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I have 2016 Z51 A8. Seems like a rough shift going from V8 to V4 and back to V8 when cruising over 70 mph. I also see rpms bounce 100-200 rpm and feels like riding on a rough road for a second when trying to stay at constant speed. Is there something wrong with the transmission, or am I imagining things?
 

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Welcome. When was your car's build date?
 

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Mtheckray, welcome aboard, has it been shifting rough since owning it? Take it to the service department and have one of the Corvette techs do a ride along to determine what the cause is. Hope you get the situation squared away and get back to wheeling around Houston care free.

SF
Rick
 

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FWIW - I have a M7 but on a recent long road trip i experimented with ECO and 6th and 7th gear. In ECO, to me when i was in 7th, the engine seemed very "boggy". The RPM's were so low the car just bogged down in 4 cyl mode. You could feel it go in and out of 4/8 mode but it wasnt a hard jerk. I was lucky to average over 30mpg on the interstate, and this was with and without using. AFM. Overall the car felt better in 6th gear at about 75mph w/o AFM. I might have left 1mpg on the table but i was ecstatic at the 30 i was getting in full V8!
 

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I have 2016 Z51 A8. Seems like a rough shift going from V8 to V4 and back to V8 when cruising over 70 mph. I also see rpms bounce 100-200 rpm and feels like riding on a rough road for a second when trying to stay at constant speed.
Many people can tell, on some occasions, when the C7 switches between V4 and V8. Personally, I can feel the transition when engine load changes quickly, like when you transition from flat ground to a reasonably steep hill, at constant speed.

I'm more concerned about your RPM pulsation, which doesn't sound normal. Others have also encountered this, but I never have with my M7. I'd have the dealer check it out.
 

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I have a 2014 A6, I never feel a thing when it goes from 4-8 or from 8-4 cylinders. I have even kept the screen up to watch it and still don't feel anything. Either I have a fat butt or a great transmission. Good luck on your quest to get it fixed.
 

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Switch between V4 & V8 is a mechanical transition which can only be muted through the power train, never eliminated. If you can't sense the transition then it’s not working.

Although I seldom use the V4 ECO with my M7, when I do the transition is almost imperceptible. I say almost because under certain driving situation the transition becomes harsh. For example accelerating just as the engine wants to go into V4 mode. The transition back to V8 mode is normally harsh.

Remember, in V4 mode the engine barely exceeds 120HP so it works best on a level road and cruise on.

Unlike the M7 the A6 and A8 owners have less control since AFM is always on, as a result the driver should be more aware of V4/V8 transition.
 

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Remember, in V4 mode the engine barely exceeds 120HP so it works best on a level road and cruise on.
Do you recall where you saw this number published? Most of the articles I've seen estimate V4 HP as 460/2 = 230 HP...
 

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Do you recall where you saw this number published? Most of the articles I've seen estimate V4 HP as 460/2 = 230 HP...
I believe it was a youtube video describing the LT1 and the number I remember was 129HP. I assume that the transition is not directly proportional because of parasitic effects. But that is just a guess.
 

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Did a google search.........

2014 Corvette C7 LT1 Engine with Direct Injection, Variable Valve Timing and Active Fuel Management; Statistics

Active Fuel Management
Fuel economy is kept under control by shutting down four of the cylinders during subdued driving conditions, turning the LT1 6.2 L V8 into a 3.1 L V4. Cylinder shutdown is accomplished by deactivating the lifters in cylinders 1, 4, 5 and 6. V4 operation, in the case of manual transmission C7 Corvettes, is only available in the Eco mode. Output is a meager 125 horsepower.
 
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You have to remember, the 8 cylinder 460 hp is achieved at full throttle. If you go full throttle in V4 mode, it switches back to V8 mode.

So there is no way that V4 mode would produce half the horsepower of V8 mode. You cannot stay in V4 mode at full throttle.
 
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Horsepower, even low amount really has no bearing if 4 or 8 cylinder

DOD (AFM working window is when speed is over 5 MPH and less then 3,200 RPMs
So it is lower speeds and non PE or WOT modes

Here below is I recording OBD data with cruise testrun of a 2015 Z06

As seen below even with 40 to 70 MPH the output horsepower is low as very little torque output and this was with DOD turned OFF in tune so DOD has no part of the data shown is very little horsepower
which would be about the same amount IF cruising with DOD on
Once to cruise speeds is achieved then little Tq/HP is needed to maintain moving the mass

Where DOD is a bummer is when having to pull the total weight to get car moving ( why GM has DOD off below 5 MPH), pulling up hills or the loss of performance due to elevation
Look at the collected PID data below and see how little Tq and HP was at 40-70 MPH speeds even with 8 cylinders in part throttle


Switch between V4 & V8 is a mechanical transition which can only be muted through the power train, never eliminated. If you can't sense the transition then it’s not working.

Although I seldom use the V4 ECO with my M7, when I do the transition is almost imperceptible. I say almost because under certain driving situation the transition becomes harsh. For example accelerating just as the engine wants to go into V4 mode. The transition back to V8 mode is normally harsh.

Remember, in V4 mode the engine barely exceeds 120HP so it works best on a level road and cruise on.

Unlike the M7 the A6 and A8 owners have less control since AFM is always on, as a result the driver should be more aware of V4/V8 transition.
 

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No wonder it felt like my lawn tractor at 75mph ;) Cool data thanks for sharing and the explanation!
 

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I used the ECO mode for only the second time in over 17K miles. The first time I used it on the highway car had about 5K miles. The cruise was set to 70 MPH with a lot me using gas/brake because traffic volume was pretty high. I noticed the "bump" going from V4 to V8 and back. It was very annoying.

Over a year later, last weekend to be precise, I tried the ECO mode again. The scenario this time very different. Heading back from northern Maine practically no traffic. I set the cruise control to 81 (highway speed limit was 75) and put it in 7th gear. For over 60 miles I never once had to hit the gas or brakes. I watched the V4/V8 change on the display but barely felt the slightest change and many times nothing.

Below is a picture taken by my wife at the 52 mile mark. Also at picture at the 400 mile setting. I'm thinking the ECO mode improves as the engine ages.

Gus

IMG_1216.JPG IMG_1225.JPG
 
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Yep that's about what I get here in Texas under similar setup.
 

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I've used a AFM disabler since taking delivery back in 10/2015 to eliminate the 4 cyl mode. Since that time it has never gone into V4 mode.
 

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I've used a AFM disabler since taking delivery back in 10/2015 to eliminate the 4 cyl mode. Since that time it has never gone into V4 mode.
If I owned an A6 or A8 I too would install an AFM disabler. There really is no good reason for the AFM to function in all modes in the A6/8 and only ECO in the M7.
 
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