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post #1 of 21 (permalink) Old 04-08-2019, 11:35 PM Thread Starter
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Question MPGs from MDS

I just made a 6 hour drive with the MDS disabled on my 5.7L, and I was surprised to see that I averaged 25 MPG (rounded down; something like 25.3 actual).

I am surprised because back when I had the MDS enabled, the best I could do on the highway was 28 MPG (rounded up; something like 27.6 or 27.8 actual).

It surprised me that the MDS technology, for all its innovation and efficacy in general, is only giving me 3 extra MPGs on the highway. (I don't remember my MPG average for city driving with MDS, but city MPGs without MDS is averaging 19 MPGs.)

It may have been unrealistic, but I honestly thought the MDS system was supposed to give these cars more of a MPG boost. I don't know what I thought that increase was, but I probably have been assuming something like 5-6 extra MPGs on the highway and 3-4 in the city. Obviously that is not the case, for me and my car anyway...so I will definitely be leaving it disabled going forward. I see no reason to enable it as of this time, that's for sure.

Anyone else share my unrealistic assumptions about MDS' benefits, or were y'all all already hip to its real contributions, meager as they are?

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post #2 of 21 (permalink) Old 04-09-2019, 01:54 AM
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We all bought V8's to run on 4 cylinders, yeah that's it!

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post #3 of 21 (permalink) Old 04-09-2019, 02:21 AM Thread Starter
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We all bought V8's to run on 4 cylinders, yeah that's it!
But if it is operating on only 4 cylinders, would it be fair to call it a Semi Hemi??
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post #4 of 21 (permalink) Old 04-09-2019, 02:43 AM
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Nah, I think its a Chevy now.
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post #5 of 21 (permalink) Old 04-09-2019, 08:50 AM
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By observation using an OBD2 code reader/data viewer and monitoring engine load and over a number of vehicles its takes around 45hp to move a car down the highway at 65mph.


A 400hp V8 -- such as was in my 2006 Pontiac GTO this 45hp represents a bit over a 10% load. At 10% load the engine is not working very hard to begin with. As a result gas mileage with that car was rather good at 65mph as long as I behaved myself.


The same was true with other cars with engines from 1.9l, 2.0l, 2.7l, 3.6l, 6.0l, 6.4l and 6.2l in displacement.


Actually in the case of the 1.9l engine this was a turbocharged direct injection diesel engine in my VW Golf. At 65mph its load was nearly 50% because the engine's HP rating was just 90hp. But because it was a turbo charged direct injection diesel fuel consumption was light. Over 150K miles the engine averaged better than 40mpg.


The 2.0l engine is in my 2018 Mini JCW. It is a turbo charged engine producing 228hp. At 65mph its fuel consumption is quite low. With my normal usage the engine delivers 30mpg. I have not had time to take it on a long road trip to see what it can do at steady highway speeds but I suspect fuel consumption would be even less, above 30mpg. My Boxster (2.7l engine) on the highway would average around 28mpg. When driven like I drive my JCW this would drop to around 20mpg.


My point is whether all 8 cylinders are firing or just 4 the engine is having to produce the same amount of power to move the car down the road. There is some efficiency in that the "half an engine" is under a higher load at a rather low RPM which results in a wider opened throttle. This reduces pumping losses which improves gas mileage some. Because the throttle is opne more the engine is getting more air into its cylinders the cylinder filling is better. This requires more fuel but the engine controller can use more timing advance and this can really improve gas mileage.


I recall with my GTO in 6th gear engine RPMs were quite low, well, at legal speeds at any rate. Timing observed using the OBD2 code reader/data viewer -- was quite high, in the 35 degrees area. Instantaneous fuel economy was around 30mpg.


And yeah, any increase in throttle had this drop quite a bit. But the point is operating the engine at low RPMs with a bigger throttle opening was the engine's sweet spot when it came to fuel economy.


Well, that is what MDS offers one. It forces the engine to a high load, wider throttle open position, with better cylinder filling operating mode and the ability to use more timing to wring as much mechanical energy from the chemical energy of the burning gasoline as possible.



The deactivated cylinders still represent some loss due to friction but the active cylinders are operating in a much more efficient fashion.


Given how much fleet fuel mileage matters nowadays just a few MPGs improvement over a sub set of the vehicles offered by an automaker is important. In this context, a 3mpg improvement ain't nothing to sneeze at.
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post #6 of 21 (permalink) Old 04-09-2019, 09:31 AM
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I've turned mine on and off over the last 9 years and 148k. I've tried it with all three grades of gas. I've tried it with the stock tune, 91 CAI tune, and 93 CAI tune. It all figures out to roughly 30 miles per tank farther driving with the MDS on. For those inquiring minds.... the 91 CAI tune with 93 gas, no MDS is the most fun.

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post #7 of 21 (permalink) Old 04-09-2019, 09:37 AM
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So that is about a 1.5 MPG improvement. I think FCA claimed anywhere from 10-20% improvement in MPG.

I know all car manufacturers use DFCO which leads to MPG improvement of ~2%.


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post #8 of 21 (permalink) Old 04-09-2019, 10:13 AM
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Quote:
Originally Posted by Nuke View Post
But if it is operating on only 4 cylinders, would it be fair to call it a Semi Hemi??
Well...you know what they say about folks not operating on all cylinders.
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post #9 of 21 (permalink) Old 04-09-2019, 12:44 PM
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Quote:
Originally Posted by Nuke View Post
I just made a 6 hour drive with the MDS disabled on my 5.7L, and I was surprised to see that I averaged 25 MPG (rounded down; something like 25.3 actual).

I am surprised because back when I had the MDS enabled, the best I could do on the highway was 28 MPG (rounded up; something like 27.6 or 27.8 actual).

It surprised me that the MDS technology, for all its innovation and efficacy in general, is only giving me 3 extra MPGs on the highway. (I don't remember my MPG average for city driving with MDS, but city MPGs without MDS is averaging 19 MPGs.)

It may have been unrealistic, but I honestly thought the MDS system was supposed to give these cars more of a MPG boost. I don't know what I thought that increase was, but I probably have been assuming something like 5-6 extra MPGs on the highway and 3-4 in the city. Obviously that is not the case, for me and my car anyway...so I will definitely be leaving it disabled going forward. I see no reason to enable it as of this time, that's for sure.

Anyone else share my unrealistic assumptions about MDS' benefits, or were y'all all already hip to its real contributions, meager as they are?
Not sure what kind of drive you did, but the most I can muster on the same road, going 65mph with MDS on/off is 32/25.

Unless you have the self control of a saint (I don't), your trip MPGs will never be the same regardless of what settings you have on and off.

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post #10 of 21 (permalink) Old 04-09-2019, 01:24 PM
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Since my Shaker has the six speed in it, it doesn't suffer from mad dog syndrome like the autos do. What I've found is that certain performance mods help to increase gas mileage. On my last trip to the Dallas area from here I got 29 mpg on the freeway, but that was before I added the supercharger, and with a custom tune and a full performance exhaust system. Later this year I may make the trip again, and then I'll find out how much my additional mods have affected the mileage. Other than that, I don't much care one way or the other...

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