I love it when educated people post links to articles or post equations but ignore so many other obvious variables when it comes to a specific engine, driving style, altitude where the poster lives, etc etc etc. Just because something was demonstrated once given a certain set of conditions doesn't necessarily mean it's going to have the same results once nearly ALL of the conditions are different. That said, some of you also want to jump to tiny details while ignoring the general rules.
Le sigh.
OP, generally speaking: you probably won't see much of a difference on your particular vehicles with the modifications you're listing. I've installed 2.25" exhaust systems on 1.8L SOHC 8V engines and seen dyno verified gains across the entire rev range, so I'm hard pressed to think you'd see LESS power as someone suggested. That said, this was not done for economy. Another trap people get into is they tend to drive the car harder to hear the intake and exhaust, therefore canceling out any potential gains.
While the factory airbox and exhaust is restrictive by design (erring toward a quiet cabin and driver comfort), neither those engines you have nor the way you're driving them (ecomodding) are trying to pass more CFM of air through them than the factory parts can handle. I doubt you're hitting a point of restriction yet until you're at WOT bouncing off the rev limiter. But at 1,900 RPM cruising, you won't see significant gains from those parts...that's my educated guess. They can flow a certain amount of air for a given RPM, and I don't think you're asking them to flow anywhere near their limits yet. It's at THAT point where those parts become beneficial. I wouldn't waste the money on the exhaust, that's for sure...I wouldn't want the added drone in my daily driver. But for the low cost of a DIY intake system, I'm gonna do it for ****s and giggles to pass the time on a Sunday.
Note though, that many hypermilers are on factory intakes and exhausts getting double the EPA MPG's for their cars. There are bigger fish to fry.
But hey, I'm just a nut job with a 3" exhaust on a 2.3L. No, that's not the eco car. That's the corn-fed weekend car.
