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Old 08-11-2014, 07:37 PM   #14 (permalink)
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While you can improve exahust efficiency, most off-the-shelf systems are tuned for mid-range to high-rpm use. It's not the lack of backpressure that's the issue... it's flow stagnation that is. A bigger exhaust is horrible at cleaning out the cylinders at low rpm. It's like trying to suck soda through a giant straw (say, for drinking smoothies?) slowly. Not easy.

The soda straw stock system usually gives you best flow at lower rpm... which is what you want for best efficiency at a cruise.

I could see an exhaust designed for efficiency, if you have the header designed for proper scavenging at low rpm, as Mazda does with their SkyActiv and its long-tube header systems.

But, as with Mazda, you would have to retune the engine to take advantage of this by leaning out (by leaning out the charge further). On an otherwise stock system, having the charge lean out due to better exhaust efficiency (and remember, you're still injecting the same amount of fuel... you're just getting more air with it) will cause feedback from the O2 sensors to encourage the stock ECU to inject more gasoline to fix what it sees as an unhealthy lean condition.

Like the guys say... focus on other stuff first if efficiency is the goal. If you want to improve fuel efficiency engine side, breathing mods can only improve economy by accident (read any debate on K&N, ever). The only sure way to improve economy is by retuning your motor with a chip or a reflash. And even then, the cost will most often outweigh any economy benefits.
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