Intakes: An Introduction to Induction

What is an intake, anyway?

The term "intake" has been applied to a bewildering array of parts, many of which bear no resemblance to each other at a glance. Technically, the intake on an engine contains the entire system of passages that incoming air runs through, from the air inlet up to the cylinder head, but the term "intake" is sometimes used to refer to a smaller portion of the intake tract. An Iceman on a Honda B16A inline four, a GT-40 intake on a Ford 5.0 V8, and a Super Six intake on a Chrysler RG-series inline six are all sometimes referred to as intakes, but they are all very different parts. The first is a single tube with an air filter at one end, and is actually a cold air induction system rather than a complete intake. The GT-40 intake looks like a knot of aluminum snakes; it's a tuned runner intake manifold designed to work with multi-port fuel injection. The last on the list consists of a cylindrical air cleaner and carburetor mounted on a piece that looks like a six-armed cast iron candelabra, which is a somewhat less exotic kind of intake manifold.

As you can tell, there are many kinds of modifications that can be done to an intake to improve power. Most of them attempt to allow more air into the engine, either by decreasing restriction, cooling the air so it is more dense, or occasionally by taking advantages of shockwaves formed in the intake tract to compress the incoming air. Some intake modifications, such as aftermarket manifolds or fuel injection conversions, also attempt to improve fuel distribution and mixing, although most aspects of those modifications are beyond the scope of this article. Also, this article does not cover diesel engines, turbochargers, or superchargers.

Anatomy of an Intake

First, it's helpful to understand the parts to the car's intake system. On many cars, the first part of the intake tract the incoming air encounters is a tube of some sort designed to channel cold air from behind the grille or inside the fenderwell into the engine. Other cars lack this tube, and draw in hot air from under the hood. The air then passes into the air box or air cleaner; an air box is boxy shaped and usually found on fuel injected cars, while air cleaners look like overgrown tuna cans and are usually found on carbureted cars. Either one will contain an air filter to remove any incoming dirt, insects, and any other contaminants the air might have picked up off the road.

The next object the air is likely to encounter is either the carburetor or, on fuel injected cars, the throttle body. Some engines will have multiple carburetors, or rarely multiple throttle bodies. Either one contains a butterfly valve which controls the amount of incoming air allowed into the engine. Carburetors and some throttle bodies will add fuel to the incoming air at this point, while multi-port fuel injection systems add the fuel at a point further downstream. The throttle body or carburetor will be bolted onto a manifold, which distributes the air to the individual cylinders. Engines with multi-port fuel injection will have a set of fuel injectors bolted to the manifold near where it attaches to the cylinder heads, usually accompanied by a fuel rail, which is basically a fancy looking pipe which delivers fuel to the injectors.

Getting a Good Filter: The easiest intake mod

The stock paper element air filter is cheap, but a K&N cotton filter will offer significantly more airflow capacity. These filters are available for almost all applications, from 60's era muscle cars to the newest Japanese imports, and they seem to have decent results in almost every application. Hot Rod reported a 5 hp increase at the rear wheels when testing one of these on a ‘96 Mustang, while Overboost found that one of these made a 2.5 hp improvement on an otherwise stock Dodge Omni. While these gains aren't huge, these filters are very inexpensive compared to other modifications, and easy to install. Plus, they last much longer than conventional paper elements, so you could conceivably save money in the long run. Accel also makes a line of low-restriction foam filters which offer comparable airflow characteristics to the K&N filters.

Cold Air Induction

If you've taken chemistry classes, you've probably seen the ideal gas equation, PV=nRT. This relationship between the pressure, temperature, and volume of a gas indicates that if the gas is colder, it's denser, and denser air will provide more oxygen, allowing your car to burn more fuel and make more power. A common rule of thumb holds that decreasing air intake temperature by 10 degrees F will increase horsepower and torque by 1%. The converse is also true; a 10 degree rise in intake temperature will cost you 1% of your horsepower. Some aftermarket "short-ram" airboxes for newer imports draw in air from under the hood, which might work decently with an engine that's been sitting overnight, but once it warms up, the aftermarket "performance" airbox will decrease power. An additional advantage of aftermarket airboxes and cold air induction systems is that stock airboxes are frequently designed with restrictive systems for reducing noise. A system designed entirely for performance can add a couple of horsepower over the stock airbox, but beware, as there are quite a few aftermarket cold air induction systems and similar modifications that do not improve performance at all!

