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Old 06-15-2013, 11:46 PM   #1
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RPM Red Line?

So what is the red line on the 8.1? When using the grade brake, typically how high do you let the RPM's go before braking? Thanks
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Old 06-16-2013, 12:21 AM   #2
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My 8.1 has a broken red line @4500 and a solid red line @ 5000. I hate to go there but sometimes gravity just takes over and I wind up braking. Never been past 4500. It shifts then. I do not have a grade brake. Maybe someone else will chime in for you.
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Old 06-16-2013, 04:59 AM   #3
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4,000 is my personal limit. Heard too many stories of broken engines from those doing higher on a regular basis.
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Old 06-16-2013, 06:35 AM   #4
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4000 is also my personal limit. Why? Because I think I read it here years ago and it seems reasonable and safe.
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Old 06-20-2013, 07:29 AM   #5
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There are practical limitations to RPM, mainly piston speed and valve train performance. If the Mean Piston Speed (MPS) is too high, the engine will not last long. If the valve train (cam followers, pushrods, valves) cannot open and close the valves without floating, once again the engine will not last long.

In the GM 8.1 Vortec we have a stroke of 111mm. At 4,000 rpm this equates to a MPS of 2,902 feet per minute (fpm). This is sustainable all day long.
At 4,500 rpm the MPS is 3,264fpm, once again sustainable without a problem.
At 5,000 rpm the MPS is 3,627fpm. This isn't hitting the limit for modern designs with modern lubrication but in an engine with a bore of 109mm the pistons are quite large and I wouldn't want to run at this RPM for long periods.

At 5,000 rpm valve limitations are not a concern.

I know, this is geeky engineering stuff, but I enjoy quantifying these things rather than going by seat of the pants or internet wisdom. Also, you can tell by my mixing of metric designations and old money "feet per minute" that I'm an old school guy.
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Old 06-20-2013, 10:06 AM   #6
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Great post Mike!
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Old 06-20-2013, 04:49 PM   #7
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in GM trucks and USVs the line shows 6,000 but i am with you guys 4.500 is where i would step in
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Old 06-20-2013, 05:54 PM   #8
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Great post Mike!
Thanks. We could delve deeper into rod angularity and acceleration forces, but for the scope of this forum I think the point is made.

However, please don't mis-read what I said. When I say 4000 rpm is sustainable all day long I don't mean it's OK to hold the throttle wide open going up a grade at 4000 rpm all day. Unreasonable engine loading is detrimental for sure.
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Old 06-20-2013, 08:25 PM   #9
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Did you ever calculate and compare cylinder pressure differences on the engines rotating components.....under power vs deceleration under load at 5000 RPM?
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Old 06-20-2013, 08:35 PM   #10
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So Mike, say you're on a steep grade, at what RPM would the 8.1 be reaching it's best work or should i say peak horsepower/torque? I think i ask that the right way, not sure now.
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Old 06-20-2013, 09:42 PM   #11
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I enjoyed the Koop geek read immensely and was glad to have someone quantify what I felt I knew.

If I recall correctly from my manual my 8.1 reaches highest torque at 3700rpm and highest HP at 4200 rpm, roughly on both accounts.


For me the uninitiated there is a sweet spot for my driving, in particular hill climbing, and that is the torque range ,a seat of the pants determination. The pucker factor, another seat of the pants experience, takes over at 4200rpm going by what I hear and feel. I can go faster, as if that was important, running up hill at 4200/4500 but I am personally as uncomfortable as I would be if I were lugging the engine.

So I guess I am a torque man by default.
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Old 06-21-2013, 12:59 AM   #12
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Norm, I appreciate your thoughts on what RPM to run the 8.1. In April we bought a 2004 8.1 and these numbers are of concern to me. I have been only running up to 3000 or 3400 up and down the hills. It sounds like pulling a hill in direct at 2200 is not good- lugging. I retired with 40 years of selling diesels and pulling a long hill in a semi at 1200 was normal with the newer engines. Now I need to retrain the brain. We live in Kent, Wa and it looks like you are in Federal Way. If you have time I would spring for coffees so I can learn more. 253-350-6054 Thx
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Old 06-21-2013, 01:36 AM   #13
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Quote:
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in GM trucks and USVs the line shows 6,000 but i am with you guys 4.500 is where i would step in
Dodge, for a couple of years , used the same dash layout and tach, on 3 different truck gas engines . The OHV 5.2,5.9 and the OHC 4.7 the first year the Cammer 4.7 was available all the tachs showed it's 5700 RPM red line , the other V-8s , were rev limited at 4800. Of course we got complaints " My engine won't pull to the red line" Next year the red line was gone off the tach , the rev limiter spoke for it's self on all engines.
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Old 06-21-2013, 07:06 AM   #14
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So Mike, say you're on a steep grade, at what RPM would the 8.1 be reaching it's best work or should i say peak horsepower/torque? I think i ask that the right way, not sure now.
This could spark a debate on peak torque (3200rpm) vs peak horsepower (4200rpm) or volumetric efficiency etc. But I really don't want to go there.

The Vortec 8.1 has a relatively flat torque curve IIRC, I don't have a dyno chart handy. My advice for climbing a steep grade is this;
If you have to mash the accelerator pedal to floor to maintain speed, you need to slow down and select the next lower gear. End of advice.
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