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DIY intake recipe for NA/NB MX5 Miata
CAI (Cold Air Induction)

copyright Supermiata LLC 2018

The OEM intake, while having some restriction, is also tuned to help fill that dip at 3800 an otherwise bone stock NB will have when you ditch the airbox and plastic pipes. The tradeoff for that dip with low restriction intake pipe and cone filter is more power everywhere else.

BP pipe should be about 21" long x 2.5", aluminum, largest radius you can make fit with your particular car setup. Keep the beginning of that first turn as far from the TB as possible. Generally that's difficult because the pipe hits the hood, radiator, everything. Simple but high quality cone filter on the end. No pep boys or ebay filters. I suggest Green, K&N, AEM, etc. Filter doesn't need to be huge. We measured no power loss from a huge 6x8" AEM to and old 3.5x5" round K&N or no filter. B6 pipe the same but about 19". Length measured along centerline of tube from TB plate to filter element or area where cross section increases, not measured to filter flange. Getting the length wrong is not a a huge problem, just costs 1-3 whp. You'll lose more than that by using a tight radius silicone U coupler stuck on the TB. We prefer a larger radius aluminum U bend with a very small straight section off the TB before the bend begins. 

Not always possible with headlights and other stuff in the way but we like to route the intake back towards the RF shock tower. On race cars with headlight removed, you can sometimes just fit the required length in the abandoned headlight area. That reduces the bend on the pipe and maybe worth 2whp or so over a U bend. If you are wondering if a straight pipe sticking out of the hood makes the most power, ah yep. 4whp over anything with a U bend. Did we try huge 3 and 4" pipes? Ah yep, no gain.

Engine moves around, particularly with rubber mounts. Pay attention to how your fancy new intake pipe wears holes in the rad top hose, pulls loose your IAT, bangs against the hood prop and generally tells you"do over".  We won't provide specific sources for the materials here because google.

The single easiest gain for the dollar (or euro or peso) on the NA/NB is definitely an intake that eliminates the airbox. If you can, replace the flapper AFM (Air Flow Meter) on your NA6 or MAF (Mass Air Flow sensor) on your BP engined Miata. This will require a new programmable ECU and may render your car illegal when it comes to emissions testing. Eliminating the AFM/MAF is worth several HP on its own, due to the reduction in airflow restriction. In some cases, you can extend the wiring harness and relocate AFM or MAF to the coldside of the engine to facilitate a CAI if replacing the ECU is not an option for you.
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