Why Won't the Cold Air Intake on a 2007 Scion TC Work?

Cold air intakes aren't unique to Toyota's youth-oriented and mod-friendly Scion; in fact, they're not even unique to this or the last century. Cool air is oxygen-dense air, and oxygen-dense air makes for more horsepower and greater engine efficiency. Toyota's no stranger to making horsepower through efficiency, which is part of the reason a CAI is, in and of itself, kind of a pointless upgrade. But don't give up yet -- there's more going on here than just cooler airflow.

The Basic Problem

The primary problem with counting on an aftermarket CAI to make power is that Toyota beat you to the punch and installed one at the factory. Like many new cars, the tC uses an airbox that connects to the inner fender well with a short tube, drawing outside air into the engine from the area between the inner and outer fender. The inner fender well is almost completely sealed from the engine bay and sucks air in from a duct in the wheel well. This is an ideal location for an air intake, since air pressure tends to build up on top of the wheel at speed. This high pressure helps to shove air into the engine for more horsepower, and reduces front-end lift at high speed by dropping pressure in the wheel well.

Intake Horsepower

While it may come as no surprise that Toyota's billion-dollar engineering division has already done a pretty thorough job of optimizing intake location, there's still a bit of room for improvement over stock. Import Tuner magazine found that installing a Fujita F5 CAI on their 2.4-liter 2007 tC netted an additional 3 horsepower and 3 foot-pounds of torque at 3,500 rpm, 7 horsepower and 6 foot-pounds at 5,500 and 8 horsepower an 7 foot-pounds from 5,500 rpm to redline. That 5 percent power gain is nothing to sneeze at, especially considering that the test tC already had a cold-air intake from the factory.

What Gives?

So Toyota went through all that trouble to create a factory CAI and position it in the perfect location but left quite a bit of power on the table by creating a less-than-optimal intake tract. Most or all of the gains achieved by using an aftermarket intake come from its smooth-bent, large-diameter tubing, high-flow air filter and lack of a sound-deadening intake resonator. In this sense, the CAI isn't doing anything the stock intake doesn't do; it's just doing most things a bit better. There's a trade-off here involving higher noise levels, but that isn't necessarily a bad thing if you like noise. But that's not the primary issue with aftermarket CAIs; there are a few other, more important factors to consider.

Considerations -- Filter Location

Aftermarket intakes have three basic problems in this application. The first has to do with the exposed, conical air filters most CAIs use. Mounting a cone-style air filter on the end of an intake tube inside your wheel well is one of the dumbest things you can do to a car. This area gets inundated with water thrown off by the tire, and that water will soak into your air filter and effectively clog it. Odds are good that you won't get enough water through the filter to hurt your engine, but you can expect power to drop precipitously till it dries out. And that's assuming you don't get any mud-forming dirt on it in the meantime, a distinct possibility given the filter location.

Considerations -- Intake Resonace and Pressure

Resonators reduce sound by controlling pressure pulses that travel backward out of your engine and through the intake air charge. The resonator itself is a kind of expansion chamber, giving those high-energy pressure waves a place to expand into before they can stack up in the intake tube and restrict flow. The larger-diameter CAI tubing will help to reduce this pressure stack-up, which means reversion waves should have a minimal impact on power output. But changing the resonant frequency of the intake tube can negatively affect idle quality and throttle response at low rpm. Placing the air intake at the bottom of the wheel well instead of at the top also reduces its pressure-siphoning effect, which may reduce intake effectiveness and decrease stability at high speed.

What You Can Do -- the Ideal Intake

So while a standard aftermarket CAI can pick up some horsepower over the stock configuration, it's far from ideal in most other ways. The ideal would be to use big, smooth tubing from the engine to the airbox, a big, high-flow airbox, a high-flow panel-style filter and a big airbox extension tube that draws air in from the top of the wheel well. In a perfect world, you'd just cut a hole in the bottom of the airbox and mate it to a matching hole in the top of the wheel well; but, then you'd have a problem with water intrusion into the filter media. Connecting the airbox to the top of the wheel well with a 90-degree elbow will keep most of the water out, while preserving airflow and pressure siphoning. You'll still lose idle quality because of the lack of a resonator, but them's the breaks.

References

• Alldata: 2007 Scion tC Intake Specifications and Removal Procedures
• Engine Airflow: A Practical Guide to Airflow Theory, Parts Testing, Flow Bench Testing and Analyzing Data: Harold Bettes
• Racing Engine Builder's Handbook; Tom Monroe
• Import Tuner Magazine: Behind the Scion tC -- Fujita T5 Cold Air Intake Test