I'm in the no hear no feel camp. Coupe.

 

Mine did it from the factory, have not had an oil change yet. What does that say?

 

the only noise mine makes when it changes is like a woosh noise, I dunno it is a little annoying if I'm driving with the top down.

 

My car had a terrible CREEEAAAACK when it went to V4… went away a while back.

 

AFM cannot be disabled, either you drive around with the engine /transmission in track mode all the time or drive in Manual mode which keeps the crappy V4 mode from happening. It can be tuned out now if you remove your ECM, send it to HP tuners to be unlocked / reinstall and have a HP dealer/tuner tune it out.

 

I had that noise on my C7. Installed a Range to eliminate the V8 to V4 shift. Sold my C7 to get the C8 and of course the Range does not work on the C8, but I really can't tell when the car shifts modes so I'm good.

 

YOUR LUCKY to not hear it!

 

Guess I'm lucky tooo

 

I question just how much fuel V4 mode saves. I drove my C8 from Florida to Ohio back in May with the car in manual mode the entire trip. The best fuel economy still reached slightly more than 35 mpg highway driving. I can't imagine V4 mode would have raised it significantly higher. I'm sure V4 mode helps GM meet CAFE standards but under normal driving conditions, I doubt it saves that much fuel.

 

V4 save fuel in situations where the engine is "loafing", steady cruising on flat ground. Something on the order of 40 hp (I don't know exactly what it is for the Corvette) is needed to maintain a highway speed, so the engine is throttled way back, and the manifold vacuum is high. Its about 5 hp per cylinder in V8 mode. The engine is not achieving its full charge of fuel and air (what it would be capable of drawing if the throttle was wide open) due to being severely restricted by the throttle plate position. Some call it a pumping loss. In V4 mode, the 4 active cylinder must produce 10 hp/cylinder, and the throttle plate opens a little more to allow that, resulting a bit better efficiency. Here is how Tadge described it:

"Our AFM system is very effective on the small block V8. It lets us close the valves on the de-activated cylinders so the cylinders act as air springs. When the engine rotates we get energy back as the compressed air in the cylinder expands. Other manufacturers just stop fueling the de-activated cylinders and so don't get much fuel savings. The reason 4-cylinder operation is more efficient is that the active cylinders are worked harder with a greater throttle opening. That's like taking a restrictor out of the system saving pumping losses. On Corvette, AFM saves more fuel than start/stop, a feature becoming very prevalent in the marketplace. For many cars, start/stop is a great choice because it is almost transparent to the occupants. Not so on a Corvette. Our engine has a lot of character, so when it stops running it is very noticeable. We have received a lot of very strong input from customers that they would not like it."

The savings depend on lots of factors, but there are general statements that it saves between 4% and 10%. And, there are technical papers about it - with similar numbers. But...the savings are less for engines that already have other advanced fuel saving features. The C8, for example, already has variable valve timing, which also saves fuel. At 4%, and roughly 35 mpg you might see about 1.4 mpg due to V4.

 

I don't understand AFM operation at all. It doesn't do V4 when coasting or when idling. Why be in V8 mode in those cases? p.s. mine makes a loud clunk when switching to V4 sometimes and it's even felt through the car. I think that is when it coincides with an upshift.

 

When coasting, the fuel injectors are all shut off, there is no additional fuel to be saved at that point. Basically, when there is a low power demand, like steady cruising, 4 active cylinders can produce the necessary power more efficiently than 8 active cylinders. As for why its not engaged at idle, where almost no power very little fuel is required, I could only speculate (make stuff up), but 4% of nearly nothing, is nearly nothing, so the saving at idle is perhaps small or non-existent. Also, at idle, if V4 was engaged the exhaust note would be weird. Folks like to hear the V8 rumble at idle. Auto stop appears to be the way to save fuel at idle - then its saving of 100% of whatever was consumed at idle. Based on the following from Tadge, maintaining the V8 character at idle was a factor in not using AutoStop.

"On Corvette, AFM saves more fuel than start/stop, a feature becoming very prevalent in the marketplace. For many cars, start/stop is a great choice because it is almost transparent to the occupants. Not so on a Corvette. Our engine has a lot of character, so when it stops running it is very noticeable. We have received a lot of very strong input from customers that they would not like it."

I think one could extrapolate and say that deactivating 4 cylinders at idle (even it helped ever so slightly and I do not know if it would) would also interfere with the characteristic V8 sound at idle. I get that many can also detect it when cruising. I can, but it is, in my opinion, very subtle, and it does not concern me. I use manual or engine shift set to track when I want to. But for steady long distance cruising, I just use Tour and automatic shift mode.

Anyway, here is the list of conditions to enable V4, followed by a list of conditions that prevent V4, from the Service Manual.

