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Old 02-09-2012, 03:21 AM   #36 (permalink)
mcrafarm
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Red face ECMs Intake air, and so on

Maybe I'm missing something here, but I don't see your logic leading to better fuel economy, all else held equal. I think you're taking the lesson of drag due to poorly designed engine area air flow and applying it to the engine air intake. Once cooling air is moving through the radiator, that's enough, it doesn't need more which ends up causing drag hitting a wall in the engine compartment. For the engine intake air, it's a different matter; you want no restrictions there, to avoid running fuel-rich on hills and under moderate acceleration.

The ECM adjusts mixture continuously, thousands of time per second in response to the O2 sensor; that's your "short term fuel trim". If for some reason the ECM sees that you're running rich or lean habitually, it will bias it's commands to the injector pulsewidth so as to achieve the desired 14.7:1 air fuel mix; that's your "long term fuel trim". The ECM stores familiar conditions and matches what it sees to what it remembers, and commands the corresponding mixture. Not thousands of times per second, but it can still respond fairly quickly by stepping to different remembered conditions. It can even learn a new condition in the time you're concerned with.

On the flat you can get by with very little throttle and maintain 45-65 mph. But consider a hill. You're going along flat and gradually slow down given the same throttle position. So to keep some speed, you add more throttle. The restrictions in your air intake can't deliver enough air to keep an economical mixture, so to deliver what you're asking for with your right foot, it runs fuel rich. You go a bit faster but pay a fuel penalty. With less restriction in the air intake, you achieve that speed with a better mixture, and better fuel economy.

So by restricting the air intake on purpose it will run fuel-rich more often when you throttle too much for the (less) air available. At best you end up doing the same thing you could achieve by being lighter on the throttle, and at worst, you're running fuel rich more often. Another change is that you no longer can accelerate as well, which for me would be a safety issue. I can stay off the gas in routine freeway driving, but when at 62-65 mph I slowly pass the vehicles cueing up in the right lane for an exit, I'd rather go to 68-70 mph for a few seconds to be able to get out of the way of those yahoos busting my tailgate than stall in the slow lane and have to accelerate again to get to 62.

My notion is that fuel economy is about driving fewer miles with less weight, keeping inertia (including with aeromods and coastdowns before traffic lights), being light on the throttle from a stop, and running a good air-fuel mix (by letting it breathe).

The irony here is that I was looking at ways to decrease the air drag to the intake, so that the mixture is less often running fuel-rich while towing, or on hills, or worse, both...
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