what RPM to shift?

[MCummings]

Originally posted by HeavyHauler

... anytime I run mine over 2000, I get loads of smoke...

That doesn't sound right. You should get smoke at lower RPM, when the turbo isn't spooled properly.

... to the guy that said peak torque is at 2900 on these engines. You need to recheck your figures...

How much more convincing can a dyno graph be? I think Keith would know more about the power the Cummins can make, and at what RPM it makes that power at than anybody else on this board.

... If your statement was true, our engines would be rated at some 830 lb. ft. of torque at your figure (proportionally)...

I'm at a loss from this statement. How does this work?

... It is widely accpeted that running your motor anywhere over 2000 RPM is killer to your fuel mileage...

In one issue of the TDR mag, it was also pointed out that running below 1,600 RPM was also detrimental to MPG.



Running below 1,600RPM is also known for spinning the 5th gear nut off the shaft on 5-speed trannies.



Cummins also states that running the motor below peak torque is detrimental to engine life. (Lugging)



HeavyHauler, No flames here.

These are my thoughts on the matter. I don't say what I'm thinking very often, mostly because I am not very swave of putting what I am thinking into words in a way that sounds like I am not fighting. (my thoughts come across as punches, compared to conversation)



Am I TDR'ized to much, thinking the higher RPM isn't going to hurt these motors and drivetrain?



I am confused how Cummins would say anything above 1,500 is "spinning", but then rate Marine motors for constant operation at 2,600RPM, and give the Dodge /Cummins a 3,200RPM full power redline.



My $. 02.



Merrick Cummings Jr

 

[carson6]

little smoke after turbo spool's...

... 3rd 2200rpm and than a nice shift into 4th it drop's to 1800rpm egt's are low and my power is high... if this wreck's my motor than let it blow. :--)

 

[RustyJC]

Unloaded

When starting out with a cold engine, I generally use a very light foot on the throttle and shift around 1900 - 2000 RPM. As the engine warms up, I might move my shift points up to 2300 RPM or so to keep RPM from dropping too low in the next gear.



Towing

When towing our 13,500 lb 5th wheel, I'll use all 6 gears and shift such that, after the shift, RPM doesn't fall below 1900 - 2000 RPM in the next gear. This generally means I'll shift somewhere between 2600 and 2800 RPM.



Unlike the 12V engines, the 24V Cummins doesn't mind being revved. With the 4. 10 rear axle in my truck, I can run 75 MPH all day when towing (21,180 lbs GCW versus 21,500 lbs GCWR), and this equates to roughly 2500 RPM in 6th gear. The truck does this with cool engine temperatures and EGT's and has never used a drop of oil between changes.



Rusty

 

[dremelts]

Originally posted by HeavyHauler

anytime I run mine over 2000, I get loads of smoke and my engine sounds like it's going to fall apart or send a rod flying through the side of my engine. ... they all sound like that and I don't see how you guys could shift at nearly 2500 RPM. It sounds pretty bad to me, and I don't see how it couldn't to you.

My 5 speed with 4. 10 rear end is running about 2400 RPM at 70MPH and it does not sound to me like it's going to fly apart - maybe I'm just used to the sound.



Also, when I shift from 3rd to 4th (especially under a load) I lose almost 1000 RPM. If I shifted at anything less than 2500 I would be really lugging after the shift.

 

[AMF]

RPM'S

HEAVYHAULER



Iam in your conner! Cummins told me the same thing. Since I put there suggestions in practice I jumped 2 mpg! Also I asked them about that being border line lugging ? They said yes, but since the engine would idle that it was'nt lugging! How could we all love are Cummins so much and not take the advice of the folk's that build them?

 

[HeavyHauler]

It's about time!

Finally an ally!

 

[rbattelle]

Cummins told me the same thing

Are you guys sure they knew you were talking about the ISB in the Ram? Because in industrial applications the ISB5. 9 is governed to 2600 RPM, in which case the RPM recommendations they gave you seem more appropriate.

 

[AMF]

CUMMINS RESPONSE

I'LL get my son to cut and paste Cummins e-mail over here this weekend if I dont forget. I don't do it enough to remember how one time to the next.

 

[Texas Diesel]

One more angle here. From the engines point of view, lower RPM is better on the enginebut the clutch and transmission must be considered too. The angular momentum of the crankshaft has more more variance at lower RPM than at higher RPM, that is why the clutch has dampener springs in it, to help absorb the power pulses. Lug the engine and destroy the clutch AND beat OD gear to death.

