This isn't actually true - when you switch to MDS, those 4 cylinders will run at higher load. That load transition is why there's a "bump" - you're switching from 8 cylinders putting along to 4 running almost as hard as they can
If you think about it, a V8 running at 30% load can't just switch 4 cylinders off without suddenly slowing down and the driver noticing. It has to change throttle position and run the four at 60+% load to make up for the dead cylinders. The magic here is that cylinders at high load have a lower Brake Specific Fuel Consumption (BSFC), so it's more efficient overall.
That being said, the ECU has loads of tables it uses to tell when it's optimal to engage MDS - BSFC is not linear. I'm sure it's not *entirely* optimized for fuel consumption since they know it creates NVH; it's probably true that placing efficiency over comfort would yield positive results at the pump... but there are other areas where all 8 cylinders probably have a better overall efficiency. I'm honestly not sure if you can even idle properly with MDS engaged, for example.
Thank you for the thought provoking comments.
I’m not sure the explanation is completely valid since it applies a simple rule and leaves more questions than answers.
As I recall Brake Specific Fuel Consumption is used to compare the maximum power efficiency of engines to other engines using a fixed brake horsepower load on a test stand. The subject at hand for this discussion is a little different. Instead of comparing one engine design to another (two completely different engines), we are discussing one engine that switches off one-half of its cylinders on a common crankshaft. There’s an assumption that the BSFC of just four cylinders is in ratio to all eight when not in MDS. I don’t think this is true. Ignoring all other possible influences for the moment, when in MDS four cylinders are now supplying power but with twice the amount of internal frictional load, i.e.: it has to overcome the friction induced by the non-power producing cylinders.
“If you think about it, a V8 running at 30% load can’t just switch 4 cylinders off without suddenly slowing down without the driver noticing it. It has to change throttle position and run the four at 60+% load to make up for the dead cylinders.”
Except that does not appear to happen. On all three of my MDS engines the transition from eight to four cylinders (MDS mode) more often than not, is totally imperceptible. I have never observed a change in engine RPM so I don’t believe the throttle plate changes position. In my current MDS model with the 3.21 axle sometimes the slightest throttle pressure will kick it out of MDS.
“The magic here is that cylinders at high load have a lower Brake Specific Fuel Consumption, so it’s more efficient.”
Okay, but for the scenario in this discussion the torque requirements will vary considerably as the normally aspirated vehicle is driven in normal driving where there are launches from stops and power required on grades, not to mention the varying cargo and passenger loads. As you pointed out, BSFC is not linear. I am very certain that running on just four MDS cylinders in a zero grade, wind-less environment the MDS engine will indeed outshine with respect to fuel consumption. But I think in a purely practical usage and environment of day-to-day driving there will be times that all eight cylinders will be contributing to a more efficient use of fuel.
There may be somewhat of a demonstratable example. The Silverado can be equipped with a forced induction, four-cylinder engine. The EPA rating of 20/23 (2WD) and 19/22 (4x4) is really quite unexceptional, especially when one considers the Silverado is 300-400 pounds lighter than the current 5th gen Ram.
All the best,
Dusty
2019 Ram 1500 Billet Silver Laramie Quad Cab 2WD, 5.7 Hemi, 8HP75, 3.21 axle, 33 gallon fuel tank, factory dual exhaust, 18” wheels. Build date: 03 June 2018. Now at: 053971 miles.