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Old 05-31-2013, 09:17 PM   #26
OrangeVert
"Catharine"
 
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Drives: '11 2SS/RS LS-3 IOM/IO vert
Join Date: Mar 2011
Location: Southern NH
Posts: 2,132
Quote:
Originally Posted by knowitman View Post
Adding a turbo doesn't always increase fuel economy. The new GM Turbo V6 has near identical power and torque as the LS3 and gets the same gas mileage. The friction in an engine is a big determinant on fuel economy. A smaller engine might have to work harder, aka higher RPMs or throttle, than a larger engine to maintain a certain speed. In that case the increase in fuel economy might not be as noticeable because both engines could very well be using the same mass of air. The larger engine would require less throttle, and while it will "suck" in a larger volume of air for every stroke that volume of air will be at a lower pressure due to a greater pressure drop at the throttle and could be very similar in mass to that of a smaller motor using more throttle or spinning more RPMs. Also, when designing a car around turbos, the compression ratio must usually be lowered for reliability and longevity. This decreases the efficiency of the engine while it is not under load.

When the car goes into AFM mode, it may only be putting fuel into 4 cylinders, but it still has the friction of 8. Therefore, the other 4 have to work a bit harder, aka more throttle, which equals more air entering the 4 working cylinders. The only gain is you don't have as many combustion losses out of 4 cylinders vs 8 cylinders. That is why the gain in MPG seems minimal.

BMW went the turbo route mostly because it is a European based company. Most countries in Europe tax a car based on engine displacement. It's not a very fair system because nothing is an apples to apples comparison.
Very interesting, makes perfect sense - thanks for explaining.

Edit: I find it interesting that my 4-cyl 1993 Toyota pickup was rated 28MPG highway and the best mileage pickup said company makes in 2013 is rated 25MPG highway. 20 years of improvements in technology, and they have DEcreased 3MPG! Perhaps they measure it differently now. But still ... come on!

This is why I rebuilt the head at 316,000 miles rather than buy a new truck.
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