03 Honda Civic SI
codes PO300-304
Takes approx 2 miles to reach 50 mph. Backfiring so bad sounds like it is popping popcorn.

 

Wow...A number of things could be the issue here. First, what OBD2 codes are populating?

Are you burning oil?
Have you checked the compression in all 4 cylinders to make sure all cylinders are within roughly 5 PSI of each other?

Lets start there. Codes, are you burning oil, and how is your compression.