Oversize exhaust valves will flow better and make more power. Our valves are heavily backcut to give more airflow at a given lift and 1mm oversize for more flow. Plus, we have Inconel!
If you`re going pushing for a specific output of more than 130 hp/liter (basically, anything more than a standard FM II), it`s recommended that you change to an Inconel valve. The exotic 751 alloy can handle much higher heat than stainless steel. It`s a lot more difficult to machine and heat treat so they cost more, but it`s well worth it. Inconel valves are only available in the 1994-05 exhaust size.
Price is for one valve. You will need 8 exhaust valves for a Miata engine. 1.8 valves are made with slightly longer stems than stock to allow for more precise installation with solid lifters.