In my first column I promised I would make an effort to teach you about how stuff in the performance world works, among other things. So here I am writing the day Rob expects this to be turned in. It's been busy, typically busy. I just got done prepping a car and running it in a Time Attack event last weekend, I have to get ready for a NASA road race this weekend, and it seems like nothing is getting done. That doesn't include my Turbo and engineering gig deadlines as well. Sometimes it sucks to be me.
So while I was pondering what to write about in a fit of writers block I thought about a part that nearly every enthusiast has had some direct experience with, the air intake. The air intake is likely the first part an enthusiast upgrades after buying a car and starts his journey to glory. Not only is an air intake one of the easiest performance parts to get and install, it can also give you the biggest bang for your performance dollar.
This month I'll tell you more than you probably wanted to know about how intake systems work. You can use this information to help judge the types of intakes on the market and to pick the one that best suits your needs.
When building a car you seldom get something for nothing. Usually, parts that give you more power have a negative effect on something else, be it noise, smoothness, durability, fuel economy, or emissions. Unlike many performance modifications, intake systems usually have the least negative compromise on a car's performance, usually not impacting mileage, durability, emissions, or driveabilty for a power gain.
The intake system's job is to take outside air, filter it, and then bring it to the engine so it can be mixed with fuel and burned. Sounds like a simple job, but there is power lurking here for the weekend tuner to unleash. In vastly simplified terms, an engine is a glorified air pump; much of its efficiency is based on how easily air can get into an engine. Restriction to this inward flow is called a pumping loss by us engineer types. [How was your date night, Mike? Was it a pumping loss? - Ed.] Obviously, the least amount of obstruction will keep the pumping losses to a minimum, thus freeing up more power to the wheels.
Less restriction means more air and fuel can be ingested by the engine for a bigger bang and more power in the cylinders. The same acoustic (sound energy) phenomena that make musical instruments toot can be harnessed to assist the flow of air through an engine. The pulsating action caused by the rapid opening and closing of the intake valve can be harnessed to make more power in an intake system.
There are several different families of air intakes. Check them out below so the information can help you decide which type you need, as they all have their advantages and disadvantages.
Drop-in Air FiltersFor many, a first step is to replace the sometimes-restrictive factory paper filter element with a high flowing one. K&N makes OEM replacement high-flow air filters for just about every car made. The K&N filter uses an oil-saturated gauze filter that can flow up to 100 percent more than the stock paper element. The K&N filter is also very durable and can be washed and reused many times over. This makes a K&N filter a good money saving addition to a car. Usually a drop in-filter adds from 0-3 horsepower.
The only issue that a person may have with this sort of filter is that it's slightly less effective as a filter when compared to most genuine OEM filters. OEM filters are typically 99 percent effective in removing dirt and other abrasive particles from the intake air. A K&N is around 96 percent. You might have read stuff in chat rooms about how K&Ns don't filter well and can ruin your engine. The truth is that they do filter slightly worse but probably not enough to make a difference unless you drive in a dusty, sandy environment all the time, and even then it would be a very slight, perhaps immeasurable, difference.
Fake K&Ns, made in Asia or goodness knows where, that are sold cheaply on eBay and other places are a different story. Most of these are little better than rock strainers. I know several road racers whose engines wore out prematurely because of bootleg K&Ns. Let the buyer beware. A good filter manufacturer should be able to supply you with filtration effectiveness data.
A limitation of the drop-in filter is that it is restricted in filtration area to whatever the vehicle's manufacturer originally designed. If the car was designed with a tiny air filter, your high-flow drop-in will be tiny also. If the car has a restrictive airbox, you are stuck with that using a drop-in. A plus of the drop-in is that it is very easy to install, even the most mechanically inept of us can do it in just a few minutes on most cars, usually without any tools.
As a warning, on some cars, especially the more and more common ultra ULEV and TULEV standard cars, changing the filter may render the emissions systems ineffective and that is illegal. These cars are so clean that the air filter serves as a hydrocarbon trap; they have an activated carbon-impregnated filter element that absorbs the minuscule amount of fuel fumes that may percolate out of the intake manifold when the car sits.
Cone-Style Open Air FiltersIn the search to make modern cars more and more quiet, some manufacturers have been adding silencers to the air intake of the airbox. This can sometimes make the air intake quite restrictive. Most enthusiasts feel that intake noise is music to their ears. We do not mind adding more of it to our car's mechanical symphony, especially if we can get more filter element area and freer flow to go along with it. In that case, there are many companies that make cone-type air filters. These filters have a cone-shaped element that has a large area for filtering debris. The more surface area, the less restriction to the incoming airflow a filter will have.
