I'm currently working on my 67 289 mustang, i was thinking on putting open headers in my car but wanted to know the negatives of that but also want to put an exhaust system in there.

 

open headers are not a good choice unless you are wot all the time. Like for track racing.

The negatives of open headers are that you loose a lot of low rpm torque , they are obnoxiously loud .

A well tuned exhaust system make a far more enjoyable ride .

 

You can get Y's for the headers with electric cutouts just for the fun sound when you want it and do a regular exhaust as well. You'd have either one at the flip of a switch.