"Fuzzy Math" MPG question to ponder

[striz923]

How is it that the 6.2L V8, 16 valve, 638 hp @ 6500 rpm 2009 Chevrolet Corvette ZR1 ZR-3 gets the same exact gas mileage as the 4.5L V8, 32 valve, 325 hp @ 6400 rpm 2009 Infiniti M45 AWD. 14 city/20 hwy for both...

Does 300 more hp, but significantly less weight account for that? Someone smarter than me needs to put this into simple terms, because I would have thought the city mpg would be TERRIBLE for the 'vette, but they could put extra gear for the hwy. But then again it is a "race car" so why would they put that extra gear in??

[CakeDaddy]

How is it that the 6.2L V8, 16 valve, 638 hp @ 6500 rpm 2009 Chevrolet Corvette ZR1 ZR-3 gets the same exact gas mileage as the 4.5L V8, 32 valve, 325 hp @ 6400 rpm 2009 Infiniti M45 AWD. 14 city/20 hwy for both...

Does 300 more hp, but significantly less weight account for that? Someone smarter than me needs to put this into simple terms, because I would have thought the city mpg would be TERRIBLE for the 'vette, but they could put extra gear for the hwy. But then again it is a "race car" so why would they put that extra gear in??

The new V8 GM engine down grades to 6 & 4 cylinders depending on driving pattern? I was recently in an Impala SS rental car and I had a very good time driving it and the gas mileage I got on that thing was pretty good.
Modified by CakeDaddy at 3:55 PM 10/13/2008

[exbmwdude]

You got to give the General his due...

GM has been pretty much carrying the pushrod torch all alone and although their pushrod V6's are pretty lame, the V8's are still very competetive with modern DOHC motors. Going all the way back to the late 90's when the LS1 v8 was introduced, Chevy has been able to make the Vette keep up with its peers while still delivering great mileage and NVH levels not far from a DOHC motor. The mileage thing is simple if you've ever owned one - they literally idle down the highway. They make so much torque right off idle that they don't have to rev more than 1k rpm to push the car down the road at 60mph. I had the last of the "old generation" V8s (the LT1) in a 94 Firebird and it would get 28mpg at 65mph while turning around 1200 rpm. It's very easy with that .50 6th gear and tons of old fashioned torque.

See, it is possible to have your cake and eat it too. That's why I have beenso critical of the 5-speed auto in our current M's. These car should have had a 6-speed by now with a "highway" 6th gear to at least get the mileage up into the mid 20's. If my early 90's muscle car (14.0@100mph in the 1/4, 158mph Top Speed) could get 28mpg at 65, why can't my M35 which is better aerodynamically and has a motor about half the size do the same with another 12 years of technology ?

I won't even get into the fact that my 96 BMW 740il with a V8 and another 400lbs got better mileage that my M35...

exbmwdude

[marlin29311]

How is it that the 6.2L V8, 16 valve, 638 hp @ 6500 rpm 2009 Chevrolet Corvette ZR1 ZR-3 gets the same exact gas mileage as the 4.5L V8, 32 valve, 325 hp @ 6400 rpm 2009 Infiniti M45 AWD. 14 city/20 hwy for both...

Does 300 more hp, but significantly less weight account for that? Someone smarter than me needs to put this into simple terms, because I would have thought the city mpg would be TERRIBLE for the 'vette, but they could put extra gear for the hwy. But then again it is a "race car" so why would they put that extra gear in??

Variable Valve timing, compression ratios, active fuel management, weight, drivetrain losses, etc...you can go on for a while. It all deals with the overal architecture of the powertrain, from the intake all the way down to the exhaust. Your M is running a lot more systems and also requires a lot more electricity to be generated, where as the 'vette doesn't have all the fancy gadgetry and is focused primarily on the driving aspect. There are a lot of variable that cause this fuel discrepency - you can't just point out one thing.

[palincal]

No doubt putting a sixth gear for highway cruising will increase the fuel economy. M runs at 2800 RPM at 70 MPH whereas my Odyssey minivan which is 600 pounds heavier runs at 2200 RPM at 70 MPH. Big difference. My minivan has more gadgets to run.

The whole idea of running at higher RPM is completely performance driven. Higher RPM means higher instantaneous available horsepower. That is what makes M a fun car to drive at the expense of fuel economy.

[MagicM]

what's even funnier is that the 7.0 Liter V8 z06 can get 18 city and 28 Highway with 500+ horses. My friend has a new C6 corvette and he actually manages around 25/26 on the highway and that's in the mountains at high elevations, The engine literally idles when cruising around 120 KPH / 70 mph, and it takes almost no effort to get going.

[Nismo1182]

5th and 6th gear on the LS motors are both overdrives which helps out A LOT in the MPG area. Plus the ZR1 weighs 900 lbs less than a M45.

My Z with 300HP with similar gearing as a M35/M45 manages 28-30 MPG on the highway.

[MinisterofDOOM]

You got to give the General his due...

GM has been pretty much carrying the pushrod torch all alone and although their pushrod V6's are pretty lame, the V8's are still very competetive with modern DOHC motors.

GM has definitely not been carrying the pushrod torch all alone. Chrysler has continued to use pushrod V8s (and V10s--Viper!) as well (although their smaller V8 is an OHC design). Ford is the only American automaker to abandon pushrods in it's V8s, and they suffered for it early on as they adapted to the different tech. You're very right that it's still competitive, though. People ignorantly assume that older means worse. The reality is that pushrod is just a different way to do things, not an inferior way.

Fortunately, GM has moved on from their sub-par pushrod V6s. In fact, GM's current crop of DOHC V6s are on par with the best Japan has to offer. Ford simply upgraded their sub-par pushrod V6 to be SOHC and the result was a VERY sub-par SOHC motor. Wooops.

As to the original question, you have to look at more than just those few bare numbers. Regarding a suggestion earlier in the thread, no, the ZR1 does not have cylinder deactivation.

The apparent difference is probably a combination of the VK achieving lower than normal fuel economy while the LS9 is better than normal. But also, the LS9 makes a LOT of torque, and a lot of torque means you don't need to push the car very hard to drive normally. The car can effortlessly do what most cars would struggle to do. Driven casually, I'm sure the ZR1 achieves that 14/20 with ease. But if you drive it hard, I would expect that 14/20 to plummet. Also, due to the use of it's supercharger, GM was able to make low-end performance better than they usually can with their higher-performance V8s. As with most pushrod motors, high-end power usually comes in exchange for low-end roughness (which is why most powerful muscle cars have chunky idles). Since the supercharger was there, GM didn't have to make that sacrifice, and the car has a much more daily-driver friendly low-end. And since that's where you achieve your best mpg stats, I'm guessing that's played a decent role in things as well.