DIY Miata Intake
DIY intake recipe for NA/NB MX5 Miata - CAI (Cold Air Induction)
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. A good header will help the midrange and pretty much fill in that dip from replacing the OEM intake.
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. We suggest Green, K&N, AEM, etc. Filter doesn’t need to be huge. We measured no power loss from a huge 6×8″ AEM to and old 3.5×5″ 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 from optimal, but still much better than stock intake. The larger the radius the better the flow. Large radius aluminum U bend will flow much better than a tight radius silicone coupler. Much better that two 90° couplers to create a 180. Ideallly a very small straight section off the TB before the first bend begins.
We like to route the intake back towards the RF shock tower but it isn’t always possible with headlights and other stuff in the way. 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, yep it does. 4whp over anything with a U bend. Good luck getting that past tech. Did we try huge 3 and 4″ pipes? Yup, no gain over the 2.5″. Changes in velocity are bad so any turn or change in cross sectional area are to be avoided between the throttle plate and air filter. That’s why the 3 & 4″ tubing didn’t help.
The engine moves around a lot, 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 indicates “do over”. We won’t provide specific sources for the materials here because google. Ebay a good scource for piping and couplers.
It’s generally easier to fit the intake pipe and reduce bends by routing it towards the hotside (header side). Problem is its no longer a “cold” intake. If its getting air from underhood on the hotside, the IAT (Intake Air Temp) is generally going to be 20-40° hotter than a cold side (intake manifold side) intake. Higher IAT’s reduce power dramatically, which is one of the primary reasons we build CAI’s in the first place. So really try to design yours to locate the filter on the coldside.
The single easiest gain for the dollar 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.