"The conditions listed below determine when cylinder deactivation is enabled.
Engine has been running for greater than 30s
Engine speed is between 700 and 2800rpm
Engine oil pressure is between 187-455kPa (27-66psi)
Engine coolant temperature is between 40-129°C (100-264°F)
Engine oil temperature is between 16-128°C (61°F)
Throttle angle is 6% or less
Ignition voltage is greater than 11V
Transmission is not in first, second, or reverse gear
Vehicle speed is greater than 25kph (15.5mph)
Brake booster pressure is greater than 42.0kPa (6psi)
Vehicle is not in fuel shut off mode
Vehicle is not in heater performance mode
Vehicle is not in tip in bump acceleration mode
Vehicle is not in oil aeration mode"

and

"Cylinder deactivation may be inhibited for many reasons including the following:
Engine coolant temperature out of range for cylinder activation
Engine vacuum out of range
Brake booster vacuum out of range
Transmission gear incorrect or shift in progress
Accelerator pedal out of range or rate of pedal application too fast
Engine oil pressure and temperature out of range
Engine speed out of range
Vehicle speed out of range
Minimum time in V8 mode not met
Maximum V4 mode time exceeded
Decel fuel cutoff is active
Reduced engine power is active
Torque management is active
Catalytic converter over temperature protection is active
Piston protection is active, knock detected
Cylinder deactivation solenoid driver circuit faults"

 

That doesn't sound right, or safe to say that when coasting there is no fuel being delivered to the engine.

 

It cuts off as noted in the Service Manual. It may not immediately cutoff but will during "extended" coasting. But if too much fuel is provided, during a coast event, it can damage the catalytic converters. But, the context was why does it go into V8 mode when coasting. There is no significant additional fuel saving to be had when coasting, and, engine braking is better with all 8 cylinders pumping against the closed throttle plate, rather than just 4 cylinders. Again, the fuel saving in V4 is a low percentage, perhaps only 4%. It saves fuel during a steady cruise, but when coasting, using little, or no fuel, 4% of nearly nothing is nearly nothing.

I tried to find some evidence of what "extended coasting" means. Looks like it might be as quick as 2 seconds of no throttle. This is based on a diagnostic procedure I found in the Service Manual. The procedure says drive the car for at least 5.5 miles between 28 mph and 70 mph, with speed reaching at least 50 mph, then release the accelerator for at least 2 seconds, and the car will enter the decel fuel cut-off mode. So there is that. Here it is:

"22. Continue to drive the vehicle for at least 5.5 miles between 45-112 km/h (28-70 mph) with the vehicle reaching at least
80 km/h (50 mph).
23. Release the accelerator pedal for at least 2 seconds. This will allow the vehicle to enter decel fuel cut-off."

 

That would be...interesting. I did not think that while in a moving vehicle that it would be safe to have the engine turn off.

 

I have an idea that the spark plugs are still firing in the fuel cutoff mode during coasting, but I have not found anything that states that specifically for the fuel cut off mode. But ...the reason I think that is because during V4 mode the spark plugs continue to fire in the deactivated cylinders - in order to "help prevent spark plug fouling". I think the same logic - to "prevent spark plug fouling" - would apply when in fuel cut-off mode while coasting.

When the car does go into V4 mode, its like an orchestrated dance with perfect timing. For each deactivated cylinder, the exhaust valve is deactivated (shut) first. After a full gulp of fuel air mixture, then the intake valve is deactivated (shut). The deactivated cylinder then fires - but since both valves are closed - the combustion charge (and pressure associated with that) is captured for the duration of the V4 mode. The deactivated cylinders "act like air springs" and as " the engine rotates we get energy back as the compressed air in the cylinder expands" according to Tadge.

From the Service Manaul:
"Although both intake and exhaust valve lifters are controlled by the same Valve Rocker Arm Oil Control Valve, the intake and exhaust valves do not become deactivated at the same time. Cylinder deactivation is timed so that the cylinder is on an intake event. During an intake event, the intake cam lobe is pushing the valve lifter upwards to open the intake valve against the force of the valve spring. The force exerted by the valve spring is acting on the side of the lifter locking pins, preventing them from moving until the intake valve has closed. When the intake valve lifter reaches the base circle of the camshaft lobe, the valve spring force is reduced, allowing the locking pins to move, deactivating the intake valve. However, when cylinder deactivation is commanded ON, the exhaust valve for the deactivated cylinder is in the closed position, allowing the locking pins on the valve lifter to move immediately, and deactivate the exhaust valve. By deactivating the exhaust valve first, this allows the capture of a burnt air/fuel charge, or exhaust gas charge, in the combustion chamber. The capture of exhaust gases in the combustion chamber will contribute to a reduction in oil consumption, noise and vibration levels, and exhaust emissions when operating in V4 mode cylinder deactivation mode. During the transition from V8 to V4 mode, the fuel injectors will be turned OFF on the deactivated cylinders. To help prevent spark plug fouling, the ignition system secondary voltage or spark is still present across the spark plug electrodes on the deactivated cylinders. If all enabling conditions are met and maintained for cylinder deactivation operation, the ECM calibrations will limit cylinder deactivation to a cycle time of 10 minutes in V4 mode, then return to V8 mode for 1 minute."

 

Certainly a lot of complexity and technology for virtually no difference in MPG.

 

$1.50 more expensive than what? E85? What's your mileage like on that stuff? If you're talking $1.50 more from regular to 91 octane (all we can generally get at a pump in metro Phoenix), that's laughable. The spread in A.J. is $.60.

John B.

 

No Johnny, not talking about pricing between Regular compared to 91 octane. Speaking of having my ECU unlocked to run on E85 with a Flex Tune Global B Tune.

This Flex Fuel E85 picture is from April 2024. E85 is always less per gallon than our AZ crap 91 octane.

 

For those annoyed by the AFM transition noises:
https://www.corvetteforum.com/forums...the-cabin.html