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I have a 96 civic dx auto. I get 29 mpg mostly highway. I think my stock manifold has a crack and the cat maybe a little clogged. I ordered a header and im gonna instal wothout the cat. Yes,I know its gonna throw a code but I dont care about that. So....my question is do you think ill pick up some mpg with this header vs cracked manifold,clogged cat?

 
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You might. What other mods have you done to the car besides this new header you're thinking about getting?

Also, using a defouler on the forward O2 sensor will get rid of the CEL.

If you're in Cali, there are HUGE fines if a cop finds out you're not running a Cat. My suggestion to you is to turn the car off immediately after you're pulled over should you get pulled. The cops in my town are really cool about this kind of thing and we have one guy from Cali that told me this. If you get pulled, turn off the car quick so that cop can't smell the exhaust. If he/she smells the exhaust, you're off to EPA fine hell.

I should add that when I went with my intake, header exhaust, I get better gas mileage IF I keep my foot out of the pedal.

 
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i dont use this car for a racecar. i have a 01 ss camaro for that. this is a beater car that gets me good mpg. im only using this header cause i think the manifold is cracked and cat is clogged. I need a cheap replacement to get at least the same mpg. so that will be the only mod. like i said,im useing it for a cheap replacement.

 
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IMO run a catalytic converter, creates a decent amount backpressure to give you eh *cough* torque

 
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A turbo would give good back presser to...