If you didnt have the front O2 sensor in the down pipe would it effect anything other than gas milage?

Peak power?

I need to know this asap!!

 

Drive it without one, see how it runs. I've never tried it before, but I don't recommend it since your car relies on that sensor to adjust air/fuel mixture to your engine. I wouldn't risk anything stupid like that on a huge investment like my evo

 

What if you have a utec or AEm ,anagement system and have it tuned would you still need that front sensor?

 

Yeah, the front O2 sensor is vital to everything, unless the AEM or Utec replaces the stock O2 sensor.