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Old 05-05-2017, 05:02 PM   #1
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High RPMs

Hello All,

I'm curious I feel like my Cummins runs a little harder than it should. It's a 2002 built engine ISC 350. Around 60 mph I'm at 1800 rpms. Anything past that and 1950 and 2000 are right there. Came down a nice grade at about 70mph and I was at 2100 rpms. The exhaust break wasn't on. What could be making my engine run harder or is that normal? Oil was just changed along with fuel filters. Temperatures never go above 190. It idles at about 850. Any help would be great. Click image for larger version

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Old 05-05-2017, 06:20 PM   #2
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I certainly don't claim to be an expert here, but I'd say given the year model you are running, you are running normal. It wasn't until around 2009 when the emission rules really started kicking in that engine manufacturers concintrated on lowering the RPM at max horse power.
I am running a 1999 ISC 300, max. hp @2200 RPM. Click image for larger version

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Old 05-05-2017, 06:24 PM   #3
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I'm fairly new to my 400 ISL. Harder (higher rpms) is not necessarily a bad thingy​. Harder is more a term for engine LOAD, which is the stress on the engine... Low RPMs on an uphill grade for example. On uphill grades you want a lower gear and more RPMs for more HP. On decending grades, your trans will stay in high gear unless you downshift and/or use the engine brake. If you don't, you run a risk of over reving the engine. These are generalities, I will defer to those with the ISC and whatever trans you have.
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Old 05-05-2017, 06:28 PM   #4
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Well, your sig says ISL in a 2007 fleetwood, so thats kind of confusing.
But my ISC350 is running 1550-1580 rpm at 61. And it pulls just fine from there.

Most rear gears on DPs are like 4.88 or 4.56. once you combine that with an Alison 3000 and a 22.5 wheel by 275 or at most 295 that puts them close to 1600 at 60 normally.

So, any idea what Alison you have ( I guess its a 6speed) and tire size?
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Old 05-05-2017, 06:49 PM   #5
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I've been back in forth with that signature for months. This isn't my original coach I did have a 07 that I lost in a fire and i bought this one from a older gentleman who was getting up and age couldn't use it as much. He was the original owner. Got it for pretty much nothing.
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Old 05-05-2017, 06:49 PM   #6
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I run 295 80 R 22.5 and have the Allison 3000
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Old 05-05-2017, 07:07 PM   #7
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Running those kind of RPMs won't hurt a thing. You need to keep in mind that these engines are designed to run up to the governor (3,000 RPMs) and hold them all day long with no adverse effects. I had to run my 5.9 for almost 850 miles at 2250+ RPMs and the only thing it did was cut into the fuel mileage by about a mile or so per gallon.
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Old 05-05-2017, 08:11 PM   #8
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No, it,wont hurt. It wont be making best torque or mpg. .

I think isc govs are set for like 2400. Isbs are like 28 or 2900
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Old 05-05-2017, 08:18 PM   #9
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Thank you all just wanted to be sure. My 370 ran around 1650 RPMs at that speed so I wanted to be sure I was not overworking it.
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Old 05-05-2017, 08:32 PM   #10
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Quote:
Originally Posted by USMC_1345 View Post
Hello All,

I'm curious I feel like my Cummins runs a little harder than it should. It's a 2002 built engine ISC 350. Around 60 mph I'm at 1800 rpms. Anything past that and 1950 and 2000 are right there. Came down a nice grade at about 70mph and I was at 2100 rpms. The exhaust break wasn't on. What could be making my engine run harder or is that normal? Oil was just changed along with fuel filters. Temperatures never go above 190. It idles at about 850. Any help would be great. Attachment 160779


You might consider investing in one of the after market systems that read engine operating parameters from the engine ecm as dash gauges are notorious for being inaccurate. Scan gauge D is one and Silver leaf is well thought of. That being said, rpm/speed is a function of total drive train gearing and unless you have a major problem with you transmission is not subject to change.
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Old 05-06-2017, 01:50 PM   #11
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Max HP on most older engines is at around 2200 rpm, max torque is some where between 1600 and 1800 rpm. We run our agricultural engines @ 2200 rpm all day long with no ill effects at all. This is working them hard burning as much as 12 gallons of fuel per hr in 100*+ heat some day. We run both ISB and ISC as well as older 5.9 & 8.3 mechanical engines. Many have close to 10,000 hours on them and still run great. The only problem I have ever had was when an engine lost the piston cooling jet and scored the cylinder. Oil consumption got high so we over-hauled it. This was at 8,000 hrs.

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Old 05-06-2017, 02:00 PM   #12
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Are you sure it's shifting into 6th gear? At 60mph, engine RPM is dictated by the rear tire size, axle ratio, and what gear the trans is in. The coach/chassis manufacturer uses guidelines provided by Cummins to select the rear axle ration based on weight and tire size.
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Old 05-10-2017, 10:11 PM   #13
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Quote:
Originally Posted by clyon51 View Post
I'm fairly new to my 400 ISL. Harder (higher rpms) is not necessarily a bad thingy​. Harder is more a term for engine LOAD, which is the stress on the engine... Low RPMs on an uphill grade for example. On uphill grades you want a lower gear and more RPMs for more HP. On decending grades, your trans will stay in high gear unless you downshift and/or use the engine brake. If you don't, you run a risk of over reving the engine. These are generalities, I will defer to those with the ISC and whatever trans you have.
That highlighted is red does not happen. The ECM and TCM are linked together, when the ECM tells the TCM the engine has reached its upper RPM safe limit for braking, the TCM upshifts one gear to protect the engine from damage. This applies to electronically-controlled Cummins engines and Allison transmissions. This is explained in the Cummins owners manual.

This is also a concern for novice mountain driving, when speed is allowed to climb excessively, the transmission upshifts regardless of what you want, to protect the engine, leaving you with only service brakes to slow the vehicle enough for the transmission to downshift again.
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Old 05-13-2017, 05:29 PM   #14
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My ISC 350 in the '02 DSDP ran 1650 rpm's at 60 mph with no problems. Present ISX runs 1330 rpm's at 60. ISC was redlined at 2200 under power and 2400 when using the exhaust brake. Seems the ISX is 2100 both under power and when using the engine brake but it's hard to tell due to the quick deceleration.
I think the OP's tach is off or it's simply geared that way. I always used a VMSpc in the Dutch Star since it was more accurate.
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