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Yes they do by impoving the exhaust system. They tend to relieve back pressure on the exhaust stroke making the engine more efficient. You may have heard the term "tuned headers" which are designed for specific engines.
I am not sure about that, but I do know that excessive back pressure will degrade the performance of an engine. It can be caused by a plugged muffler, cat system, or a handfull of snow (my youthful indescretions are catching up). What I do know is that an engine must be able to "breathe", and the exhaust stroke is just as important as the intake. The stuff has to get out to make room for the next stroke.
do those k&n air filters that replace the stock (in air box) help enough to be worth the $40-$50. i have already taken out the air muffler and replaced it with a stainless steal pipe.
Let's cut to the chase. What are you trying to do? I'm guessing this is for your 4 cyl Ranger? I do not know about the K&N filters, but I think there are several posts on this forum about them. Try a search to see if there is something you can use.
Too much of anything isn't good. Too much back pressure, as hreed was saying, is bad. Not enough back pressure is bad too. It kills your low end power. Once you get over 3500 RPM's though, watch out. I don't know if a K&N is worth $50 to you, but good quality immitators sure are for me. Mine helps a bit on performance but mainly just makes it sound cool. As far as headers, depending on your year and engine, you may have factory headers, like mine.