EGT's & Rpm

[BigWheel]

This snippet from the gear change thread sparked some curiosity:

Originally Posted by tbsimmons

If I let it shift to fifth, then it is below 2000 and the EGT's start to climb and the mpg goes down.

I know very very little about diesels but this surprises me. Can someone shed some light on that phenomenon? Logically I'd expect EGTs to be lower as rpm drops. Or is it something like reducing exhaust velocity allowing heat to build up?

I may need to bump my gear upgrade project up the list. 3.73's with 40's sucks but I didnt realize I may be causing other issues for my truck.

Thanks!

[hanky2k5]

i believe that it is just related to engine load and airflow... airflow is what keeps egt's down, for example if you constantly have high egt's you could put on a larger turbo to improve that. so by being in a fifth gear instead of forth you would likely have less airflow through the engine and a higher load on it, both of wihich will raise egt

[wdwkr]

I notice that climbing hills in a high gear and low rpms the egt will be higher than a lower gear with higher rpms. This is one way I use my egt gauge as a engine load indicator. Basically there is a higher load on the motor at lower rpms when climbing a hill or accelerating. Kind of like lugging the motor. Now shift down a gear and bring the rpms up and the motor is working easier because it will be closer to peak torque. The motor will breath easier and I will notice that the boost is lower. Think of it as riding a bicycle up hill in a high gear. Your legs are struggling and peddling slowly. Now shift down and your legs will be going a bit faster and it is easier to peddle.

[InnovativeDiesel]

Originally Posted by wdwkr

I notice that climbing hills in a high gear and low rpms the egt will be higher than a lower gear with higher rpms. This is one way I use my egt gauge as a engine load indicator. Basically there is a higher load on the motor at lower rpms when climbing a hill or accelerating. Kind of like lugging the motor. Now shift down a gear and bring the rpms up and the motor is working easier because it will be closer to peak torque. The motor will breath easier and I will notice that the boost is lower. Think of it as riding a bicycle up hill in a high gear. Your legs are struggling and peddling slowly. Now shift down and your legs will be going a bit faster and it is easier to peddle.

Agreed. When there is a heavy load at low rpms, the truck will fuel heavy(low boost and increased timing) and lug. This is due to the converter being locked. Once the converter unlocks and the rpms jump up, the load goes down, along with the EGT's.

[BigWheel]

Very interesting, thank you.

[tbsimmons]

It was probably me that posted that.
3.73's with 40's would suck.
I had 3.73's with 35's and it was alright.
But after the gear change the truck is happier towing.