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I know I've read on here that the safe upper limit for a 6.2 is 2500 rpm. What I'm wondering is what really happens to the engine above that.
After reading on a diesel forum many people saying the 6.2 will run against the rev limiter all day without harm I'm questioning why I believe I should keep the RPMs down?
Quote:
Originally Posted by jeeper
Self destruction comes to mind! No diesel likes to rev very high. Lots of heavy parts swinging around and huge compression.
That is probably from my post that you have read that 2500 rpm limit. I call that my self imposed red line. I will explain why after I give a few examples of engines that rev higher.
The M1008, M1028, M1010 and M1031 trucks all have 4.56 gears. 55 mph is 2700 rpm in them with stock tires. Owners routinely drive that speed. 60 mph is almost 3000 rpm.
Heath Diesel has a land speed truck they run at 5400 rpm.
I have no idea what the HMMWV turns rpm wise at any speed.
Banks Turbo website talks about how the turbo doesn't do anything until you get over 2000 rpm and really can't be felt until over 2500 rpm.
Randy's Ring and Pinion which has what I think is the best web site for calculations:http://www.ringpinion.com/Calculators/Calculators.aspx
Has the acceptable rpm range for towing with a 6.2 at 2800 rpm.
The CUCV manuals list 3450 as the regulated top rpm of the 6.2.
With all of those examples, here is why I try to stay below 2500 rpm. The 6.2 is an indirect injection engine. Which means the fuel is injected into a little bitty pre combustion chamber inside the head. The flame travels through a small throat into the combustion chamber over the piston where it spreads and then expands to push the piston down. Diesel fuel burns at a slower rate than gasoline all the time as it is. Making that slow fire move into a different room to meet up with the air as it begins to burn makes the process even slower.
A 4 stroke engine turning at 600 rpm means 10 revolutions per second or 5 power strokes per second for each cylinder. That is at idle. 2400 rpm means 20 power strokes per second per cylinder. That is an awful lot of fuel changing rooms while going down the road.
That is the physics of my 2500 rpm red line. Spinning it faster just makes it more and more inefficient.
Then there is the fact most of these engines are 25 or 30 years old. Add in a known weak point is main bearing webbing castings and harmonic balancers coming apart. You can start to see why I want to keep it below 2500 rpm.
Personally, I have the M35 Whistler that being a straight 6 diesel is for sure red lined at 2600 rpm, I fly air planes with 2550 rpm redlines. The NP200 keeps my M715 at 2000 rpm in 5th gear at 54 mph and everything else is mostly 6.2 powered. I am just used to keeping it below 2500 rpm. Only my Mini is designed to be run faster so keeping the rpm's down isn't a problem for me.
Sorry if my self imposed red line has morphed into something seen as fact.
Thanks for the reply Tim.
I never assumed the self imposed 2500 rpm limit was fact, rather just a good idea.
The reason I'm asking is because when I put the 6.2 in my truck I didn't think it would bother me to have a top speed of 45. After driving it for a while I realise it does bother me. I really want to be able to go 55 which is the minimum speed for the expressway here in MIchigan.
Doing 45 on the back roads doesn't bother me. I actually enjoy it plus it is safer around where I live because we have so many deer. I just want to be able to get on the expressway sometimes and am concerned I'll end up walking if I blow my engine apart.
Make sure your balancer is new or at least in good shape. Then let it rev if you want.
Well I guess I won't be revving it up any time soon.
I took my Jeep to the lumber store today and on the way home it started making a bad lifter noise. It has had a small tick in it for a while but I think something let loose today.
I got the engine cheap and figured I'd drive it until it died but was hoping it would last until I had the other engine rebuilt.
I guess it's time to do something but I'm not sure what yet. This engine oil pressure drops low when it's warmed up anyway so I probably won't just do heads.
Anyone have any suggestions?