Fuel Efficiency differences 870 vs 871

[DLC]

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?

[tree98]

I know the turbo plays a big part

[Chamberpains]

Air actuated turbos compared to electronic controlled is huge plus the Truck design plays a part. The 870's were put in the starts of the aero-cab style's. But a lot of them were inside old heavy box style rigs.
I also believe that the ECM is more complex and able to adjust more on the 871.

I think it's a case of where they are almost identical but it's a lot of little changes piled on top of each other that makes a big difference in the end.

[Rawze]

(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.


The biggest difference is the Air operated turbocharger just does not have the response like newer engines with full VGT's do.

[DLC]

An angle grinder seems to be a good upgrade for the nozzle.

[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.

[Rawze]

I think your over-thinking it a bit there.

Last I checked, there was hot inert gases and and an EGR valve restricting flow though the venturi portion getting in the way of that effect, therefore there is no real efficiency gain. Block the EGR pipe (just commenting on what most 870 owners do) and all you end up with is a bit of unnecessary turbulence.

(speculating here) ...
The most likely reason that newer engines do not use a full Venturi pipe design like the 870 has is the fact that exhaust manifold pressure is almost always much higher than the intake pressure because of turbocharger inefficiency. There really is no need to "suck" the air from the exhaust into the intake by very much, it mostly wants to go that direction any ways.

[DLC]

The more I read the more questions I have. I would so like to have a chassis dyno, various trucks, TIME, and a few of you guys to test and check out different things. I think it would be a lot of fun. I'm sure we could enjoy a few frosty beverages, good Canadian ones, and figure out a few things.

Every application is a little different something may work on one and not another.

[Chamberpains]

(06-15-2018 )Rawze Wrote:  I think your over-thinking it a bit there.

Last I checked, there was hot inert gases and and an EGR valve restricting flow though the venturi portion getting in the way of that effect, therefore there is no real efficiency gain. Block the EGR pipe (just commenting on what most 870 owners do) and all you end up with is a bit of unnecessary turbulence.

Oh I'm sure I'm over thinking things. It's what keeps me entertained on the long hauls. Your site is great for this.

Without the EGR nozzle in the way (completely removed) wouldn't this pipe act as a de Laval nozzle? Or probably not because all the valves and intake ports would just kill any effect it would've had. They are used more in turbine engines with a straight shot right into the combustion chamber.

After researching Detroit's early EGR systems, they used a Venturi pipe of some sort. I'm guessing this was probably used more for thoroughly mixing the exhaust gasses with the clean air. The extra turbulence was probably thought to be needed.

Some times I just need to stroll down the rabbit hole just to see what all is there.