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Discussion Starter #1
Does anyone have information on the effects that increasing the maximum RPM limit of the Duramax would have on durability?


Obviously large changes would impact durability, but would an additional 500-600 RPMs increase wear substantially?


Are there special considerations for diesels that make them different from gasoline engines?
 

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Any time you place more stress on an engine, you are reducing its life. Although I don't know by how much. It may or may not of been built for a higher rpm range. Only time will tell.
 

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Discussion Starter #3
>> It may or may not of been built for a higher rpm range.


You almost have to wonder if the current RPM maximum wasn't chosen to be just beyond the RPM that peak horsepower occurs (on a stock engine). If that was in fact the reason, then 500 or 600 more RPM probably wouldn't have much of an effect on durability.


Personally, a linear or near linear decrease in ring/bearing life wouldn't bother me. A catastrophic rod or main bearing failure would, however.
 

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being that a typical diesel engine like the Dmax is built
with extra stout bottom ends to withstand the high compression
and torque produced by them, raising rpm by a small amount
isn't an issue. What IS an issue is where the cam stops making
power.........what we need to do is dyno someone's truck
who's running a higher rpm program, like what Quad offers,
to see where power really begins to drop off. Going any higher
than that is just a waste..........
Tom
 

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at 4k the dmax is still making good power but the line is starting to slope down a bit next time il see how far ir goes if the motor and tranny last that long
 

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king d said:
at 4k the dmax is still making good power but the line is starting to slope down a bit next time il see how far ir goes if the motor and tranny last that long

Are we expecting a catastrophic failure?????????


kingd what's up??????????


GMC
 

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GMC-2002-Dmax said:
king d said:
at 4k the dmax is still making good power but the line is starting to slope down a bit next time il see how far ir goes if the motor and tranny last that long

Are we expecting a catastrophic failure?????????


kingd what's up??????????


GMC
well i have read that 600 hp is approaching the limit of the rods ao like i said we will have to see if she holdsor if she lets lose...not planning on brekin,but gotta be gettin awfully close to doing just that....jess
 

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if i blow it up on the street just driving around il take the stuff off and let gm fix
if i blow it up at track or dyno il eat it and pay for it myself..
.only seems fair....jess
 

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I think we would find that the valve train is the ultimate RPM limiter on this engine. IMHO it would be easily safe to take the engine up to the point that the tach goes to a soild red. In stock trim the grade braking feature of the allison will take it there, heavily loaded and descending a grade.


I believe that 3200 rpm was chosen for reasons other than structual. I also believe that with the stock fuel system that the structual limit is beyond the max power point. Remember we are a direct injection engine, the burn starts the moment fuel is introduced into the combustion chamber. A stock fuel nozzle can only deliver fuel at a certain rate, Upping the RPM limit reduces the "open" time that an injector has. IMHO with a stock fuel delivery system power will drop off prior to reaching mechanical failure. Just my .02c


On edit,


I believe that doubleing the RPM Squares the stresses on a rotating component, in other words a 10 percent increase in RPM is more than a 10 percent increase in stressEdited by: a64pilot
 

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Discussion Starter #11
king d: Have you taken your truck to the drag strip? If so, what kind of ETs/mph have you gotten? You have got to be close to maxing out a stock lower-end -- thanks for testing it for us



a64pilot: Excellent point about grade braking -- I hadn't thought about that. Making *good* power past 3200 rpm might be difficult, but having the option to over rev to avoid a shift is always nice. Top speed appears to be limited by RPM as well once the limiter is disabled.
 

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In grade braking the motor will run up to 4800 rpm. The internals of the motor can handle that much rpm. My truck will turn about 4500 if you hold it in a gear (it sounds wicked too). I've seen dyno sheets where the power starts dropping off after 3700rpm. On the pulling track it doesn't seem to want to run over 3500 going down the track because the motor won't go past peak hp.
 

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Lets not forget emissions reasons for GM setting specific rpm limits.

I read a story about GM engineers taking a Chevy Cavalier four cylinder gas and upping it to 500 hp before the heads started to lift. In the 300 hp range it held up fine. They used drugs to get the power boost.

There's a lot of padding in todays engines.


