Is there a way to keep this at about the same loudness as the borla only, or do I need to go with shorty header and high flow cat? How much more power would a Race header make over a shorty/high flow cat? Thanks!
Couple things. First the only hi-flo cat available that bolts to the short headers is from Random Technologies and it cost as much if not more than the header, so cost wise, it's a bit much... Bosal is a few weeks(maybe longer) away from having their bolt on cat available, not sure how much it's going to cost.
Take a look at the FocusSport headers section, they publish their street header will give you 2-3 HP improvement, and their race header will add 6-7 HP on a "lightly modified" Focus.
Similar numbers should be expected on other street/short and race headers available.
Where you will pick up more HP is after you've done your exhaust mods(header, cat/no cat, flexpipe, catback), and you do adjustable cam gears. Tune them in and find lost HP.
Sound, you will definetly be louder with a race header. Short header and hi-flo cat, probably not as loud, but still louder than stock header, cat, and Borla...
Ask me again in a few weeks when I've put on my Ford Racing short header and Bosal cat, and flexpipe with the Magnaflow. Currently running just the Magnaflow.
You can use the race header with the borla. Yes it will be loud, but there is a straight section in the exhaust that you can get a Magnaflow muffler or some kind of resignator installed to make it quiet without loosing the performance.
It's Loud. Like Cop bait loud. But it's a deep loud. Now if I can stop that damn chattering sound from my fluke (It seems to be local only to my car) I'll be happier. But it's fast. Not scary fast, but fast