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for what it's worth,my experiences with headers and less restrictive exhaust have been similar to Johnunit's.most exhaust parts are sold on their ability to increase top end power,and many can get carried away with too large pipe diameter.small tube headers also increase the bottom end of the rpm range,and I've done exhausts on everything from motorcycles to trucks,and seen similar results.for example,my 90 Cherokee 4.0 would lug down any time I tried to drive it 2500 rpm or lower.After the addition of a header,and a free flowing exhaust,the shift up light would come on at 2500,and the rig will go down the road just fine at 2000.Lower rpm power means ability to shift sooner,lower rpms and increase fuel economy.Plus,a free flowing intake/exhaust means less parasitic load on the engine to bring in air and expel exhaust.I have seen economy improvements on every vehicle I have done this type of thing to.As long as you don't get carried away with pipe diameters or intake runners so large that you lose volumetric efficiency,they do indeed help.
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