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hes talking about variable exhaust valve that sits in the exhaust pipe
(most are butterfly valves like your throttle body butterfly valve)
it closes the butterfly vavle at low cruizing loads to improve low end response (as you now have more back pressure so gasses escape easier compared to a bigger exhaust)
then it opens as load increases (like w.o.t)
all this is controlled either by your ecu or by engine vacuum
as for an increase in fuel economy well i wouldnt know but if you ask me you could invest your money in something abit more worthwhile, like a "economy grind camshaft" or a some intake mods and stripping emissions devices to increase fuel economy.
do a google search and some reading on it and report back as you have me interested on if this might do something or not (if it does then i'm moving upto a bigger exhaust hahahaha
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