by MazdaManiac

The intake system on the RX-8 is arguably one of the best-designed, original manufacturer systems of its type on a contemporary vehicle.

It flows extremely well, has ample plenum volume, is relatively easy to service, reduces noise without reducing performance and filters the intake air more than adequately under the wide variety of operating conditions that the average owner might encounter in normal daily use.

The OE intake design is also perfectly matched to the mass airflow sensor, which is the single most important feature of any intake because, without accurate airflow measurement, performance and economy suffer and any tuning efforts are rendered difficult or impossible.

That said, there are benefits to been realized by installing an aftermarket intake system, but only if that system is carefully chosen for the task at hand and a comprehensive understanding of the effect that system will have on the overall vehicle functionality is considered.

This would mean a review of the things the original manufacturer system does not provide.

First, let’s look at the functions performed by an air intake system.

The primary function of the intake is to acquire air. This seems fundamental, but it’s not such a simple action as not all air is created equal.

As it is explained in the tuning section, air density is paramount. That means air that is allowed to flow freely at ambient pressure and air that is as cool as possible should be provided to the engine to create maximum power.

The OE system acquires its air source at the end of a “sock” positioned above the front bumper core support which has its open end pointed at the right side headlight. This “sock” is somewhat smaller in diameter than the tube that actually feeds the motor through the throttle and doesn’t have direct access to the flow of fresh air around the outside of the vehicle, except through cracks and crevices in the body work. On the high-power versions of the RX-8, this “sock” is supplemented by a device Mazda calls the “VFAD” – Variable Fresh Air Duct – which opens under PCM control when the engine speed is above 5250 RPM. The VFAD allows the effective flow area of the intake “sock” to nearly double when required, which eliminates any concern about the somewhat reduced diameter of the “sock” itself. This is mainly a noise abatement technique that Mazda employs to help keep the intake quiet until you really mean it when you step on the gas.

However, the VFAD doesn’t change the source of that incoming air, which is still the somewhat stuffy confine of the inside of the front bumper. Getting the air from an area of higher pressure, cooler flow would have a positive effect on power under all operating conditions.

After the air passes through the “sock”, it enters the air box where it is confronted by a series of directional baffles that send the air through a large, panel-type dry air filter. At roughly 6 times the surface area as the port area of the intake tube before the throttle, this filter is arguably large enough for the task at hand. Certainly, anyone that has taken a look at his filter after only a few tens of thousands of miles of normal service can attest to the daunting task that it faces and the admirable job that it does as one can typically pull a fist-full of dusty flora and even rocks and gravel out of the filter after this interval.

After passing through the filter, the incoming air is aimed by another set of directional baffles and then confronted by bell-shaped “horn” that marshals the freshly-scrubbed air into a smooth column, pointed directly at the mass airflow sensor. This “straightening” of the airflow is further aided by a wire mesh screen placed in the mouth of the horn, just a few inches before the sensor itself – a proximal relationship that is important in any air intake system that one might consider in this application.

After passing by the airflow sensor, which is suspended in an ever-so-slightly oval tube measuring 3.375 inches (roughly 86mm) in its interior diameter, it passes through a flex bellows, which isolates motor vibration from the chassis-fixed components of the air box, and then into the motor through the throttle body. Affixed to the bellows is a small plastic box, which further reduces intake noise through a resonant effect. Also attached to the flex bellows are three hoses that pass air to the crank case vent on the oil filler neck (which was later rerouted to the lower intake manifold to eliminate oil pudding that would often occur in the bellows because of this arrangement), the jet air bleed (which helps atomize the fuel being delivered by the primary fuel injectors) and the oil metering jets.

The diameter and, to some extent, shape and dimensions of the tube in which the mass airflow sensor is suspended is critical. A known diameter tube will flow air at a known rate, which is compared to the output voltage of the mass airflow sensor to produce an accurately calculated accounting of nearly every air molecule that goes into the engine. If you change the shape and/or diameter of that tube, you change the accuracy and calibration of the mass airflow sensor and, therefore, radically change the precision of the fuel and ignition delivery choices made by the PCM. Even a minuscule change of just a 1/16” can throw the calibration of the MAF off by 5% – 10% or more. At low airflow rates – like those encountered at idle and light cruise – this variation can make the engine stumble, run rough or even not run at all. Of all the variables that might affect the enthusiast’s choice of intakes, placement and mounting of the MAF is the most important.

So, we can see from all of this that the main considerations of intake design from the original manufacturer’s point of view are engine longevity through proper filtration, air-metering accuracy for most economical power production and noise abatement for passenger comfort.

