

Each camp believes they understand it correctly, and some will not be convinced that they are wrong.

I don't fully understand the relationship myself and it doesn't matter to me as my rig has plenty of horsepower and torque.

I don't care about being first up the hill as long as I get up there.
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Old 10-26-2008, 06:34 AM   #13
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The relationship between torque and horsepower used to be the following.

At any given rpm.

HP = (torque x 2 x 3.14159 X rpm) / 33,000

HP is an measure of the rate at which work is done.

Torque is an indication of force or rotational force.

1 HP = 33,000 ft-lb/min.

2 x 3.14159 is the distance a force is applied to a one foot long lever (Torque) through one rotation (2 x radius x pi)

The more revolutions in a minute at a given torque, the more work that is done, i.e., HP.

Or more torque at a given rpm, the more work that is done, i.e., HP

Back to a previous post, seems that no torque = no horsepower.

Maybe these principles of physics have changed, sort of like the new math.
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Old 10-26-2008, 09:11 AM   #14
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The relationship is as Eaglesview describes and is fixed. If you were somehow able to increase the torque of an engine at some specific RPM range, the hp would also be greater. But we are buying engines, not designing them, so we have what was designed in. When evaluating and engine for likely performance, look at the torque & hp it generates at the RPM range that your transmission and rear axle gearing need for normal driving. Typically in the 1400-2200 rpm range. Torque & hp that are generated at higher rpms is of little value to you. That's why diesels, with their high torque at low rpms, pull better. Gas engines typically have to wind up to around 4000 rpms to generate their rated torque & hp and you get by with maybe half their rated output at more normal rpms.
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