 

[SpicyJam]

With all this worry over RPM, does that make it so I can't drive my truck on the HWY? Cause if I am too shift at around 1400-1600, then I shouldn't be able to go over that in OD on the road. Gee I am going to get a lot of back road driving. No wonder I pass so many people, I am over reving... ... ... ... lol!:--)

 

[carson6]

more rpm equal's...

more pressure holding my clutch plates together and less stress on other engine componets... so let her rev boy's

 

[nickleinonen]

not on a cummins engine, but the little 4. 7v8 that i have in my 1500 ram gets shifted in all different places. if i am just driving nicely around town, i'll usually hir 3rd gear at the other side of a street with a stop sign/light shifting at 1400-2000 rpm. if i am driving a little more agressively, 2500-3500 is not uncommon for me. and if i am in "let's play ricer boy" mode, and my engine and trans and everything else is at operating temp, i will shift at about 5500rpm [with the rev limiter at 5700 rpm]... . when downshifting, i will usually not go past 4000 rpm



i say run it like you stole it when you want to, and granny it around when you want to... . just don't do too much of one or the other... :-laf

 

[AMF]

rpm's

We know how much fuel the engine will burn, under full load, on the

engine dynamometer, which is measured in pounds per horsepower hour, but

when the engine is installed into a chassis, those figures no longer

apply, due to the various chassis components. Cummins is unable to

troubleshoot fuel mileage (MPG) problems on Dodge trucks.



Our engines are certified to within 5% on power and fuel rate, which

means that - at 20 mpg even - the 5% is a half-mpg difference. For

overall vehicle efficiency, you could install a bed cover, you could put

synthetic diff 75W-90 lube in, you could use SGO-50 (or whatever

synthetics Dodge recommend) in the trans if manual; finally, you could

assiduously make you upshift point be where the engine is at 1,400 rpm (torque

peak) for the next gear - and less than that for partial loads.



Fuel economy varies by individual truck and owner; we don't know what

else to tell you. But don't accelerate too slowly; the engine has it's

peak efficiency at high torque values, and spinning it easily (at 15%

torque and running beyond 2,200) will result in at least 10% less

efficiency than loading it at/above torque peak RPM, but on the low end of

the power band. If you don't have one of the 7,500 lb. behemoths with

duallys, jacked up 4WD and drive 75 mph, you should be able to get

high-teens ... . and you're really close - we're not sure you can do anything

really.



Thanks for your email regarding your Ram truck MPG.

-- email address removed --

 

[HeavyHauler]

Thank God!

Man I'm glad you got that on here. Maybe some of these guys will shutup now. Of course I'm sure someone will manage to find something wrong with the official response by Cummins.

 

[rbattelle]

Heavyhauler,



With all due respect, I read nothing in that email that helps your argument that we need to shift at 1400-1600 RPM. Furthermore, their response makes perfect sense to me: you'll get the best efficiency under load if you keep RPM below 2200. After that, efficiency begins to fall off. Where does it say RPM>1600 is just spinning the engine?

 

[mgonske]

Re: rpm's

Originally posted by AMF

<snip>... finally, you could

assiduously make you upshift point be where the engine is at 1,400 rpm (torque

peak) for the next gear... <snip>

I think the "for the next gear" is the key phrase here--you want to shift at a point (seems like that's somewhere in the 2000-2100 RPM range for most of my gears) so that you will be at 1400 RPM when you let the clutch out in the next higher gear.



Does that make sense?



I still don't understand what Cummins is talking about with the reference to "spinning" though.

 

[dremelts]

Re: Re: rpm's

Originally posted by mgonske

I think the "for the next gear" is the key phrase here--you want to shift at a point (seems like that's somewhere in the 2000-2100 RPM range for most of my gears) so that you will be at 1400 RPM when you let the clutch out in the next higher gear.

NOW I think we're getting to the heart of the matter!!! If you don't want to be lugging the engine after the shift you have to shift above 2000, and if you've got the 5sp you have to be well above 2000 when shifting from 3rd to 4th.

 

[Rman]

I tried shifting at 1400 just to see what it is you guys are agruing about. It is utterly too low. I was bogging the engine, i could feel the clutch fighting to hold... the transmission didn't get into gear very well either.



So after one pattern of "short shifting" i went back too normal shifting from 1800-2100 and everything was just peachy. I guess everyone needs to do what they need to do, but 1800-2100 unloaded... works just fine for me.

 

[MCummings]

With a big turbo, big injectors, tall gears, and big tires, I shift my Auto around 2,200 - 2,800 RPM.



My 6-speed, I shift around 2,000 - 2,500.



All this goes out the window at the track. My Auto will touch 3,900, and my 6-speed will only go to 3,100 before I powershift.



I think you need to shift INTO your peak torque, not, out of.



I Know I have heard Cummins state that operating your motor below peak torque is detrimental to engine life.





My $. 02 (again)

Merrick Cummings Jr



P. S. , does that make it $. 04 now ?

 

[KBennett]

I'll add my $.02 to make it $.06

My truck is a bone stock 2004 HO with 6-speed and 3. 73 gears with 450 miles on it.



If I am just cruising around town and shift at 2100 to 2300 RPM it feels the smoothest. If I shift at any less than 2100 the RPMS drop to less than 1500 after the shift and the motor feels like I am lugging it.



On the highway 2100 RPM seems to be the smoothest.



Maybe after it breaks in this will change.