What is the best RPM range?

[chevss442]

Hi, what is the most econimical/efficient RPM range for the 6.5TD to operate in, say for cruising at 65 - 70 MPH?

[john8662]

Depends on the rig, but a lighter truck will do best with RPM's ranging 1800-2000 RPM.

[chevss442]

It is a 1996 GMC 2 wheel drive, single rear wheels, crew cab. Light to no load virtually all the time. It has 4.10 gears now with stock 245-75X16 tires.

[moondoggie]

Good Day!

"...what is the most econimical/efficient RPM range for the 6.5TD to operate in, say for cruising at 65 - 70 MPH?" Well, you really don't have much choice, you can cruise at the most efficient rpm OR 65-70 mph, but not both at the same time.

WWWWaaaayyyy back in the early days, I read here on the Page that there were three rpm's to remember for an unmodified 6.5: 1800 rpm - peak torque (& likely best mpg); 2200 rpm - best rpm to cruise while pulling heavy; 2800 rpm - best rpm to pull a hill while pulling heavy. So, you might want to cruise at 1800 rpm for max mpg.

A formula I've presented before here on the Page:

rpm = (mph) X (1 hour/60 minutes) x (63,360 inches/mile) / (tire dia in inches x pi) X (driveshaft rev/tire rev [axle ratio]) X (0.75 engine rev/driveshaft rev [AT OD ratio])

From this:

mph = rpm /((1 hour/60 minutes) x (63,360 inches/mile) / (tire dia in inches x pi) X (driveshaft rev/tire rev [axle ratio]) X (0.75 engine rev/driveshaft rev [AT OD ratio]))

mph @ 1800 rpm = 1800/((1 hour/60 minutes) x (63,360 inches/mile) / (30.47 x pi) X (4.10) X (0.75 engine rev/driveshaft rev [AT OD ratio]))
mph @ 1800 rpm = 53.1

Substitute a different rpm in the mph equation to find what speed you'll be going at various rpms; substitute what speed you want to go into the 1st equation to find the rpm at any speed you'd like. For example, your rpm @ 70 mph:

rpm @ 70 mph = (70) X (1 hour/60 minutes) x (63,360 inches/mile) / (30.47 x pi) X (4.10 [axle ratio]) X (0.75 engine rev/driveshaft rev [AT OD ratio])
rpm @ 70 mph = 2375

And you wonder why I bought my Gear Vendors Aux. OD? I'm turning ~ 1750 rpm @ 70 mph - eat your heart out, RJ! :D

Blessings!

[DLang]

to make this sort if thing easier, I put togeather a page a few years ago to produce graphs (especially useful when thinking about different transmissions), the aerodynamics were for a International Scout, but these trucks probably aren't much worse (a little large, a little more rounded, and more stuff hanging in the breeze unerneith, but it wan't _that_ precise to start with :p )

http://lang.hm/scout/nph-performance6.cgi

I recently added the 6.2 and turbocharged 6.2 engines (pulled from the graphs on the banks website)

in the last couple of weeks (while looking for graphs of the torque curve of these engines) I stumbled across a graph that showed fuel consumed per HP at different engine rpm's, according to that graph (which I can't find again) the most efficiant rpm's were in the 1800-2200 range.

now this assumes that you have enough power that you aren't maxing out your power (becouse when you do that you tend to overfuel the engine, drasticly dropping the efficiancy, and producing black smoke), so if you are hauling heavy loads you may not be able to operate in that rpm range. ideally you would operate just under the point where you start produceing smoke, as you would be useing all the air that the engine is pumping through it, but in reality you want to back off a little from that to give yourself a margin for error

one of the things I would like to rig up is a O2 sensor hooked to a gauge to show how much unburned air is getting out of the engine, with that you could operate just shy of running out of air, sacraficing almost no power, but avoiding wasting fuel. after som experimentation I'll bet you could calibrate a "you're wasteing fuel, back off the throttle" light

[CareyWeber]

to make this sort if thing easier, I put togeather a page a few years ago to produce graphs (especially useful when thinking about different transmissions), the aerodynamics were for a International Scout, but these trucks probably aren't much worse (a little large, a little more rounded, and more stuff hanging in the breeze unerneith, but it wan't _that_ precise to start with :p )

http://lang.hm/scout/nph-performance6.cgi

I recently added the 6.2 and turbocharged 6.2 engines (pulled from the graphs on the banks website)

in the last couple of weeks (while looking for graphs of the torque curve of these engines) I stumbled across a graph that showed fuel consumed per HP at different engine rpm's, according to that graph (which I can't find again) the most efficiant rpm's were in the 1800-2200 range.

now this assumes that you have enough power that you aren't maxing out your power (becouse when you do that you tend to overfuel the engine, drasticly dropping the efficiancy, and producing black smoke), so if you are hauling heavy loads you may not be able to operate in that rpm range. ideally you would operate just under the point where you start produceing smoke, as you would be useing all the air that the engine is pumping through it, but in reality you want to back off a little from that to give yourself a margin for error

one of the things I would like to rig up is a O2 sensor hooked to a gauge to show how much unburned air is getting out of the engine, with that you could operate just shy of running out of air, sacraficing almost no power, but avoiding wasting fuel. after som experimentation I'll bet you could calibrate a "you're wasteing fuel, back off the throttle" light

Hi David it's good to see you around. :D

Carey

[More Power]

Efficiency... As luck would have it, we'll have a brand new story on the web site in a few days that discusses fuel effiency as a function of load and RPM - as part of the continuing 6.2/6.5 fuel economy series we've been involved in.

We had a "Fuel Mizer" 6.5 on Peninsular's engine dyno while we were in Michigan this past June. Dr Lee & I measured fuel consumption at various RPMs and loads.... :)

Jim

[TurboDiverArt]

Efficiency... As luck would have it, we'll have a brand new story on the web site in a few days that discusses fuel effiency as a function of load and RPM - as part of the continuing 6.2/6.5 fuel economy series we've been involved in.

We had a "Fuel Mizer" 6.5 on Peninsular's engine dyno while we were in Michigan this past June. Dr Lee & I measured fuel consumption at various RPMs and loads.... :)

Jim
Did you get a chance to try it at different boost levels/RPM as well as at different MAT temperatures (simulating intercooled and non)?

Art.