I wanted to simplify the intake path on the Miata as soon as I converted it to Super Touring 6. After fruitless research yielded conflicting results and very little non-anecdotal data, I decided to hack my own system together. The result is the shortest intake possible while retaining the factory AFM. And it works remarkably well.
Parts Used:
- 90 degree 2.5-inch silicone elbow
- universal crankcase breather
- short cone filter (a mushroom-style filter will work as well)
The Factory Intake and Spec Miata Trim
While running this car in Spec Miata, I was required to keep the factory intake, which is located on the hot side of the engine near the headers. The factory air box is removed in spec trim, and a cone filter is fitted after the AFM (Air Fuel Meter). An extension is allowed to run the filter closer to the firewall, but the result is still a hot-side intake.
Building the Cold Side Intake for the NA Miata Racecar
Now that the car is running in ST6, I can get creative. I had already deleted the IACV and the AIC under the throttle body. Remember, this is a racecar, and I love simplifying as much as possible. And without headlights, I was determined to place the intake on the ‘cooler’ passenger side.
To do this, I ordered a 90-degree 2.5-inch silicone elbow, a 3-inch compact cone-style filter, and a universal crankcase breather. The 2.5-inch elbow is run from the throttle body to the AFM. The AFM opening is slightly larger than 2.5 inches, but I found the hose could be worked onto the end regardless. Then, the shorter cone filter was fitted after the AFM.
Extending the AFM Plug and Loose Ends
At this point, the AFM plug no longer reaches the AFM, and the car will not start. The plug must be cut and extended from the driver’s side to the passenger. I initially used crimp connections to extend the wires for testing. The signal wire was unhappy in this configuration, and the car started but ran erratically. The solution was to solder the extension wires properly. This fixed the issue, and the engine revved out great. In hindsight, the racecar has no dash, and I believe there was enough slack in the wiring loom to pull the AFM plug from the driver’s firewall and run it through the passenger’s side without extending the wires.
Regardless, the only thing left to do is to place a breather filter on the hot side valve cover vent.
Street Car Considerations
When adapting this to a street car, there are a few considerations. First would be the headlights; if they are there, there really isn’t a way to replicate this intake. With a factory radiator through, you could probably turn the elbow downward from the throttle body and place the filter either in front of the radiator or use ducting to draw in cool air. Also, the AIC under the throttle body connects to the factory intake tube, as does the valve cover vent. To keep these two, an appropriate nipple with tee fitting would need to be attached to the elbow.
Results
My concern was whether the AFM and throttle body proximity would affect the AFM. It does not. I won’t necessarily say the throttle response is x% better as this car lives at 6000 rpm. But I will say that I dyno’d the car for compliance at the National Corvette Museum last year to the tune of 119hp. I dyno’d again this year to similar conditions, on the same dyno to 120hp. That doesn’t necessarily mean there is a horsepower gain from this intake, but it does mean there probably isn’t a loss. Unfortunately, this dyno had issues reading torque from the car in both years. As a result, I don’t have a good before and after torque curve. However, I don’t think that changed much based on lap times and the seat of the pants.
In conclusion, this intake works. It is the most straightforward cold side intake for the 1.6l NA Miata. The NB guys have been doing this intake forever with their tiny MAF sensors. But as long as the headlights are gone, there is plenty of room to mount the AFM in this configuration on the NA.
