also, what do you mean by if my car is a F1 or N/A? What do those mean?

 

f/i - forced induction (turbo, supercharger)
n/a - natural aspiration (motor only)

you dont want backpressure. backpressure is the last thing you want..

what you want is to conserve velocity, too large a pipe and you lose exhaust velocity and torque.

too small a pipe you choke up the top end because it can't flow enough exhaust.

 

so since both have downsides, what should I do? Get 3 inch piping or keep it at 2.5 with custom single exhaust? I want more power, and dont wanna lose power.

 

even though 3 inch is mainly for turbo cars, would I still gain atleast some power with a 3 inch piping with a test pipe? My Apex N1 muffler is for 3 inch only I was told. I have a stock header at the moment too, and will probably have a stock header for a while until I can afford a better header. They aint cheap!!! So will I still gain with a stock header?

 

im pretty sure you will gain up top. im guessing in the 9-14whp range with a test pipe. but i'd be worried about the midrange. you might be sacrificing some power there.

your best bet would be either a 2.5inch (63mm) or 2.75inch (70mm) system.

 

would 2.75 piping still work on a 3 inch inlet for my muffler?

 

i dont see why not. a muffler shop would just expand the piping to fit.

i had a similar setup on my old car. 2.5 inch piping, 3 inch muffler.

 

I agree about too small a pipe chokes up top end but for the rest I don't quite agree.

There's common misconception about exhaust back pressure and exhaust gas velocity. Exhaust flow and exhaust pressure wave flow are 2 very distinct properties. Pressure waves travel at about the speed of sound (it's basically sound wave really), while exhaust gas itself (the particles) travels at a much lower speed.

To speed up exhaust gas, that's the job of header tuning while piping diameters of primaries and secondaries matter. Further down in the exhaust system, small diameter piping becomes more a factor to impede exhaust gas flow than to scavenge exhaust gas out of exhaust ports from the increased speed. Once past the header, the exhaust gas will almost always flow better in a bigger catback diameter piping to not impede exhaust gas flow. Too big catback diameter comes in when the diminshing return gets overcome by negative effects(noise, weight and clearance).

Pressure wave is something totally different. For exhaust back pressure, we're talking about resonance to help scavenge exhaust gas out of the exhaust ports at low-mid RPM. This resonance is NOT determined by catback piping diameter. It's more about resonance chamber(s) and total pipe length. This is why resonators are strategically put in an exhaust system. They don't just work in any location in the system.

I do my own welding and I had had quite a few iterations of test pipes welded for my miata for track use over the years. It's proven that while everything stayed the same on my exhaust system from header back, my resonated test pipe dramatically retained torque over my straight-thru' pipe. The resonated version was basically the straight thru' version with slots cut into it and inserted into a hollowed-out hi-flow cat. Go figure!

Since resonance tuning does NOT depend on piping diameter, you can have back pressure while NOT impeding exhaust gas flow to not affect high-end hp. People mix up the 2 distinct flow characteristics (exhaust gas and waves) and mess up with exhaust systems in the wrong direction.

For example, the worst catback system is to run small diameter piping with a straight-thru' muffler and no resonator. While this kind of system is the cheapest to make, it hurts both low-mid torque and high-end hp. First small diameter piping rreduces maximum exhaust gas flow and therefore hurts high-end hp. Then, striaght-thru' muffler with no resonator means bad resonance tuning and hurts low-mid torque.

The best system is to use big diameter piping, a strategically placed resonator, and muffler(s) with strategically convoluted exhaust path inside for resonance tuning.

 

ok, so you would agree with my setup Race Miata? By the way, thanks for the info!!

 

I don't have as much experience about exhaust piping diameter for the S than for the miata, but I think 3" reaches the point of diminishing returns where cost/noise/fitment make it not worth it for NA use. For max power, I would try 2.75" first. But then with your 3" N1 muffler it's gonna be really loud. Don't cheap out on a resonator. I kind of remember reading some simple trick in the old days to use spray paint on the exhaust pipe and locate the most burnt area after some hard-driving. This area suggests the hottest area due to resonance where a resonator (or cross-pipe for a dual-exhaust for a V engine) at the exact location will work best. If you run a test-pipe, a resonated one will most likely work better.

If you want to custom-make a catback system just because you want to use an existing muffler and for cost reason, that's OK. If you want to custom-make a catback system to outperform some of the best off-the-shelf systems, prepare for some trial-and-error work.