Exhaust Temp vs. RPM

[JMichelson]

I was towing my trailer and noticed that if I tow at 2300 RPM my exhaust temperature is 600° if I shift into O/D RPM drops to 1700 and exhaust temperature climbs to 750°. The truck seems to tow fine either way. It’s difficult to do a apples to apples comparison of towing both ways to check fuel consumption. I know that the optimum RPM for fuel economy is between 1800 and 2000. With an auto and 3. 54s, it’s sometimes hard to tow in the optimum RPM range. Here’s my question. Is fuel consumption directly related to exhaust temperature or RPM ? What is best for fuel economy higher RPM or higher

exhaust temperature. Thanks, Jeff

 

[Fireman Dave]

If ya don't mind a 24valver chiming in...

I believe lower RPM = higher mpg - to a point. Lower EGT = better for the engine. So, higher RPM = lower EGT = maybe a little lower mpg, but better for the engine. I think there's a considerable RPM range where higher RPM does not necessarily = lower mpg. I tow with OD off and am in the sweet spot for a 24v. It might cost me a little mpg, but the EGT is lower and the engine is happier.

 

[Steve Campbell]

Mr Cummins, Clesslie that is

... once said that the best fuel mileage was achieved at 600 degrees..... if I remember correctly.

 

[SlyBones]

-- Whats the boost at each of these RPM readings? I believe the best way to tell which is giving better fuel economy is to use the boost gauge. Which ever has the lower boost reading is giving better fuel mileage.



-- I believe your cummins can run at 600* or 750* all day long.



-- I believe the higher EGT reading in OD is related to the lower RPM's because you have less airflow at the lower RPM's. The engine is simply not pumping it thru as fast.



-- I would be OK with 1700RPM towing on level ground and maybe the small inclines on the freeway. Big hills, long grades, etc. I would lock out the OD to keep the RPM's up to help keep the temps down and add some life to your TC.

 

[Pthomas]

heck you can run a 1200 all day long !

yes even under a load

before you all get temp crazy have you ever seen any damage done from towing at 1200 and below? this is pre turbo temps...

 

[illflem]

Sly is right the boost gauge is your best mpg gauge.



Pthomas is also right, you won't damage your engine except that running at low rpms will drastically shorten your transmission life, auto or manual (5th gear nut)

 

[milehiscott]

Guys,

You lost me on the boost. I thought that more boost was a good thing (to a point).

Scott

 

[illflem]

More boost is good for power less for economy.

 

[nfox]

I agree with watching your boost gauge and seeing at which rpm the boost is the lowest when towing that's when your getting optimum fuel mileage.

 

[JMichelson]

Transmission Temp

Thanks for the information. You guys are the reason why this is the No#1 source for Dodge/ Cummins information. Almost all of you mentioned potential transmission problems when towing on the low end of the RPM range. I have a transmission temperature gauge and highest temperature I’ve seen when towing is 160 degrees taken in the pan. Is temperature the problem or is it damage to the torque converter which you wouldn’t notice until the damage is done. Thanks again. Jeff

 

[tschwab]

Originally posted by SlyBones

Big hills, long grades, etc. I would lock out the OD to keep the RPM's up to help keep the temps down and add some life to your TC.

SlyBones is exactley right on this - You will run lower EGT's and lower trans temps. After a while you can just tell by the sound of the motor and keeping an eye on the guages (boost, egt & trans temp).