V6 vs V8 gas mileage

[Learpilot]

On the car ad's, they now are saying V8 power with V6 gas mileage.
If the V6 and the V8 have the same cu. in. and compression ratio.
Would they have the same power and gas mileage ?
I would rather have a V8 just because they are smoother.
Rick

[wayne petty]

many newer motors have multiple things to help MPG..


displacement on demand...

variable valve timing..

they might have actually figured out how to properly moderate the electro throttle valve opening..

when people are driving a car.. and they want to go.. with drive by wire.. they mash the pedal to the floor in an attempt to accelerate faster..  they hold the throttle down and reduce the fuel economy.. in an attempt at acceleration...  this sends the injection pulse width wide... but the OBD2 computer is looking at the upstream and down stream oxygen sensors... and will reduce the injection pulsewidth to keep the cats working properly...

in OBD1 motors.. during acceleration.. the system dropped into OPEN LOOP.. did not rely on oxygen sensor data to calculate the injector pulse width.. allowed you to make more power so you got up to speed faster and were able to back out of the throttle earlier..  


this is just my conjecture from working on production units.. i am NOT a fuel injection engineer..

there were some GM midsize trucks that would not reach expected MPG figures..  seems.. for some reason.. the AC compressor ran ALL THE TIME in these.. never cycled on and off.. even had the AC pumping when the system was commanded to hot..   i was not able to verify this in person. but since that fact was found out on another forum with gm customer service reps also involved.. there have been NO MORE posts.. seems all it takes is some reprogramming and to reflash the systems to avoid this issue..

when will they be able to get some serious MPG out of cars and trucks.. probably when they use inductive heating to either heat the exhaust flow just before it enters the cat to keep the cats at operating temps...  or using inductive heating to keep the monolith at operating temps..

some of the loss of fuel economy might have been to an SAE paper that mentioned that it takes more fuel to reach higher intake manifold vacuum levels...

if you have higher intake manifold vacuum.. you have less volume of air.. so less fuel is needed...  but with OBD2 cats and oxygen sensor monitoring.. you need more air and fuel to keep the cats operating at proper temps to keep them working to put out clean air..

have they got smart and reduced the size of the cats..

will variable displacement valve trains help...

will variable valve timing help..

what did they do besides advertising increased Economy?

not a clue yet..

sorry if i have mislead anybody in thinking that this is an easy subject..

ethanol fuels also reduces fuel economy... as there are less BTUs to build heat to expand the air and push the piston down..

[enjenjo]

I think most of it is hype, they have improved the fuel mileage on V8s to where a V6 was a few years back, but the V6s have improved too. I have a Ford with a V6, and a balance shaft, and it is very smooth, and has enough power to break the tires loose when starting out from a stop to the point that you have to feather the throttle when starting out.

[Warpspeed]

Yup, the very latest technology is always going to be better for both power and economy than even just a few years ago.

The latest thing now, is direct fuel injection, that is still in the future for most of us, but it is certainly well on the way.

I doubt if the actual number of cylinders makes very much difference, apart from smoothness.  
But even that can mostly be cured with balance shafts and very clever engine mounts.

[29abone]

`I had a couple of Thunderbird Turbo coupes with the 2.3 litre Pinto 4 cyl engine.  They put out v8 power on the turbo, but cruising at lite throttle was  like running the Pinto eng.  There was a Ford engineer who raced a 2.5 litre turbo coupe that ran under 12 secs and would pull the front tires off the ground.

So the deal is,  when cruising on the smaller displacement engine,  you get better mileage, and when on the turbo boost, you get V8 power.  The wave of the future!!