Rpm Range, And Rear End Gears

[chrisrmerritt]

Hey guys, I am just curious what's a safe RPM rage to drive these trucks on the freeway? I have a 4:10 ratio which I really want to change to 3:55 which brings me to another question anybody know where to get a good set of 3:55 gears? Regarding the RPM's I don't let it get passed 2 RPM's which with my ratio is 60 to 65 mph. So since I 'm new to the Diesel world what would be a safe operating RPM rage for these trucks? Just want to know what everyone thinks.

[dripley]

2k rpm is a good cruising rpm. You are not going to hurt going higher. When I tow I regularly take it to 2400 2500 before shifting. I dont get any benefit going any higher even pulling long grades. Empty it just depends on how quick I want to get up to speed or how much of the 18 year old  have in me that moment. Now i have a 6 speed with 3:54 gears and dont have to deal with the 4:10's like you. If I had them I would be seeing higher rpm on the long road trips I make. There is no way I could make a 600 mile trip home for the weekend at 60 mph. I am very glad I dont have them. 

 Your mileage is going to suffer, but you are not going hurt it. Your best bet on on the gears would be to find a 3:54 rear end and just swap the whole thing. If you are 4X4 you have to swap both. I am sure others more knowledgeable than I will have plenty to add.  

[Mopar1973Man]

Thing is the most torque is typically produced around 1,900 to 2,100 RPM's This is where the efficiency is the highest for towing. For empty running your best be down lower between 1,500 - 1,700 RPM's...

 

With that being said there is little benefit to going above this range. You can run the engine against the governor all day long and not hurt it. (In theory - I personally don't suggest it!)  But now the efficiency is going to really low.

[MnTom]

Your engine is designed to run up against the governor all day long. In other words you can run 3000 RPM and not hurt the engine, but your fuel mileage would really suck. Keeping it under 2000 RPMs yeilds the best mileage.

[chrisrmerritt]

Thanks guys for your thoughts in this! Crazy to know you can run these things against the governor all day long at 3000 RPM's and not hurt anything but then again it is a Cummins!!

[joecool911]

Just remember that you'll get much better fuel economy at 55.

[chrisrmerritt]

Yea I usually keep it around 60mph. But good to know I can run her at 75 mph if I want to and not hurt anything!

[MnTom]

Just keep an eye on the EGTs and you can run a long ways.

[chrisrmerritt]

Yea. Speaking of EGT's this is going to be an obviouse tell of me being new to the Diesel world but what are EGT's? lol see told you I was new lol.

[TFaoro]

Yea. Speaking of EGT's this is going to be an obviouse tell of me being new to the Diesel world but what are EGT's? lol see told you I was new lol.

You have to start somewhere! This site is one of the best on the net for beginners and experienced diesel owners. 

 

To answer your question, EGT stands for Exhaust Gas Temperature. Running too hot of EGT's for extended periods of time will melt the pistons off.

[chrisrmerritt]

Oh ok thanks Tfaoro! Makes sense.

[chrisrmerritt]

What's the correct EGT range?

[Mopar1973Man]

Aluminium melts at 1,220*F (660*C). There is cooled oil sprayed at the pistons from underneath.

 

So extended 1,200*F is safe all day long. But you'll notice the coolant temps will be elevated still normal but higher than typical.

 

1,000-1,100*F you will see less increase in coolant temperatures typically float about 193-197*F.

 

Optimal for 20+ MPGs you want to get below 600*F and 5 PSI of boost.

[tsmithnw]

if you are looking for some where to look for gears randy's ring and pinion. http://www.ringpinion.com I have a set in my truck I was recommended by couple friends who are pro street drag guys. 

[chrisrmerritt]

MoparMan, let's see if I'm understanding this correctly. So for MPG's I want to keep it below 600*F but not to let it get up to 1,220*F due to the pistons melting right?

Tsmithnw, thanks for the link!

[Mopar1973Man]

Lets say I've crossed 1,400 a few times on fire calls and passing vehicles on the highway. But its only for max of about 10 seconds. 1,200 is safe to run. It just once past that no one knows how long any thing will last. One pyrometer gauge might be + few degrees and the other might be - a few degrees so who is to say what is safe? Like I said pistons are aluminum and they are cooled by engine oil so how long... Don't know. But as long as your below aluminum melt point you should be safe.

 

I typically float 550-600*F at 55-60 MPH at about 2-3 PSI of boost. Typically can reach the 20's for MPG's...

[chrisrmerritt]

Ok thanks mopar for the info! And thanks to everyone else that chimmed in