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a bigger exhaust gives more horsepower, but less efficiency at part throttle and lower RPM.
Sucking on the exhaust, if you can maintain it, will build more top end horsepower, but have less efficiency.
partly Plugging the exhaust can very easily give better low end efficiency and mileage. This is basically what variable cam timing does - opens the exhaust sooner as RPM increases for more power on the top end, but good economy when the exhaust valves are staying closed longer.
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