Too high of RPMs?

[jaynes]

Well I"m back again regarding a post I read about high RPMs on the highway. I drive home from college everyonce in a while, and I try to stay off the highways and take back ways, but even in the back ways I hit 70 or 75 sometimes. ANd before I can even get to the back roads, I must take I-70 and to keep up with traffic and not get killed or kill anyone else, i'm going like almost 75 mph. I have a '91 with a 3 speed auto. I don't have a tac or anything, but it sounds like the rpms are getting really high. Is this bad for my engine or transmission? It doesn't really sound too good cause the rpms are so high, but then again there has been no apparent damage to the engine either, or transmission for that matter. SO what i'm asking is, is that bad for my engine or transmission going that fast with only 3 gears to work with? Thanks for any imput.



jaynes

 

[EOliver]

I can't imagine a first gen truck having a problem with high rpm damage unless you have one of those "special" fuel pumps and many other mods. I know I can only get to 2600 rpm or so on the highway at around 95 mph with my 3. 54 gears and the 4 speed auto. In neutral my governor stops right at 2900 rpm, but this is still low rpm for the 5. 9 Cummins. I know there are guys out there running 4000 rpm with no problems, granted they most likely have valve springs and teflon coated pistons, but the bottom end holds together fine. I just can't see anything under 3000 rpm doing any damage to the motor no matter how long you run it there.

 

[Midnite]

Originally posted by EOliver

In neutral my governor stops right at 2900 rpm, but this is still low rpm for the 5. 9 Cummins.

Not for a 1st gen it's not! You're not going to care a bit about how tough the bottom end is when a valve kisses a piston because of valve float at 2900 rpm. If you want to turn that fast, get a set of 60# springs, and not just on the exhaust valves. EOliver, it sounds like you missed DieselBuzz's thread. I rarely ever go faster than 70mph, but even so it was an eye opener for me!



Jaynes: what size of tires have you got? If you want to lower your rpms the cheap way, you might want to look at taller tires. Otherwise you could look at swapping your gears (it sounds like you've got 4. 10s).



Mike

 

[EOliver]

I just can't believe that the stock valve springs won't work for the stock governor settings... . ??? I would imagine that you would need the 60 pound springs if revving over 3000 rpm, but come on a valve hitting the piston at 2600 rpm?

 

[jaynes]

i'm confused

It seems that we have conflicting ideas here, guys. EOliver sounds in disbelief which makes me think he doesn't think so, and Midnite says that running my truck that fast is a bad idea. This is really important to me, cause if I'm at risk of blowing a piston because of valve float and weak springs, then I wanna know cause I'll be driving home like that again next week. . . i've been mulling over this problem for a while now, and I was actually gonna sell my truck if I could find a five speed or a mercedes or jetta with a diesel so I could go faster. So am I really at risk? Thanks for the help though, guys I appreciate it!

 

[boatpuller]

Our engines were designed for use in all kinds of commercial applications where they run at the governor 100% of the time for days on end, the only way you would float a valvespring is if it is weak or you over rev the engine on a downhill run(going beyond the governed RPM)

I think you are safe but you are probably taking more life out of your engine than if you could keep the RPMs down.

My $. 02

Happy Rammin,

Caleb

 

[R.E. Miller]

I have wondered about this too. I would run the engine slow rather then wound-out. I have heard that the rods in our engine are larger then the other 5. 9's which would be reason for the slower rpm rating they have, aside from the valve spring strength. I have the getrag, 3. 54's, and 235/85/r16's, and at 75 she sounds wound-up



If some one who knows these engines inside and out could shine some light on this subject it would really help out a lot of people.



I am looking to install an overdrive or a six-speed to help out. Yet I too might go for the valve springs when I get an exhaust brake... money money money

 

[rdodd]

After reading this article i talked to a diesel injection shop who has supertuned many first generation trucks and he said he has never seen a problem with valves floating out with stock valve springs even after turning engine speed up to 4200 RPM, and this is on a first generation with stock valves springs!

 

[bvanetten]

Originally posted by R. E. Miller

I have the getrag, 3. 54's, and 235/85/r16's, and at 75 she sounds wound-up

With that setup you should be running around 2100 rpm in OD at 75. Not to bad really. If you were to re-insulate the cab you would remove alot of the extra noise you are hearing.



Bruce

 

[Big Don]

Guys,



I think we are worrying about a non issue. These engines in stock form can be left with a brick on the accelerator pedal without any damage. They will run on the governors for unlimited amounts of time as long as they have oil and antifreeze with no damage.

Mine is not stock and I have run it at 3,000rpm's for miles with no damage. I still run stock springs without any problems.

I can tell you you will get better mileage if you can keep the engine below 2100 rpm's. Mine seems to burn more fuel at 2200 and up.

I recently installed a GV overdrive behind my 5 speed. The truck has 3. 54 gears and I can run interstate at 78 mph turning about 1700 rpm's. It dramatically reduced the in cab noise and gets better mileage. I also like the idea of running the engine slower. But this is a personal preference. Mine has 177,000 with alot of interstate miles running 80+ and it was turning higher rpm's. I really don't think you'll dmage anything, but do what makes you feeel comfortable. This is the best part about owning a truck, you can set it up to meet your needs/wants.



later

Don

 

[Midnite]

Sorry, I guess I sounded a little too paranoid in my earlier post. I'd just read about DieselBuzz's problems, so that was fresh on my mind.



Thanks Big Don and others!



Mike

 

[jaynes]

Whew!

THanks guys, now I feel better. I had no idea what I was doing to my Cummins, but now I know that I'm really not doing any damage to it (but to my financial situation from using more fuel at higher RPMs. I might have to consider either installing an overdrive or swaping for a 5-speed. . . or selling this truck for a five speed. Dunno yet. Thanks fellas i feel alot better now!



jaynes