Would a 2" exhaust system be ok for a stock or near stock 1.8? At what point would this size begin to get restrictive or would it already be? This would be a pretty straight through design with no cat.
Most spec miata exhausts are 2.25, but then again they are restricted to that as the maximum, and some 2.5 exhausts make more power/tq than 2.25 NA. I really can't say when it would hurt performance, i've never even seen a 2 inch exhaust that i know of, but you would almost definetly make more power with the 2.25, but again i'm not sure how negliable it will be sorry, my guess would be 2-3hp top end.
The PCR exhaust that I used to run was 2". It had a straight through muffler and no resonator and it was loud. I didn't notice any performances gain or loss compared to stock. I replaced it with a Racing Beat and the only thing I noticed was that the RB was much quieter. Must be I'm not very sensitive to +/- a few hp.
There was an issue of SCC a few years back where a guy with a VR6 was running a 2" exhaust after losing dyno-proven horsepower with a 2.5" and 3" pipe. I think the quote was, "There's a difference between what works and what really works."
You are not outflowing 2" on a stock engine and any wider and you are losing the savaging effect. That being said my hyprocacy knows now bounds because I have a 2.5" exhaust running into a 2.33" HKS muffler.
If 2 exhausts are of the exact same design, the one with the larger diameter will make more power. It has been dyno proven to death that the best exhaust is no exhaust after the tuned length (i.e., the manifold). There are several dyno charts at Flyin Miata and on dot.net that show that there was a slight power gain with 2.5" over 2.25" - slight, as in a couple horsepower.
However, as long as the exhaust is as straight as possible, and mandrel bent with a straight-through muffler, the diameter won't make a huge difference. Going with a smaller diameter is lighter, usually cheaper, and provides more ground clearance.
Ok, thats what i thought, and anything will be an upgrade from stock. I was aware that some exhausts from japan use 2" piping but i am unsure what kind of power levels they were designed for. So ill be missing out on a pontential horsepower or two and im sure that will be pretty high up in the revs, which isnt too important to me. Now to redirect my question a bit, how do you think 2" would fare at something like 130hp. I guess i should cross that bridge when i come to it considering it is a long way off.