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I switched from a 4-2-1 catted equal length header to 4-2-1 unequal shorter headers and my mileage actually increased. This might be because i could run hella lot more timing at lower RPMs than before. Im talking about 6-10* of further timing advance. I'm not sure why, but i used to get much more bottom end knock with equal lengths.
Stock EL headers Catted:
Opening dimension: 1.378" ID
Primaries on the cat side are 25", 21" & 20" on driver side
1.772" ID collector
20" collector to first cat
New UEL Headers Catless
Opening dimension: 1.56" ID
Cross pipe length: 17"
Driver side collector length: 7"
Passenger collector to uppipe length: 13.5"
Header to uppipe opening: 1.75"
My cruising mileage was much improved at lower RPMs however at higher RPMs it seemed that for some reason my car liked to run richer. Even in closed loop the ratios were always a tad richer than target. about 0.2:1 AFR richer. this was nothing crazy but it did affect fuel economy. note the new headers have much higher flow rate and lack a catalyst converter.
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