Regular readers will recall last month's testing of two Motordyne bolt-ons that successfully proved Helmholtz resonance can be used to increase exhaust gas scavenging and help your ride put out a few extra ponies compared to other aftermarket offerings. What we didn't get into was how problems on the intake side of our test-car equation nearly negated our power gains altogether, giving rise to the possibility that fitting your car with the wrong intake might bring power to below stock levels. Before we were finished with one tuning uncertainty, we had unwittingly unearthed another:
Can Certain Performance Air Intakes Decrease Power?
We had actually come across this problem before, when installing one reputable manufacturer's short-ram/cone filter intakes on a G37 for Power Pages back in June, '08, resulted in a loss of nearly 15 whp over stock. Since the G's VQ37DE was an all-new engine, we attributed the phenomenon to a tuning issue, re-installed the factory airbox, and left it at that. But when the problem reared its head again while testing Ken Kojima's '04 350Z last month-powered by a single-throttle VQ35DE engine-our curiosity was piqued.
The prevailing hypothesis is that cone-filter-equipped aftermarket intakes have a tendency spin incoming airflow into a vortex. If an engine's mass airflow sensor (MAFS) is positioned too close to a cone filter and doesn't feature a screen or grid before the sensor element to diffuse airflow (the 350Z one doesn't), the incoming intake charge can either strike it at a faster-than-normal velocity, tricking the MAFS into overestimating airflow, or flow around it, causing the MAFS to underestimate airflow. In the case of the MAFS reading too much airflow, too much fuel would likely be injected as a result, making for rich air/fuel ratios (AFRs) and decreased power. If the MAFS reads too little fuel, lean AFRs could occur, causing knock-sensor-tripping pre-ignition, and again, a loss of power.
"Another possibility," explained Tony Collette, Motordyne lead-man and innovator of last month's successful ART products, "is that certain short-ram intakes experience some high-speed air separation if they don't feature a built-in velocity stack." Like our hypothetical vortex spinning airflow into a fast, concentrated stream, the separation Tony's referring to occurs when air enters an intake with high speed and angle, and without the aid of a velocity stack to provide a smooth transition into the intake tubing, can't adhere to the walls of the tubing and causes air to condense into a fast-moving stream which could also prompt a MAFS to overestimate airflow. Since our AFRs became increasingly rich as engine speed increased, and the power loss was mostly top-end, "I'd bet that's what's happening," hypothesized Tony.