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Discussion Starter #1
I have read all of the threads on these questions and unless I have missed something, I did not find the answers... I am wondering if installing a quality 4-2-1 header will in any way hurt fuel economy on the QG18DE? Other than the loss in fuel economy due to you driving it harder with the new found "power", with the cat's being removed will the O2 sensors throw the fuel trims out of wack and cause poor fuel economy? And will it set a CEL due to the cat's being removed? I am rather focused on this because my wife hates the CEL, and she commutes roughly 70 miles a day with it, so I don't want it to have a decrease, but alittle more power pulling onto the interstate would sure be nice! Any experienced input would be greatly appreciated.
 

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The car will actually get better gas mileage, from what I found at least. I ran a header on my 1.8 sentra for quite some time. I noticed that if I didn't drive like a lunatic I got great gas mileage....better than when it was stock.

But exactly as you said, once you punch it your gas mileage is considerably worse with a header on...but that is the trade off for more performance.

As far as getting a check engine light, you can use the "ghetto sim" trick that is discussed here in detail. It basically involves installing a spark plug anti fouler on the o2 sensor to fool the system. I used the ghetto sim trick and had no check engine light on after that....and that includes something like 120,000 miles with no problems.

Having said ALL that...if you are in a state where they smog test your car by doing an actual sniffer on your tailpipe....you will fail horribly with a header installed. However, if they just hook up to your OBD II port to check your emissions, if you have the ghetto sim correctly installed you will pass with flying colors.
 

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Discussion Starter #3
Good deal, that's pretty much what I needed to know! Now I have one last question... on this topic anyway... my wifes is an 02 GXE, it has 4 - O2 sensors, I take it I will have to modify the bungs because it only has 2 bungs and a egr output, so I will have to add 2 more bungs... the big question is, the 2 up-stream O2's, do I need to have them reading 2 cylinders per O2 like the factory manifold, or can I just feed 1 - O2 on a single pipe and then have the down-stream O2's, 1 on each of the 2 down-pipes? If this makes any sense to you?
 
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