Oversize intake 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!
We've seen valve failures with stainless steel valves on track turbo cars. So now we offer Inconel. They've been tested on some of the most powerful turbo Miatas to see hard track time and have proven their ability to stand the abuse. 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.
Price is for one valve. You will need 8 intake 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.