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The highest RPM safe to run for long distanced??? (too low of gear's!!)

12 Valve Engine and Drivetrain Talk about the 12V engine and drivetrain here. This is for 1994-1998.5 engine and drivetrain discussion only.

The highest RPM safe to run for long distanced??? (too low of gear's!!)

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Old 03-21-2009, 12:01 PM
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The highest RPM safe to run for long distanced??? (too low of gear's!!)

I just lowered my truck back down to 35in tires, it did have 40in, (more like 38's). With the biggier tires my stock gearing just wasn't doing it, so i putt 4.56's in it. I still felt it was a little off, like it sould be lower. But i knew some day i would prob be back down to 35's or so. Yesterday i got the truck done and on the road with the 35's. Its, well, differnt to say the least. My speedo is still of, so i don't really know on how fast i was going, But i was running with traffic on the interstate (with speed limit 70) and my RPM's were around the 2500 mark.. With having stacks, it just sounded like the truck was screaming!! So, besides the poor fuel milage, can this take its toll on a motor after many miles of running that high of RPM's?? My boost read about 16-20 and pyro around 700* just with cruse set. I did do a ratio calculator on Rany's ring web site, before i did this, and it said it would be a few RPM's off than a truck with 4.10's and 235's. Dose all of this sound right?? Help me out fellas!!-Chris
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Old 03-21-2009, 06:36 PM
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Not gonna hurt the engine. Gen sets and pumps and other industrial apps, that use the same long block, run for hours and hours at sustained high rpms. 2500 really isn't all that high, but it does sound like its screaming.
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Old 03-21-2009, 09:41 PM
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If you have stock governor springs, you can run up against the governor all day long without a problem.

2500 is not a problem.
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Old 03-22-2009, 12:35 AM
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stock gov. springs = 2700 rpm max unless you have the fuel to make it go a big higher like i did.
3gsk= 3000 rpm because of valve float

id change those gears in a hurry . id hat 2500 rpm @ 70 mph
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Old 03-22-2009, 12:07 PM
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It has a 4k GSK. so i can rev past 2500 if i want. But don't want to hold it that high for very long. I guess the way i have to look at it is, its not made to run 100mph. Oh and forgot to say, the head has been pulled before, and has had EEP valves and valve springs installed along with ported and polished. and studed. So i think they said my valves could handle the higher RPM. I was just concerned about the bottom end.-Chris
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Old 03-22-2009, 12:15 PM
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I would not hesitate to run mine on the governor if I needed to I've done that before on class 8 semi's heck we used to gear them to run on the governor, manys the big cam 400 or cat that redline at 2100 rpm & turned 1900 at 70mph.
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Old 03-27-2009, 03:39 AM
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i would agree with the others. i have worked on cummins for a wile now. i was in the air force and never had a problem with one running at to high of an RPM. even our light tug trucks (aka bobtails) rated to 65k tow and run them at the governor.
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Old 03-27-2009, 06:59 AM
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With proper valve adjustment and good valve springs you are safe up to around 3,200 rpm on the B series. They will run all day long at 3,000, and can handle short full load rpm up closer to 4,000 rpm.

Never run them without a load on the engine over 1,500 rpm, as that puts too much harmonic distortion on the crankshaft and will lead to lower end failure.

The 98 is probably getting a bit weak in the valve spring department by now, so I would keep the maximum rpm in the 3,000 range.
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Old 03-30-2009, 04:52 PM
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Well like i said its got EEP valves and springs. But what do you mean by never run over 1500 without under a load??-Chris
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Old 03-30-2009, 05:35 PM
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means dont rev it in N

*ps - put a gear vendors in the thing and you will be able to cruise 80+

with stock gears n tires my truck spins 1800 rpm @ 82 mph

5th gear over
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