NA Randall cowl intake duct
Product Review (submitted on September 2, 2018):
I'm on the fence between 3 & 4 stars. Installing this was a beast. I'd rather remove rusted exhaust fasteners than install another one on a Miata. It was extremely difficult to get the snorkel through the cowl, with lots of swearing, filing, and Dremel work. I still chipped the corner of the bell mouth. It also didn't align perfectly with the air box inlet--I think the template in the instructions could be improved.
Now, the good. While I didn't notice any appreciable gains, I no longer experience loss on the autocross circuit. I co-drive my Miata, which means lots of duty and lots of under-hood heat soak. Intake air temps on average dropped 20F-25F. Throttle response stays snappy and it pulls strongly even on that 6th or 7th autocross run. The engine feels very awake at all times. I feel much better about running my advanced spark timing now, that's for sure.
There's a slight bit of inductive noise when the conditions are right (low speed, 3k rpm and you whack the throttle) but if you have any sort of aftermarket exhaust, the exhaust will be louder 99% of the time.
Do I recommend it? If you're planning to go for big power adders in the near future, it won't be as useful as the tube matches the stock inlet size and will probably be a source of restriction. But if your Miata sees hard duty cycles at nearly stock power configurations, yes. This (or something like it) is how they should have plumbed the intake from the factory.
Note from FM: We suspect that Derek installed the intake with the bellmouth on the wrong side of the cowl. There's no need to feed the snorkel through the cowl, it simply presses up against it.