Edited by: hoot
 

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There are a few things one needs to remember when raiseing engine speed on the Duramax. As engine speed increases so does the exhaust backpressure. The forces of the exhaust backpressure are a major concern as it tend's to make motors blow apart at the seams. This is one of the many reasons GM has limited the RPM. Just think about it when the exhaust pressure is equal to the valve spring pressure the exhaust valves nolonger close. As the exhaust pressure aproaches valves spring pressure the valves close slower and slower and once the pistion and valve meet it's all over. Also from a power point of veiw the higher the backpressure the lower the power output and the higher the internal heat rises in the motor. So one only need to check the exhaust backpressure Vs the intake pressure to begin to see when things get out of hand. To raise the rev limiter of the engine is no big deal, but who are you going to blame when it flys apart? Do some testing and you will find the reason why things are where they are. I donot sugest raiseing the rev limiter anthing above 3700 RPM unless one is ready to take the risk of engine damage!


Once you raise the Rev limit you also need to look at it's affect on the transmission. If you own a manual transmission it's clutch problems and if it's an automatic the transmission it will begin to cavatate internally if the output shaft speed is run to high! So unless you would like to distroy a $5000.00 auto trans be careful. I've working on and testing this engine and transmission for the past 3 years so believe me when I tell you these things as we have learned some of these lessons the hard way
Our truck is a 2001 2wd and to date we've run a 12.78 @ 106 and it's getting faster but we are willing to assume the risk when something breaks.
 

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The engine's efficiency and power rapidly falls above 3700rpm. Not that it the motor can't be built to run up there, but with stock internals and stock downpipe it's not making any power above 3700 rpm.
 

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I will say that I have ran a lot of rpm for extended periods of time without failure. Eventually anything can break.


I also don't believe anyone will run a truck fast enough and long enough to hurt the transmission. I would have to see some tests showing what output shaft speed causes cavitation, and what is cavitating?


These trucks will make power above 3700 rpms, but they need the proper fuel to do so.


Remember that when you raise the rev limiter, the only time you see extra revs, is when you force it and hold it in a gear, and when you run excessice mph in 5th gear. So ask yourself how often will I be above 3200rpms?


ALso another thing to consider is if your truck starts shifting at above 3200 more than likely your clutches are going south.





QUad
 

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"I also don't believe anyone will run a truck fast enough and long enough to hurt the transmission"


"ALso another thing to consider is if your truck starts shifting at above 3200 more than likely your clutches are going south"


Quad


So what your saying here is that if the transmission is shifting above 3200 you've hurt the transmission and it will need to be rebuilt. I believe that means people have already been hurting the transmission! Also you have failed to event look at the backpressure issues.


What I'm trying to do is warn people if you want to raise the rev limiter it's your choice but be ready to reach into your pocket book to pay for the results. This wouldn't be a GM warrenty item!
 

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I am not saying the transmission needs to be rebuilt at all. Actually I have seen trannies with glazed clutches go more than 50,000 miles without any problems. If you apply more than 110hp in most cases the clutches will slip. That doesn't mean rebuild your tranny, unless you want more power.





You are correct there are a lot of hurt trannies out there. Some of our bigger tuners have hurt them, the Juice has hurt plent, the Hot juice, and I am sure you have. The fix is either back the power off, or buy a new one.


I have not failed to look at back pressure issues. Actually I know exact ratios for a given boost number, as well as a number for every cylinder. Yes they have a lot of backpressure, but its not as bad as you think until you make stupid power. We will have a fix for the backpressure issue in the near future.





Anytime someone mods a truck, especially if the go with higher hp, they are assuming somewhat of a risk. I really do not think that rpm will make any difference, unless it is abused. Lets face it, the only way to run much above 3700 rpm's would be to put the truck in a low gear and mat the pedal and not lift off. Do you really think people are going to be doing this for anything longer thana few seconds? I think there are much easier ways of breaking this motor than a few extra rpm's for a few seconds at a time. At that I am not sure there is enough fuel available to even hurt the motor with rpm's. Have you ever blown one up? I haven't.





Quad
 

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Quad





That's where we disagree! If the transmission is sliping the clutches, you've already hurt it and it's down hill from there. How would one fix it unless you rebuild the transmission. That's the reason we've done all the research and helped develop all the fixes, we don't need unhappy customers. The back pressures are out of hand at much over 120 Hp unless the exhaust has been replaced and that then moves the point to about 150 Hp! So if you call 150 Hp stupid power we agree but if you don't we disagree again. As far as going over 3700 it gets done by the guys truck pulling and that's the worst possible case! Full load at 3700 will hurt pieces unless major modifications have been made! Yes, you can get away with it for awhile but it will catch up with you after awhile.
 
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