515 detroit, fuel ecconomy/ turbo questions

Less RPM doesn't necessarily mean less fuel. Boost is a measure of the amount exhaust energy that is driving the turbo. It takes fuel to make boost, because added fuel increases exhaust. If one is running at, say 1500 RPM, and it takes 20 lb of boost to keep pace, and the other is running at 1300 but it is taking 30 lb of boost to keep pace, the second one is actually using more fuel, even though the RPMs are lower. It also begs the issue about having a pyrometer to go along with the boost gauge. If RPM's are too low for what you are doing, and EGT temps are rising, it is because of poor exhaust flow. You are applying more fuel, and creating more exhaust that cannot get out of the engine fast enough due to lower RPM. And then you apply more fuel to gain power because you cannot overcome the restriction of the expanding exhaust gasses not being able to get out of the engine efficiently. You need to get the RPMs up to a level that both boost stays low and EGTs stay low. That is where the engine is most fuel efficient. This is why, lugging an engine on a hill climb really is less fuel efficient than bumping up the RPMs. It takes less peddle to get over the hill at an efficient RPM than it takes by lugging down. That means less fuel. That doesn't mean that you take it to the other extreme and put the RPM's on the ceiling. It is kind of an art form to be able to determine that best RPM for max performance and fuel efficiency. But that is what sets good drivers apart from bad drivers.

I will concede the low RPM side of things. But only when on flat ground and no wind. But working hills, fighting high winds, especially with heavy loads, the low RPM thing does not equate to better fuel mileage.

I am not talking road speed here. that is a separate issue. That has to do with gearing. That is why many OEM's, dealers, Fleets, and many Owners don't spec properly. They don't study the particular engine, understand the simple fluid dynamics if exhaust flow / rotational inertia / and fuel application and how it all comes together inside that engine, and how that all applies to the real world of what a particular truck is being asked to do. Blanket "one size fits all" specs put out by OEM's don't work well in the real world. They may know the engine, but they do not understand the broad spectrum of applications.

I have known owners that are putting over 700 hp to the rear wheels and getting better mpg than any fleet spec'd truck. I know several with this kind of performance that are getting into the 7's, and even some 8's, on mpg CONSISTENTLY. Why? because they know their engine, they know how to drive it, and they have the proper drivetrain specs matched up to it. And speed may or may not be the big bugaboo. I know of one O/O, who tanks fuel in Oklahoma, runs 70 - 75 mph most of the time, is putting down 725 hp to the rear wheels, yet breaks 7 mpg average. I am rather envious. But facts are what they are. He knows how to run that engine so that it is at maximum performance and efficiency, and has the right gearing in the drive train to support running the engine in at the optimum RPM range for what he is doing.

 

U need 3:73 gears and 22.5 tires for that eng/trans combo.... then your fuel mph will go way up! even at speeds u drive now

 

torque is king in these engines. You get torque at the low end, not the high end.

Jeez I love it when end users feel they know more than an engineer who spent months studying this stuff using stats and science.

And I am not just talking driving a #### truck either.

I suppose you feel you can properly install a tire on one of these trucks too without a torque wrench?