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FAQ Project - Intake
Air filters

Engines are like big air pumps; they need to breath to make power and making this task easier translates into more power.

The air filter has multiple purposes:

- The obvious one is to filter debris, small rocks, etc that could go in the engine and damage it; very small particles can be harmful, for example pollen is highly abrasive and can go in the oil, if not filtered, and raise friction between piston rings and the cylinder walls, etc.
- A second, less known, purpose is to straighten the air flow; I'll come back to it later

The filters come into two main categories: Foam and Paper

- the foam filters are best at removing dust and small particles, but they are usually more restrictive and get clogged faster too. They are uncommon on performance cars

- the paper filters vary a lot; They don't clog as fast, are cheap but the filtration is not the best because particles smaller than the holes in the paper can pass the filter; Some companies, such as K&N, use a paper filter with oil on it; The gaps are bigger, but the oil traps particles and as they add up, the filtration gets even better because it makes the oil patch slightly larger. A K&N without oil would be a very bad filtering device, so you have to make sure it's always properly oiled; You can clean them and re-oil them, but the cotton based fibers shrink over time leaving even larger gaps. Test have shown that the paper and oil filter are able to filter as least as good as regular filters while straightening the air flow more. They react badly to heavy dust though and it is the reason they are not used in industrial environments. The oil is not removed by water from rain.

Now, what is straightening the airflow about ? Less restriction is better, so competition engines should run without a filter, right ? well, not really; Air is a like a liquid, it doesn't flow straight as molecules bounce of surfaces, perturbating the flow from the other molecules, etc. As a result you get zones of turbulences that can lower the airflow; Filters tend to straighten the flow out and make it swirl, resulting in a higher speed

It has also another purposes; most modern fuel injected cars are using a MAF (mass air flow sensor). This sensor monitors how much air gets in the engine; Older engines didn't use these. The MAF works by heating up a wire to a predetermined temperature which is used for calibration; it determines how much current to put through to put the wire at a given temperature; then an identical wire is put in the airflow and the current is raised to match the same temperature; the difference in current can be used to determine the airflow, as all other variables (diameter, temp, etc) are identical.
This sensor gives more accurate results if the flow is straight; a K&N filter will not straighten the flow as well as a regular filter when the flow is low (low RPMs) and can make the readings fluctuate; that is why some cars get their idle fluctuate slightly when they put a K&N; as soon as the flow is higher, they work really good.

If you want a cheap way to make a race filter, take a nylon, dip it in transmission fluid and use that as a temporary solution; be careful because if can be sucked in the engine and jam the throttle open if not attached properly and can melt if it's close to a heat source; experiment at your own risk

Air duct (filter to intake) and air boxes

The stock air box and duct is usually the result of many compromises such as engine bay layout, noise, etc.
They are typically very restrictive, which is why racers tend to modify them as soon as they get a car; They also placed so that warm air goes in as it affects fuel economy and emissions. For races you want cold air as it is more dense and packs more molecules in the same space; For this reason there are a variety of setups available, from ram air to close to the ground air boxes, etc.
Keep in mind that these setups need to be cleaned up more often and a low airbox requires you to pay attention to water
The more straight an air duct is, the better; Also it should be in a material that doesn't heat soak, like composite plastics, etc rather than metal for example. Most of the time you have the choice between various performance replacements or can go to Home Depot and build your own; it has to be as straight as possible, as cold as possible and take air either high or low, but not at engine level; outside of the engine bay is best.

If you hear about devices such as 'the tornado', supposed to enhance the air flow, they do not work. it's the same guy that comes with a new revolutionary gimmick every two years.

Engine intake

It is common for people to call the air duct and box and intake, but it is very different from the engine intake.
The engine intake is the large piece sitting on top of your motor. It has many important roles.
The layout differs radically between a fuel injected vehicle and a carburated vehicle.
On the fuel injected one, a throttle body controls how much air gets admitted in the intake by moving the blade(s). The air gets into a large chamber, called the plenum; in this chamber there are galleries that lead to each of the cylinder's intake port. The shape of the plenum and galleries are very important because they must profide a clean airflow with now turbulences; at the same time they have to promote swirling to help with velocity. The injectors are mixing the gas with the air in each of these galleries.
With a carburated car, the carburator has a blade, like the throttle body, but the gas and air are mixed when getting in the plenum. This usually lowers slightly the air temperature too which is one of the reasons carburated engines can make more power.
Intakes have two contradictory roles too. They must stay as cold as possible to cool the air and/or avoid heating it up, but they also remove some heat from the heads; for that reason they're usually build using materials that can absorb and get rid of heat quickly, such as aluminum and plastics.

They can get dirty for multiple reasons and this affect the airflow :

- Particles passing through the filter
- Emission regulations force manufacturers to take crank case gases and put them back in the intake to burn them, which forms oily deposits in the intake to which dust sticks.
- Another problem, mostly with modified engines, is called cam reversion; for more details check out the cam faq, but in a nutshell, a performance cam will cause reversion (of flow) in the low rpm range and suck some exhaust gases in the intake during the overlap (when the intake and exhaust valves are open at the same time), bringing carbon deposits in the intake, and of course as there's oil they'll stick, etc
- EGR (Exhaust gas recirculation) is one of these emissions things too; some exaust gas is put back in the intake to lower combustion temperatures and improve emissions and fuel economy

You should try to keep the intake as clean as possible. I personally recommend SeaFoam (distributed at Napa), but be careful and use it at your risks because some people broke their engines with it.

You can do some things to keep the intake clean, but they are not emission legal:
- remove the connections that bring the crank vapors to the intake; install a breather on the valve covers; This has some drawbacks, mostly moisture that can make it in the valve cover and diulte the oil and an oily film will always build on the engine. More frequent oil changes and regular cleaning will take care of that.
- remove the EGR, but it will lower your mileage and may require to reprogram your computer; the EGR allows to fill part of the cylinders with inert gas while cruising, effectively reducing temperatures, but also artificially lowering the displacement of the engine, helping with fuel economy; the EGR valves are only activated during cruising by the computer, not while racing; Although some faulty valves that leak will cause low rpm stumbles and high rpm misses. Some heads have a kind of built-in egr feature and in that case, some exhaust gas doesn't leave the cylinder a mid range rpm; these cars don't have egr valves.

To enhance the performance of the intake, you can add a larger throttle body, although it usually not justified with mild mods as it may even lower the air velocity and lower performance.
A common modification is to port and polish the intake to make the flow smoother; Enlarging the intake ports to match larger heads, make sure that the gaskets match the port's size, etc are common performance enhancements; it's definetly possible to do all this at home with a Dremel if you know how

Usually they have up to two sensors: one of the throttle body to determine the throttle angle that will be used by the computer (fuel injection and automatic transmission); that is why changing the throttle body can throw things out of whack because a lower throttle angle (lower reading) will flow more than stock (causing problems with automatic transmissions that believe you're going slow and may let the clutches slip too much, etc)
the other sensor can be an intake pressure sensor, used to determine the relative vacuum and compute the engine load

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