Intakes

Here's a writeup on air intakes. There's really not that much to know about these things, but there seems to be a lot of confusion about them.

What is an Intake?

An intake is the source of air for an engine. An intake is made up of tubing, a filter, and a resonator (missing on most aftermarket intakes). The intake is attached to the throttle body (or turbo/supercharger). The filter filters the dirt and grit from the incoming air, the resonator makes the air less turbulent (quieter), while the piping provides a path to the throttle body.

What do intakes do?

An engine needs an incredible amount of air to function. For example, your typical 1.6L engine needs 0.4L of new, clean air per rotation (33L/second at 5000rpm).

Air is composed of many ingredients, the most important of which are Nitrogen and Oxygen. A fuel management system tries to maintain a perfect air/fuel ratio of 14.7:1 (Oxygen to Gasoline in a compressed mixture). That being said, the most important functions of an intake are:

• provide the engine with an adequate amount of air (oxygen)
• maintain a quick flow of air which pushes fresh air in to the intake cycle.

Like with exhausts, velocity and volume of the air being moved is very important to engine performance.

What kinds of intakes are there?

Short Ram Intake (SRI) - A short tube with a filter on the end. The 'ram' comes from days gone by when ram air (hood scoops) used to be a popular upgrade on carbeurated pony cars.

Cold Air Intake (CAI) - Typically, a Short Ram Intake with an extension tube. This extension tube sources air from somewhere outside of the engine bay, where the air is cooler. In most Honda/Acura's, the Cold Air extension dips into the space between the bumper cover and the wheel well. The theory behind a CAI is that the colder air is denser with oxygen.

Why would I want to upgrade my intake?

Intakes are designed by automotive manufacturers to meet several goals, some of which have nothing to do with performance. There are a few specific cars that suffer spectacular defeat at the hands of a poorly designed intake. The early 90's Neon is the best example. The restrictive, hot, and poorly shaped intake with its cheap filter rob the engine of power and performance. Honda D and B series engines have very well designed intakes, on the other hand, but people replace them anyways.

Filter - Stock paper filters restrict the flow of air at high RPM's. Replacing the filter with a 'high flow' filter (a K&N drop-in replacement, for example) is usually the best intake upgrade you can make on a budget.

Looks - #1 reason intakes are replaced. A nice shiny silver tube looks a helluva lot better than the drab black plastic tube.

Sound - An intake without a resonator sounds more throaty and confident. Particularly when coupled with a shiny tube and high-flow filter.

Performance - Take this with a grain of salt and make sure to read the rest of the article before jumping to conclusions. Unless your intake is particularly restrictive, changing your intake will give small performance gains. Consensus in the Honda D-series community is 3-5HP gains at max RPM on an otherwise stock 1.6L.

If you've upgraded internal engine components, changing the intake is a must. If you've bored, ported, upped the t-body diameter, turbo'ed, super'ed, added 1-2k to your max RPM, or altered cam/valve timing, you'll need to upgrade the intake to keep up with air demands.

Why are the performance gains so limited, even though air is so important?

The intake is only the first of 3 or 4 major components that are responsible for air delivery. The throttle body, intake runners, valves, and other components are more important, and provide much more ground for performance improvement.

Potential problems?

With intakes, there are a few small things you need to watch out for:

1. Tubing diameter. No matter what anybody tells you, unless you've done major engine work, stick with the stock tubing diameter (most likely 2.5"-3"). Increasing the tubing diameter decreases the velocity / increases the volume of the air in the tube, making it slower to respond to increased demand. The bog-down effect of a fat intake is very noticable in low-RPM operation.
2. Cold Air Intakes. They sound great, look great, give you street cred, make things a little bit more fun at 5000rpm+. But they do kill your winter fuel economy. Mine goes down from about 500km/tank to about 380km/tank, until I take the cold air extension off. There's a good reason for this: the air IS more dense with oxygen when it's cold. So your fuel management system matches it with more fuel. Basically, you waste fuel when it's cold outside.
3. Cleaning the filter. Be mindful of where you move your filter to. You may not want to take your bumper off every time you want to clean your filter. Because of that, coupled with the cold weather fuel economy, I currently use a generic short ram intake.
4. Other valves and hoses. To save $5, most people do not attach the vacuum hose and cam vent hose to their aftermarket intake like they're supposed to. The vacuum hose needs clean air too, doens't it? And not having your cam vent hooked up to your intake can cost you hundreds ($300 in Ontario) in environmental offences.