I always read in advertisements about how a cat-back and intake will increase mileage (if you can keep your foot out of the new found power) but ive never taken the ime to actually test it to see. Have any of you done a before and after test and does it really affect your mileage.
Don't expect any mileage increase. The reason is that yes the stock exhaust might be restrictive at WOT 6000 rpm with all that air pumping through...but at 70mph and 2000 rpm the exhaust can easily handle the air flow needs of the engine.
Now headers will increase your MPG 1-2 because they use cylinder scavenging (each exhaust pulse helps pull the next pulse out of the cylinder) to increase the volumetric efficiency of the engine.
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