I came across a used Borla SM exhaust for a good price. The exhaust is 2.25" pipe diameter and I plan on running RB NA header which has a slightly bigger outlet 2 3/8 pipe diameter which according to RB had a wall thickness of .0625 which then puts ID of the pipe at 2.25. The Borla SM exhaust has a small section between Header and muffler that is 2" in size. I think they did this for SM rules or stock manifold flange fitment. Since my used exhaust has a missing flange, I was thinking of just replacing the front pipe with 2.25". For a NA motor in the 130-159whp range, is this exhaust appropriate with proper 2.25" match or still a bottleneck and I should instead look for the RB 2 3/8 race midpipe & muffler? Perhaps I am overthinking this as the difference is negligible and only will be known on a dyno. Any thoughts?

 

Bigger fan of running 2.5 inch and more muffler for quiet power over the 2.25. I wouldnt want to run the SM setup on anything other than an SM.

 

So a better match would be Racing Beat 2 3/8 mated to their header same size ? What's a good header back system in 2.5"

 

we're talking like .000002hp difference lol

 

So the minor difference is worth keeping the exhaust and changing the front section to the muffler to be 2.25" ? Otherwise will post to sell in SM facebook group and look for Racing Beat

 

Street car or track car? An SM exhaust of any kind is really obnoxious on the street. If an SM and you really are after that last 0.000002HP, then Springfield Dyno or Mandrel Bending Systems are the way to go.

 

track car and based on some forum posts, the Borla X1 race (SM) was on par or slightly better then Springfield Dyno.

 

Everyone needs to remember, that Spec Miatas...suck. A lot. They run terrible suspension that is cheap and only works on smooth tracks. They have EXTREMELY loud exhaust because lighter is better, and every .5hp is a HUGE advantage in the incredibly tight front pack, or mid pack, or back field.

So stop comparing to SM, please. It only shows ignorance.

My enduro team ran a quiet (street friendly) 2" OD Borla exhaust, made 148hp. If you're a little bigger diameter, and a little less restricted muffler, yes, I'm sure it'll be a TINY bit more powerful. But we had to spend 2 hours at a time in the car, so we loved it and it made great power. Please consider your circumstances before making exhuast decisions, don't just buy the one that makes the most noise or the most power.

 

I am in complete agreement with you that SM dictates tight rules and within such confines, the SM crowd looks to lightest less restriction to stock exhausts. I happen to come across a local deal on the almost new Borla exhaust with an optional muffler. Given the $$ I spent on this exhaust and the fact that I need to change a small front section of the pipe that mates up to the flange of the header, I figured I should be ok on both functional and sound wise with the optional muffler, unless this system is too restrictive and then I'll just list it. Thanks all!