Is AFM (Cylinder Deactivation) on the C7 worth it?

[dwightlooi]

Is AFM (cylinder deactivation) worth it?

I report, you decide…

• FACT: AFM is worth 2 mpg in on the freeway, 0 mpg in the city. With AFM enabled in Eco mode, the Corvette Stingray posts 17 (city) / 30 (hwy) mpg in the EPA test cycle. With it disabled in Touring mode, the Stingray posts 17 (city) / 28 (hwy) mpg in EPA test cycles. GM chooses to put the “average of the two” 17/29 mpg on the window sticker.

• FACT: the LT1 V8 makes 460 hp @ 6000 rpm / 465 lb-ft @ 4600 rpm. This is about 45 hp shy of the 7.0 liter port injected LS7 V8, but only 5 lb-ft shy of it. The 7-liter LS7 peaks at 505 hp @ 6300 rpm with 470 lb-ft @ 4800 rpm. Most of the torque gain in the LT1 is from direct injection and the 11.5:1 compression, the LS7 actually has higher lift cams and is tuned for better breathing up top.

What’s the cost of AFM you may ask? Well, it’s simple. With about 465 lb-ft of twist from 11.5:1 compression, if it used a similar lift and duration profile on the camshafts as the LS7, it would be a 500 hp engine. It is 460 hp because is slightly hampered by lower valve lift profiles. And this isn’t because GM didn’t want 500 hp, or that somehow emissions or civility would be bad. It had to use slightly lower lift because AFM added valvetrain mass. The AFM lifters are heavy and with the same valve spring tension you could use less lift before the valves floated.

So… basically, with AFM, you are getting 2 mpg and sacrificing 40 hp (give or take).

Put another way... if you drop AFM and change the cam grind a little, the LT1 will make 500 hp @ ~6300 rpm and 465 lb-ft @ ~4800 rpm. It'll also give 17 / 28 mpg.

Is it worth it? You decide!

[daves87rs]

Nope....nobody I know drives a vette for MPGs anyways...

[David]

No and with all the forum talk on AFM issues from Oil Consumption to rough idle and weird shift issues, I am starting to be less thrilled with AFM. Not sure why Chrysler got it going so well in the Hemi, but GM does not seem to have figured out how to do AFM smoothly yet.

[Drew Dowdell]

I've never experienced AFM issues.... I'm not saying they aren't there, but I've never noticed them to be sluggish.

I have a Silverado 1500 coming as a review car next month, so I'll get another look.

[David]

Very cool, look forward to seeing your review and what you think of the AFM and if any issues seem to show up.

[jwbouch]

In my 5.7 Hemi with cylinder deactivation (can't remember Chrysler's name for it), I do not think it's as seem less as it is supposed to be. On my trip from NJ to Ohio, I could feel it going on and off as needed to maintain 73 going up the mountains. It was kind of frustrating to me, but my partner never noticed or felt it at all. But, he's not a car guy.

[regfootball]

it was either MT or C/D that noted 35.8 mpg on a new stingray for a 25 mile stretch at highway speeds recently.

[dwightlooi]

To put it in one sentence....

AFM's downside is that AFM lifters are heavy and this limits the valve lift you can operate with, which in turn reduced the redline and limit high rpm performance.

So... maybe you can have a Naturally Aspirated LT6 engine. Same pistons, same bore size, same block, but stroked to 101.6mm (same as LS7), displace 6.8 liters and operate without AFM. This engine will rev to 7000 rpm (same as LS7) and make 550 hp @ 6300 rpm, 512 lb-ft @ 4800 rpm and it'll deliver about 16 / 27 mpg in a 3300 lbs car like the vette. Not a bad engine for a Z06 or an ATS-V.

Edited November 15, 2013 by dwightlooi