T/F? Lower EGT's = Better MPG

[JRG]

Is it true that if you lower your EGT's, it will increase your MPG's?



ie: @ 47 mph selecting a lower gear to increase spool/air. (with or without inclines)



ie: Adding twins/more air without other mods.



What do you guys think?



JRG

 

[CLAYTON]

I always seem to get the best MPG when my boost is lower. When trying to get good milage on the freeway I'll try to keep in under 10psi.

 

[RMachida]

Is it true that if you lower your EGT's, it will increase your MPG's?



ie: @ 47 mph selecting a lower gear to increase spool/air. (with or without inclines)



ie: Adding twins/more air without other mods.



What do you guys think?



JRG

Interesting, I was wondering the exact same thing yesterday evening (after filling up @$2. 51/gal). It would make sense that lower EGT implies less fuel burning in the exhaust manifold.

 

[BK]

The less your into the pedal, the less oil your burning.

Boost is power and more boost results in higher EGTs.

But at what point is best with out holding up traffic.



I had read some where, then again in the TDR mag I think, and a friend that use to drive for a living told me the same thing... , to make the engine last and to keep fuel economy up there, there are two key numbers Cummins taught the drivers to remember... 600 and 5 ( I think it was 5 and not 10). . keep gearing so you didn't lug it, but didn't exceed 600 Deg for EGT and 5lbs of boost.

Before I put my gauges in, years ago I notice on my dodge that keeping around 1800rpm was a magic efficiency point, but moved along in traffice well enough when I wasn't towing. . with the gauges I found that if at 1800rpm or below, in any gear, I never went above 600/5 , reached them, but never went over when not towing.

 

[Mundgyver]

If you have dyno'ed your truck and know the RPM where your peak TQ is developed, that is where you will get your best mileage. That peak TQ is where you are getting the most power for the given amount of fuel burned.

 

[Hohn]

Exactly. Peak torque almost always corresponds to best BSFC.



On a VP44 truck, that means 2000 rpm. The BSFC curves were in a recent TDR mag.



The best hwy fuel economy for most manual trans 24v 2nd gens is direct gear at 2000rpm (5th for 6 speed, 4th for 5-speed). This is about 52mph in my truck.



Thermodynamically speaking, lower EGT means better efficiency, in the sense that efficiency is measured as the difference between peak temperatures. So, if peak combustion temps are 1200 in two cases, the one with the lower EGT would be more efficient.



But peak combustion temps are rarely the same. You'll find that peak combustion temps often increase linearly with EGT (with no boost). Then you add boost to the scene, and find that it increases efficiency uncer certain circumstances.

 

[MD500E]

I would guess that excess boost when not needed for complete combustion would be a waste of energy. I would think a larger charger at lets say 3psi at 65mph empty would be slightly more efficent that a smaller charger at say 10psi. Just a thought.

 

[bmoeller]

On my truck-



Stock turbo, #10 plate as the only fuel mod, cruising on the flat, 4. 10 gears and 70-75mph, no load. Was making ~15psi and ~600* pre turbo.



With HX-40 and similar conditions, with current mods, 2-4psi and 650-750* cruising.



Can't compare mileage apples-to-apples, though. Had the dually box on earlier. Lost the mileage I had gained with the mods, going to the flatbed. Not complaining though. More or less double the available power, and the same mileage as stock.



Higher drive pressure at higher RPMs on the stock turbo, vs the 40.