FlowThe theory behind an intake system-be it short-ram or cold-air-is it sucks more air into the engine, allowing for more fuel to be burned and, therefore, creating more power. But just how much does it flow? In other words, how much air is an intake capable of moving from point A (the outside edge of the filter) to point B (the end of the pipe)? To measure flow, you use a flowbench, and to measure something the size of an intake, you need a big one. Luckily, Branch Flowmetrics in Cypress, Calif., had one that met our specs. We strapped all nine of our aftermarket systems to the bench, as well as a stock air intake assembly from an RSX Type-S, and tested them all at 12 in. of water.
The results were very interesting. The lowest number, not surprisingly, was for the stock airbox with the stock filter, posting a corrected number of 237.0 cu. ft per minute (CFM). Replacing the stock paper filter with K&N's oiled cloth filter raised that number to 243.9 CFM, not much of an increase, really, but it demonstrates the biggest impediment to flow in the stock intake isn't the filter, but the intake assembly itself.
One look at the stock intake, and it's easy to see why. The path it takes is tortuous, sucking in air from the nose of the car, but immediately bending it through different sized pipes, routing the air to the back of the engine compartment, altering the path 180 degrees as it passes through the filter, then finally shooting it over to the throttle body. Plus, the intake itself is filled with baffles and anti-resonators to keep the noise levels down. It works, but it also significantly impedes flow.
From there, we moved on to the intake systems themselves. When all was said and done, Injen posted the highest number on the flowbench, at 351.4 CFM (see graph "Flowbench Results"). Interestingly, the cold air intakes posted the lowest numbers, with DC Sports CAI coming in at 322.1 CFM and AEM flowing 323.2 CFM. The K&N Typhoon short-ram system flowed the lowest number of all the short-rams, barely beating out the cold air intakes at 323.3.
Why the disparity in numbers? Well, the K&N Typhoon quandary is easiest to answer, as it uses a smaller pipe than the other short-ram systems. From there, it's a little tricky. Cold-air intakes, in theory, result in more power, however, they do this through filter location, not flow. Long pipes and multiple bends are anathema to good flow numbers, and cold-air intakes have plenty of both, with pipe lengths at least twice as long as the short-rams.
Imagine sucking milk through a straight, standard-length straw. Easy, right? Now, imagine that straw was twice as long and curls all over the place. You need more effort to suck the liquid up to make it to your mouth. The same basic principle applies here. The Injen intake, on the other hand, was not only the shortest of the group, it boasted a big pipe and only one easy bend, a roughly 60-degree turn that it made where it attached to the intake manifold.
Then we got curious. With the Injen pipe clearly the best-flowing design, we decided to find out who was using the freest-flowing filter. Although they were all of the familiar cloth-soaked-in-oil variety, there are differences you can't see just by looking. We removed the Injen filter and flowed the pipe with the AEM, DC Sports, Jackson Racing and K&N-supplied filters (see graph "flowbench Mix and Match"). For the most part, not much changed with the AEM, DC and K&N filters returning flow numbers almost the same (and in the case of the K&N filter, exactly the same) as the Injen filter. However, when we attached the Jackson Racing filter, we got a reading of 359.7, notably higher than the Injen number. So the flow king was a hybrid of Injen and Jackson Racing. OK, you can't actually buy that combination in a store, but it's interesting to note.
TemperatureBut there's more to intake efficiency than just flow. Cold air is denser, and if an intake inhales cooler air from the outside world, rather than the hot air from under the hood, then it would stand to reason that CAIs would make more power in the real world.
We wanted to test this, especially with the RSX. There is a camp that says because of the large cutouts behind the wheelwell in the RSX, short-ram systems will be just as efficient as CAIs. The thinking is enough air circulates under the hood when driving that the advantage of the CAI is taken away, and the deficits of the poorer-flowing long pipes will come into play.