Fuel Efficiency differences 870 vs 871

[Chamberpains]

(06-13-2018 )Rawze Wrote:  

(06-13-2018 )DLC Wrote:  What causes the 870 to be harder on fuel than a 871?

I've had the intake apart on an 870, the nozzle and Venturi on them appears very restrictive. Once it has been upgraded, what else stands in the way of decent mpg?

Below 550HP and driving the holy piss out of it, the after-market intake manifold will not improve efficiency on the CM870's. For something in the air stream to be "more efficient", you have to be pushing the component above its normal flow rating. -- The CM870 after-market intake manifold does improve power efficiency (does nothing for fuel efficiency though ) slightly, but none of the other components like the exhaust manifold etc. do squat.

Im reading on wiki about the Venturi effect and the de Laval nozzle. So now I have a question about the Venturi effect and the de Laval nozzle, being this is a Venturi pipe, de Laval nozzle. If I'm reading this right, In the Venturi effect on compressible fluids, such as air, wouldn't the mass flow of air increase after the restriction and the placement of this being so close to the combustion chamber, wouldn't this pipe act like a de Laval nozzle? Could these be the principles that Cummins may be trying to use in the use and placement of this pipe? If so, was this a failed idea or a "makes no difference if its there" part, to get better efficiency? Because they didn't use the same set up on the cm871's and after. I always have to ask why something was used then not. It's like this worked somewhere in their engineering process but then didn't...

Im looking at the second half of the part on "Choked Flow".

https://en.wikipedia.org/wiki/Venturi_effect

The limiting case of the Venturi effect is when a fluid reaches the state of choked flow, where the fluid velocity approaches the local speed of sound. When a fluid system is in a state of choked flow, a further decrease in the downstream pressure environment will not lead to an increase in the mass flow rate. [u]However, mass flow rate for a compressible fluid will increase with increased upstream pressure, which will increase the density of the fluid through the constriction (though the velocity will remain constant). This is the principle of operation of a de Laval nozzle.
Increasing source temperature will also increase the local sonic velocity, thus allowing for increased mass flow rate but only if the nozzle area is also increased to compensate for the resulting decrease in density.