Cone filters typically have an adapter that bolts directly to the car's intake pipe or airflow meter, replacing the stock air filter and airbox. The best adapters have a radiused inlet to help smooth the airflow into the intake tube. These cone filters typically add from 0-5 horsepower and are much noisier than stock at wide-open throttle. Usually, at part throttle they are reasonably quiet. As a warning, poorly engineered cone filter adapters on cars with mass airflow meters can cause many mysterious driveabilty problems as they can create turbulence that makes the mass airflow meter give inaccurate signals to the ECU. This is typical of non-engineered and tested bootleg eBay filters.
On some cars the stock airbox is a well-designed one, as with the Subaru WRX. Those that suck cold air from outside the engine compartment, like the 350Z or the G35, are even better. When the factory airbox is good, it is possible to lose power with a simple cone air intake as these usually suck in hot, less dense, lower-power-producing-underhood air. Better cone filter conversion kits have heat shields that prevent this.
Air Intake SystemsFinally, there are cold-air intake systems. These systems have the most potential to make large horsepower gains. Most air intake systems use a long mandrel-bent tube to replace the typically restrictive stock convoluted rubber pipe. The best of these systems use the harmonics and resonance of the incoming pulses to create a zone of denser air around the intake valve during the camshaft's overlap period. This improves the scavenging effect in the cylinder and helps to increase the density of the air/fuel charge.
If the intake's main tube is at the right length and diameter, the tube resonates and the incoming pulse of high pressure air arrives at the intake valve right when it is opening and the exhaust valve is closing, i.e., the overlap period. This helps the engine ingest more air at this moment than it normally could.
The tuning of the length of the intake pipe can be best compared to the tuning of a musical instrument. A flute is longer than a piccolo. This makes the flute play low notes and the piccolo, high notes. [Mike apparently went to Band Camp. - Ed.] Similarly, a long air intake pipe is usually better for bottom end power and a short pipe better for top end. Most of these intakes range from 2.25 to 3 inches in diameter with lengths from 12 to 30 inches.
The better companies that design these systems tune them like an organ pipe on a dyno, experimenting with tube diameter and length until the greatest area under the power curve is obtained. Some intakes, like the AEM V2, have another chamber that communicates with the main tube through a passage. This chamber has a different resonance point than the main tube and allows the intake to be more effective over a wider rpm range than a simple straight pipe. It should be noted that the intake dimensions are directly affected by the cam's specifications and an air intake that works well with stock cams might not do so well when installed on a wildly cammed out engine.
Since increasing the density of the intake air and changing the flow pattern in the intake tube can change how mass airflow meters and MAP sensors measure the volume and mass of intake air, the better companies also log long- and short-term fuel trims and observe the air/fuel ratio with wideband O2 sensors to make sure that the mixture is not dangerously off. You can bet that the cheapo bootleg eBay part may be checked only for fit at the very most, and probably not even for that.
Air intake systems can and should be plumbed outside of the engine compartment to take advantage of cold air, unheated by the engine, radiator, or A/C condenser. A rule of thumb is that for every 10 degrees you can make an engine's intake air cooler, you get 1 percent more horsepower. The colder the intake air is, the denser it is and the more oxygen it contains, and this is where the additional power comes from. A properly designed system can use the reduction of restriction, tuned length, ram air, and cold air to get tremendous increases in power. A properly designed air intake system can also increase gas mileage by providing the engine with cooler, denser air, and subsequently reduce pumping losses.
Ram air can also be incorporated in an air intake design. A ram air system uses a scoop of some sort to force air into the air intake from the forward passing of the car through the air. Although you might think this makes a big difference, it is typically only a 1-1.5 percent difference in power if the scoop is designed and placed correctly. Thus, cold air is more important than rammed air.
As a warning, many air intakes can scoop up water during a rainstorm, which can damage your engine if you suck up enough water to hydrolock it. Look at the designs carefully and be careful in rainy weather if they can pick up water. Many cold-air intakes, like AEM, have a removable section so the air filter can be quickly mounted higher in case of rain. AEM also has an "anti-suck" bypass valve that prevents water ingestion. The valve does interfere with the intake's tuning slightly and may cost a couple of horsepower but I feel that this feature is a plus that can easily be reversed for dry weather.
Overall, air intake mods are one of the best bang-for-the-buck items you can buy, and is an excellent first choice for enthusiasts-you get more power with little sacrifice.
Until next month, happy motoring!
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