i have done a 4k kit on my truck and it will hit 4600 or so quit easy. i have read about people putting girdles and such on the bottom end to make it all hold together. is it nessary? i have 249k on it now and the 4k kit has been in 4 or 5k. i have spun it to 4500 plus at least 50 times or more. am i asking for a disaster? i have no idea how fast this old truck will run, but 120 is nothing anymore.

 

Who knows. Too many variables. I did that all the time in mine and never broke anything in the engine. I did lock the P pump up twice. Stuck plungers. Man that gets old having them put back together. It generally wears the pump cam flat on one or more lobes. 2 grand in pump repairs in two months is tough to stomach.

Cams like to snap off behind the number 1 bearing journal on P pumped trucks at higher rpm. Or the gear will walk off the end of the cam from harmonics.

Rods generaly hang in there, but if you have a high wear engine you could roach a crank or rod bearing. Valve float is not bad in the 12 valve if the springs are up to par.


The reason I am rebuilding now is the engine was wasted from hot rodding. It ate three HX40's. Scuffed the crap out of the cylinders and had about zero compression in two or three of them.

Its gets expensive


Don~

 

Pulled this from another site of Cummins info.

-Destructive torsional harmonic resonance occurs at 4100 RPM
-Crank and rods fail at 5400 RPM (if you make it through 4100 RPM)

Not to say that its definately going to happen cause people do run higher than that, but i guess if the motor is stock, Cummins thinks problems can occur.

 

mine runs right past 4100 like it was nothing

 

I remember the Gator used to Snap that poor needle down so hard i thought it would spin around....say GunRacer...How do you know it's 4500? it only goes to 4k on the meter don't it? or is that after a few beers?

 

my tach doesn't stop like the 98 and up trucks do, mine will go way past 4k to about straight down.

 

So what kind of sustained RPMs would you guys say is safe for a 96 12Valve?
towing and not towing?

 

I have found that the stock locks on the valves will pull through in time with HIGH rpm usage.

J.R.

 

Can I ask a dumb question here? Why would anyone want to rev so high? Isn't that way past the torque curve and engine efficency range?

 

you won't belive how easy it is to spin it that far, heck a simple burn out tags it every time in second. but it is not much faster to spin it past 3800 or so. but it was great for the tractor pulls.

 

Past the torque curve.....would depend on the engine - way past a stocker. If you can provide the air and fuel it will probably not be past the HP curve. I would say the higher you could rev it, the higher your peak HP could potentially be. HP is what makes you go faster - not torque. That is why these guys are looking for revs.

 

If your not gaining much speed after 3800, then I wouldn't push it past that. No sense in putting extra strain on the engine.

 

I wanna here what 5K sounds like in a Cummins in person. Godasound good.