How does AFM save fuel
Thread Starter
Instructor
Joined: Jan 2022
Posts: 121
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From: Simi Valley,Ca
How does AFM save fuel
I am sure it does work but how? The way I see it if you are cruising at 75 MPH in V8 mode which requires "X" GPH of fuel to produce "Y" HP. When the engine switches into V4 mode you still need "X" GPH of fuel to produce "Y" HP to maintain going 75 MPH. So where does the fuel savings come from? The only thing I can think of that is different is half the engine valves aren't opening. Is the power required to open these 8 valves enough to give the increase in fuel mileage? Or is there something else that increases the mileage that I am not considering?
Melting Slicks
Joined: Apr 2021
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From: Texas
I am sure it does work but how? The way I see it if you are cruising at 75 MPH in V8 mode which requires "X" GPH of fuel to produce "Y" HP. When the engine switches into V4 mode you still need "X" GPH of fuel to produce "Y" HP to maintain going 75 MPH. So where does the fuel savings come from? The only thing I can think of that is different is half the engine valves aren't opening. Is the power required to open these 8 valves enough to give the increase in fuel mileage? Or is there something else that increases the mileage that I am not considering?
Personally I dont give a rats patoot about the fuel economy so I keep it in V8 mode all the time. Even if I set my cruise control at 75mph, I prefer to have all 8 cylinders firing, all the time.
Racer
Joined: Apr 2013
Posts: 376
Likes: 322
I am sure it does work but how? The way I see it if you are cruising at 75 MPH in V8 mode which requires "X" GPH of fuel to produce "Y" HP. When the engine switches into V4 mode you still need "X" GPH of fuel to produce "Y" HP to maintain going 75 MPH. So where does the fuel savings come from? The only thing I can think of that is different is half the engine valves aren't opening. Is the power required to open these 8 valves enough to give the increase in fuel mileage? Or is there something else that increases the mileage that I am not considering?
Last edited by KentCNC; Jul 22, 2022 at 08:45 PM.
Racer
Joined: Apr 2013
Posts: 376
Likes: 322
Your question made me curious. Below is a link that describes better efficiency due to less pumping loss
https://driving.ca/auto-news/news/ho...r-deactivation
https://driving.ca/auto-news/news/ho...r-deactivation
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Brett Foote
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From: Converse TX
Thread Starter
Instructor
Joined: Jan 2022
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From: Simi Valley,Ca
I understand your thinking. Horsepower is force time speed. Runnign down the road at 75mph requies the same force regadless of the amount of power the engine is capable of producing. I believe the savings comes in by it being more effcieint to produce the energy in 4 cyclinders than 8, but I am not an "engine" person and only speculating on that.
Thanks for the thoughts.
Thread Starter
Instructor
Joined: Jan 2022
Posts: 121
Likes: 24
From: Simi Valley,Ca
Your question made me curious. Below is a link that describes better efficiency due to less pumping loss
https://driving.ca/auto-news/news/ho...r-deactivation
https://driving.ca/auto-news/news/ho...r-deactivation
Burning Brakes
Joined: Dec 2016
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I think I read somewhere that with the valves closed the air in the deactivated cylinders essentially act as springs and help to efficiently turn the crank in low load situations. But alas, a mechanical engineer I am not.
Drifting
Joined: Mar 2005
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From: Placitas New Mexico
Le Mans Master
Joined: Jun 2002
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From: Commerce Texas
One thing no one has brought up is how little HP is required to keep a car moving at a steady speed on the highway. IIRC it only takes about 20HP to maintain a constant speed between 60 - 70 mph.
Thread Starter
Instructor
Joined: Jan 2022
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From: Simi Valley,Ca
I found a youtube video that tested a 2020 C8 mostly on the freeway at 73 mph in cruse control. He filled up using the 3 click method at a gas station near an on ramp. He got on the freeway and drove 30 miles, turned around and drove back then filled up again. He did this once in "turning auto" and once in "turning manual". When driving in manual he shifted at 2000 rpm. He compared the car's speed and millage with a stand alone gps. He calculated the millage and compared it to the cars reported numbers. The results of a 60 mile trip was with AFM he got 28 mpg and with out AFM he got 27. These numbers were rounded to the nearest whole number which is 1.67%. You can decide whether this is good or not but it is interesting.
Team Owner
Joined: Oct 2004
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I'm going with GM testing over a single datapoint, YouTube video produced by someone who failed to report variables he/she did not control and might have had an "agenda." Obviously, a company is not going to go to great expense and added complexity to implement a fuel saving feature that produces less fuel economy.
Le Mans Master
Joined: Feb 2002
Posts: 5,394
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From: cumming ga
Oh by the way Marine auto tech finished in top 5 all classes. Worked at city Chevrolet Charlotte NC not my first rodeo. Retire GM Service parts Division 78 years old be 79 in October.
Last edited by z51vett; Jul 24, 2022 at 05:38 PM.
