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Keep in mind though, that hp gains aren't always good, most of the mods that increase horsepower do only that, and decrease efficiency for power.
There a thread with significant info about exhaust tuning. Bigger free flowing exhaust, or none at all can actually HURT your mileage. your exhaust basically gets turned into one big muffler, not in that it muffles sound, but how one causes exhaust to bounce around into other exhaust sounds, canceling the sound. too big of an exhaust actually can hurt performance AND efficiency, but in order to get the full just of what I'm trying to say, you'll have to find the thread.
Also, you only use about 20 HP going 60 down the highway. so 70 wouldn't be too much more. I don't think power and efficiency go together, and that torque is more important that Hp. generally, smaller exhaust=more low end torque, but less power, and larger exhaust=more power, but less low end torque.
your gonna wanna look HEAVILY into aero if your gonna wanna have a van do 70 and get good mpg. look up basjoos's aerocivic, you can find a lot about aero from there.
I have more, but i gotta run.
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