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Originally Posted by Bmxer300zx
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No I did read but you also said engine size doesnt have much to do with fuel economy its simple more cyclinders/bigger displacement will use more fuel even at idle with no mass being moved through drivetrain. Now comparing a 6.0 vortec to a old 2jz is dumb you're talking 2 different engines from different times. N2m most people who go to a v8/lsx swap have a tendancy to strip it of ANY economy related componants even such the ecu is retuned so how can it give more mpg after all that than the vehicle rated mpg it came from?
Now if you slap it in miata vs a vette I can understand a bit of a mpg gain but not to where people should be thinking " well if I ls1 my miata I'll get better mpg's than a 2jz in my 240sx cause it's lighter..
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You are still thinking from the perspective of engines. Physics doesnt care what engine you are using; it takes X amount of horsepower to push a vehicle with a weight of Y. Simple as that, end of story. Keep swapping engines all day and the mileage will be similar as long as the overall efficiency of the engines is similar. A 2j has a very similar efficiency to an Lsx; actually the Lsx should be a little bit better.
Using your example as an engine is idling. Comparing an SR20 with an LSx engine. I agree that the LSx has heavier rotating components, and should use more fuel at idle. But if you actually calculate the horsepower requirement between the two engines rotating, you will find the difference is negligible. that is, the lsx may require more horsepower to turn, but it will also probably idle at a lower RPM which will reduce the horsepower requirement. But that is also negible. What really counts is the fact that both engines require a very minimal amount of horsepower to turn, the difference is probably less than 1horsepower overall between the two. Which means differences in idling fuel requirements will also be negible.
But that is all overlooking the fact that the LSx has a
higher compression ratio and therefore extracts
MORE horsepower per ounce of fuel burned- therefore, the lSx most likely uses less fuel overall than the SR20 does at idle.