Understanding the physics/engineering of fuel efficiency (???)
Feb 9, 2011, 12:16 PM
#1
Evolving Member
Thread Starter
Understanding the physics/engineering of fuel efficiency (???)
This isn't meant to be a rant about my Evo's gas guzzling, I'm just genuinely trying to understand the mechanics of how the following can happen.
During highway driving I typically cruise just below 80mph, with my 2008 Evo X MR (mods listed below) @ ~3,300rpm or so in 6th gear. Over long distances I get maybe 23-24mpg.
In my 1995 3000GT (3.0 NA V6 base model @220hp....on a good day), again cruising @ 80mph on the highway @ ~3,200rpm (I think in 4th gear- automatic), I would easily get 26mpg.
So a big old engine driving at the same speeds and rpms gets better mileage than a new small engine that's working just as hard? Huh?
Questions:
-At similar RPMs, shouldn't each cylinder in the respective engines be firing a similar number of times?
-Shouldn't a bigger-displacement engine require more fuel in the combustion chambers to function?
-Shouldn't a 5 year old engine generally be more efficient and technologically advanced than something Mitsu designed 20+ years ago?
-How does the turbocharger affect all this? Is more fuel being pushed into the engine to mate with the compressed air, thus requiring a quantity of gas disproportionate to a NA engine of same displacement? Would a comparison to the turbo'ed VR-4 be more valid (especially given similar output numbers)?
During highway driving I typically cruise just below 80mph, with my 2008 Evo X MR (mods listed below) @ ~3,300rpm or so in 6th gear. Over long distances I get maybe 23-24mpg.
In my 1995 3000GT (3.0 NA V6 base model @220hp....on a good day), again cruising @ 80mph on the highway @ ~3,200rpm (I think in 4th gear- automatic), I would easily get 26mpg.
So a big old engine driving at the same speeds and rpms gets better mileage than a new small engine that's working just as hard? Huh?
Questions:
-At similar RPMs, shouldn't each cylinder in the respective engines be firing a similar number of times?
-Shouldn't a bigger-displacement engine require more fuel in the combustion chambers to function?
-Shouldn't a 5 year old engine generally be more efficient and technologically advanced than something Mitsu designed 20+ years ago?
-How does the turbocharger affect all this? Is more fuel being pushed into the engine to mate with the compressed air, thus requiring a quantity of gas disproportionate to a NA engine of same displacement? Would a comparison to the turbo'ed VR-4 be more valid (especially given similar output numbers)?
Feb 9, 2011, 12:27 PM
#2
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Lots of factors come into play at that speed.
Coefficient of drag, Rolling resistance, BSFC (Brake specific fuel consumption)
http://en.wikipedia.org/wiki/Brake_s...el_consumption
Coefficient of drag, Rolling resistance, BSFC (Brake specific fuel consumption)
http://en.wikipedia.org/wiki/Brake_s...el_consumption
Feb 10, 2011, 07:20 AM
#5
Evolving Member
Weight, draive train resistance from dual clutch setup, drag, AWD losses, MIVEC ingagement, more air with mods= more fuel...
turbo spinning is realtive to the volume and speed of exhaust gasses coming through. Typically, the more air+fuel burned by your setup= more exhaust gases which in turn, should spin the turbo more than stock... which is why guys running stock w/ tune get better gas mileage vs mods/tune...
smaller vs bigger displacement means nothing when comparing NA vs turbo, it more becomes a question on volume of air at x throttle NA vs volume of air at x throttle w/ turbo.
Im not a tuner/mechanic, but thats how i understood it.
turbo spinning is realtive to the volume and speed of exhaust gasses coming through. Typically, the more air+fuel burned by your setup= more exhaust gases which in turn, should spin the turbo more than stock... which is why guys running stock w/ tune get better gas mileage vs mods/tune...
smaller vs bigger displacement means nothing when comparing NA vs turbo, it more becomes a question on volume of air at x throttle NA vs volume of air at x throttle w/ turbo.
Im not a tuner/mechanic, but thats how i understood it.
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