It is probably safe to assume that the typical individual that contemplates the replacement of the original intake system with an aftermarket alternative is not primarily concerned with noise abatement. In fact, it’s a safe bet that some would prefer a noisier intake that only flowed air as well as the original intake to the original intake itself for that reason alone.

To that end, one could simply remove the noise abatement controls (the air box interior baffles, the “sock” and VFAD and the resonator box on the flex bellows) and call it a day. However, that would not be what I would consider a performance modification, despite the tendency of some drivers to perceive noise as performance. We’ll leave that to the Honda crowd. Instead, we should turn our attention to reducing any restriction on the flow of air to the motor and to increasing the density of the intake air mass.

The first goal is typically addressed by straightening the path that the air must travel on its way to the motor, reducing the number of changes in volume and shape of the air pathway itself and reducing the obstruction that the filter creates. This last concept needs to be looked at rather carefully as any reduction in air filter restriction may be accompanied by a commensurate reduction in filtration capacity – something that doesn’t bode well for the long-term life expectancy of the motor.

The second goal – increasing intake mass – can be accomplished in two ways: by reducing the temperature of the incoming air and by pushing it into the intake by force.

Reduction of the intake temperature is most easily realized by finding a place where the air temperature is lower than it is in the factory “sock” position. The most likely candidate is the “mouth” of the bumper where air enters the front of the vehicle and travels to the radiator and air conditioning condenser. The air found in this location is usually at the ambient temperature and plentiful. It is also available at an elevated velocity when the vehicle is moving forward at any appreciable speed, something that helps us realize the other possible method of increasing the intake air mass – pressure.

The drawbacks to this position are exposure to the elements and potential blocking of airflow to the radiator, which might affect engine cooling system efficiency.

Another possibility would be to create an opening in the skin of the bumper cover itself in the area just in front of where the VFAD is located. This would eliminate any airflow obstruction and provide maximum pressure, but it also requires cutting the bodywork of the car, which is an opportunity some RX-8 owners might choose to forgo.

By the way, the idea of “pressurizing” the intake should not be construed, as having any significant performance benefit – the likes of which can only be realized by traditional forced induction methods like turbocharging and supercharging – beyond what it does by way of reducing the effort the engine must exert to induce air into the combustion chamber. Nature attempts to push the air into the motor at around 14.7 PSI, so we are just looking to not impede this natural supercharging effect that surrounds us and supplement it by whatever means we can.

The Mass Airflow Sensor (MAF) is a precision instrument. It provides a voltage output based on a 5-volt source that responds directly on a roughly exponential curve to the mass of the air that passes by it. For the PCM to compute actual airflow value, it must take into account a known, fixed value for volume and that value is correlated directly to the inside diameter of the tube in which it is enclosed. Change that diameter and the calculation will be skewed accordingly.

The original equipment MAF tube has an inside diameter of 3.375 inches (86mm). This gives the MAF a calibrated limit of roughly 370 grams a second of mass airflow, which is about 49 pounds of air a minute, at that 5-volt maximum.

Based on the PCM’s MAF scaling and calibration, an output of 1.25 volts would correspond to 6 g/sec of air going by it at idle. Increasing the MAF tube inside diameter by a mere .067 inches would change the PCM calculation to 5.88 g/sec at the same MAF output. This tiny difference (about 2%) is enough to change the target fueling air/fuel ratio value by up to a half point (for instance, from 14.7:1 to almost 15:1)! This is not insignificant. So, the inside diameter of the MAF tube is a measurement that must be absolutely known for accurate fuel delivery and load calculation.

This is also the area where some of the aftermarket intake manufacturers have worked their “magic”. By slightly increasing the inside diameter of their MAF mounting tube, they have effectively lowered the airflow calculation at all points on the internal MAF calibration chart, which has the effect of making the target air/fuel values leaner and, in open-loop operation where the effect of stoichiometric-seeking fuel trims is negated, producing more power in most instances. This slight increase may also increase the actual volume of air the intake can pass to the throttle body. It also has the effect of convincing the PCM that the load on the motor is ever so slightly lower, which leads to slightly more advanced ignition timing.

Just as important to the accuracy of the MAF as inside diameter is consistency and total length of the MAF tube. The MAF needs the airflow that passes it to be as smooth as possible. To that end, it is important that the tube in which it is suspended to be of a constant, uniform inside diameter and direction for a length that is roughly equivalent to twice its inside diameter both before and after the MAF itself. Anything shorter runs the risk of producing eddies and swirls in the airflow that can cause unpredictable results.

The original equipment tube contains a mesh screen several inches before the MAF. It is helpful, if not imperative that this design feature be emulated in any aftermarket intake.