So I Could Just Put a Cone Filter on a Pipe?

In a word, yes! The aftermarket cold air intakes are frequently just a cone filter on a tube, and there's nothing magic about them. A cone filter positioned to draw cold air and connected to a suitable tube can get just as good results as an aftermarket setup for a lot less money. Possible choices for a homemade intake include plastic drain pipe, metal drier hose, ABS plastic fittings, and fiberglass or carbon fiber (both of which can be fabricated at home with a little practice and some raw materials from an aircraft kit company or boating supply store). Although PVC pipe is easy to work with, I don't recommend it as PVC doesn't have the temperature resistance of other plastics.

Manifolds, Throttle Bodies, and Carbs

You might be able to buy these for your engine, if it's got a big enough aftermarket. A larger carburetor or throttle body will enable the engine to draw in more air, but too large can be as bad as too small. On an injected car, a throttle body that's bigger than the engine needs will simply waste money, but on a carbureted car, an oversized carb will make it perform worse at part throttle. There's a simple formula for determining the correct carburetor setup: The amount of air your engine requires, in CFM, is equal to the displacement (in cubic inches) times the maximum RPM, divided by 3456. In most cases, it's usually best to round up to the next largest off the shelf carburetor than round down. For example, a Chevy 350 built to redline at 6,000 RPM needs 608 cfm of airflow. A Holley 4150 carburetor rated at 650 should deliver good performance without compromising part-throttle drivability.

For carbureted V8 engines, there are two popular designs of intake manifolds, usually designed to work with a single four barrel carburetor. One is called a dual plane because it is split down the middle, so as to feed four cylinders from two barrels, and the other half of the cylinders from the other half of the carburetor. This usually works best for an engine tuned to make plenty of torque down in the low RPM range, for good driveability and pull off the line. The downside of dual plane manifolds is that they become restrictive at high RPM. Single plane manifolds have a large area, called a plenum, under the carburetor which draws air from all four barrels and then distributes the air and fuel to all the cylinders. This design allows better power at high RPM, but does not perform as well at low RPM, in general. There are some single plane manifolds that offer good low RPM performance, and occasionally dual plane manifolds that deliver power higher in the rev range than most, so check with people who are experienced with your particular type of engine before selecting a manifold.

There are many other designs out there. Fuel injected manifolds often bear no resemblance to traditional single plane or dual plane manifolds, and inline engine manifolds frequently don't resemble designs used on V engines. However, usually the same rule about manifolds that feed half the cylinders from half the carburetor performing better at low RPM, while manifolds that feed all the cylinders from the entire carburetor pull better at high RPM, still applies to inline engines. Fuel injected engines have fewer fuel distribution problems, and frequently use tuned runners to create standing waves to help push air into the cylinders, but how they manage that is beyond the depth of this article.

Conclusions

The intake of an engine is frequently the easiest portion of it to modify. Hopefully, this article has helped you understand how the intake system operates and how modifying it may increase power. For further reading, I suggest you check reviews of individual products, such as dyno tests conducted by magazines such as Hot Rod, Sport Compact Car, or whatever other automotive magazines discuss your car. If you're planning anything more serious than a cold air intake or low restriction filter, I also suggest you purchase some quality books about modifying engines, such as those published by SA Design or HP Books. Spending $20 on a book can be a very good investment, as it can save you from paying hundreds for parts that don't work.

Big Fat Legal Disclaimer

This information is free, and just something I've put together in my spare time. I've done my best to make sure the information is correct, but I may just plain be wrong. And there are many things which could prevent a theoretically sound modification from working correctly on a particular car, from an abnormal engine design to a poorly designed aftermarket part to a botched installation to improper post-installation tuning. So, this information is presented AS IS, without any kind of warrantee whatsoever.

Also, there are a lot of laws regarding the modification and operation of motor vehicles. Some modifications are only permitted on race cars which are not driven on public roads, or cars old enough to predate emissions laws. Others may void your warantee, if it didn't expire long ago. Check local laws for the legality of any modifications you're considering - especially since some overzealous emissions inspectors may occasionally need reminding, particularly in California. And be sure to drive safely and responsibly. If you feel the urge to express contempt for some of the more poorly thought out laws concerning modifying cars, I strongly urge you to do so through legal and socially acceptable methods, such as writing to appropriate government officials and supporting political groups that promote our interests.