high rpm when cruising in 60mph

[subodh]

my truck is on 1600-1700rpm in 60mph

dont really now whats under my car but is it

possible to change something to get those rpms down abit

would lowe to cruise around in 80mph or more



the thing is that my truck is rolling mostly w/o any load at all

and has some mods like smarty bigger turbo



i cant fit any bigger wheels under it 315/70/17 (35")



do i have the smaller gearing?

another thing why do my buddi have overdrive button

on the shift stick



an i have haul button?



does he have a advantage or do i just have one more mode?

 

[cerberusiam]

Those rpms are pretty low already, any lower and your way out of the power band.



What truck do you have? Year, trans, 4x4, etc. Makes it easier to understand your question.



TH is used in the newer trucks but functions the same as OD Off on the 03-04's. The 05's have an entirely different setup while the 06-07's changed again.



See why its important to know year and other things about your truck?

 

[SierraRam]

I shoot for 2000rpm at 70mph, sounds like your going to be pretty close to that.

 

[Wingate]

Thats not high RPm for your truck. My truck is an 06 with 3. 73 gearing and stock tires and I typically run between 85 and 95 MPH from East Texas to the Dallas area...

You can twist it up and pin her ears back in the wind... its not gonna hurt it.

Remember, this is not a gasoline engine nor is it an automotive built diesel. A true diesel engine can run at "Rated Speed" indefinitely. Due to the relative low speed and heavy construction, engine RPM isnt as much of a consideration as load/fuel consumption is. As you stated, most of the time, youre just logging unloaded miles. Running more RPM with less load allows the engine to breathe and cool better.

Trust me, the engine will only the max RPM that the ECM will allow.

 

[subodh]

its a 2500ram built 2005 (lift pump in my tank) 4x4 auto transmission
im thinking about stock tires because the 315/70/17 takes alot mpg
but i dont really now what kind of rpm will hurt my truck.

whats safe rpm? i was just thinking maby lower rpm will give me more mpg
when im driving abit over 50mph i can really hear my turbo charge and it puts around 40psi so if i can keep it abit lower so the turbo wont build so much i could gain mpg right?... i was thinkig its hig because its 2/3 of rpm gauge before red when i drive abit faster.
but its nothing i can do then or? dont really need so much hp when just driving around town or home/to work. any ideas how i should tune my smarty?

 

[cerberusiam]

You could run all day long at 2800 rpms and its not going to hurt the engine, they are rated around that for constant duty. The thing is you don't need to do that to make good economy.



The big tires to help with the rpm's are usually a net loss on an auto. The extra weight and rolling resistance negates any gains. In addition, they are death on the stock trans in normal driving let alone towing. About a 33" tire is all you can run and actually gain mpg.

when im driving abit over 50mph i can really hear my turbo charge and it puts around 40psi so if i can keep it abit lower so the turbo wont build so much i could gain mpg right?...

I assume this is towing a trailer, right? The boost is controlled by fueling and load. The big tires do NOT help that at all, they generate more load and ultimately more boost.



Smarty settings depend on what your doing. You really have to use different settings for different scenarios. SW #6 and timing 4 seems to be a good combo for driving empty. I am seeing around 20-21 steady cruising at around 70 mph. Push over 70 and it starts dropping mileage fast, down to around 15-16 at 80 mph. The truck is just pushing too much air at that speed.



BSFC on the CR engines is roight around 1900-1950 rpms. You need to tune the gear ratio to run 1900-2000 rpms around 70 mph for the best effect. Thats what I have found on my truck.



I have made as much as 12-12. 5 grossing 23k in decent conditions by driving the truck accordingly. Thats as good as my 1st gen will do at those weights. Can't complain about that in a truck thats heavier and taller.

 

[subodh]

i dont tow! as high as sw6? i dont have any timing boxes so i should stick with like 5 or 7 right? and the boost builds up with empty truck can really hear it but dont know how musch it builds up (it ilegal to drive a truck over 50 here) but often i drive like 60 or 70

so 2000rpm wont hurt it then

 

[cerberusiam]

i dont tow! as high as sw6? i dont have any timing boxes so i should stick with like 5 or 7 right? and the boost builds up with empty truck can really hear it but dont know how musch it builds up (it ilegal to drive a truck over 50 here) but often i drive like 60 or 70

so 2000rpm wont hurt it then

The 90 or 130 HP tune is a good choice, SW 5 or 7. Set your timing up using the timing option and you can change things up quite a bit. Advancing the timing will drop observed boost.



Boost is going to build fast if your accelerating but at a a steady cruise it should not be more than 5 or 6 psi.

 

[subodh]

2 or 4 timing then? not 3 right?

why sw5 or 7 isted of 1 or 3? same mpg?

 

[VRidgley]

Best mileage in mid-powerband, or lower?

BSFC on the CR engines is right around 1900-1950 rpms. You need to tune the gear ratio to run 1900-2000 rpms around 70 mph for the best effect. Thats what I have found on my truck.



Following up on this thought: The TDR Buyer's Guide (p. 80-81) references a discussion of BSFC curves for 5. 9 HPCR engines, stating that Cummins engineers place the sweet spot at 1900-2100 RPM.



Assuming that is true, and assuming a level road, no load, and no winds, should a driver assume that a Cummins 5. 9 running at 2000 RPM at 62 MPH in 6th gear would get better fuel mileage than running the same truck at 1550 RPM at 62 MPH in 6th-Overdrive? (I have a Gear Vendor unit which can deliver up to 22 MPG at a steady 65 MPH). I also happen to know, from Gear Vendor literature, that the Cummins 5. 9 powerband is pretty flat from 1500 to 2500 RPM.



Experimenting today, I found running in 6th, 2000 RPM required 5# boost and produced 350 EGT, while 6th-Overdrive, 1550 RPM required 4# boost and produced 400 EGT; measurable, but not very significant.

 

[cerberusiam]

Experimenting today, I found running in 6th, 2000 RPM required 5# boost and produced 350 EGT, while 6th-Overdrive, 1550 RPM required 4# boost and produced 400 EGT; measurable, but not very significant.

The difference in EGT's between the 2 is the timing, the big difference will be in the fueling curves. Given the load placed on the engine is not to far beyond the norm it will show better mileage because the engine is drastically defueled at 1550 rpms, 40% or more from what would be normal. Thats the TM in effect if you don't modify it.



That works fine on low load flat terrain, but, you will likely see that max mpg drop if you tried the same thing in heavy rolling hills where being on the back side of the TQ curve and defueled would show.

 

[VRidgley]

Best mileage in mid-powerband, or lower?

Thanks for a lucid reply. I can cerrtainly put this info to good use, since I alternate between full loads (3,000#) and empty for long distances over the same terrain. This gives me an incentive not to use GV when fully loaded.

 

[cerberusiam]

This gives me an incentive not to use GV when fully loaded.

Yeah, that would be an expensive way to find out the GV unit is not meant to take that load. They work good cruising empty but loaded is not a good use for it.