Jan 01

Tech Tuesday: Your Car’s Intake System Part 1:

The intake system on a car is often one of the most restrictive parts of the system, yet one of the most over-looked. The intake itself is usually replaced but the rest of the system goes untouched. The result is usually a minimal or no gain in horsepower and money wasted. What I hope to do here with this discussion is to just touch on other areas where attention needs to be paid in order to insure you are getting the most out of your engine.

The canvas

The Air Filter

Logically the first place for us to talk about is the air box and filter. Many filter manufactures make wild claims of double digit horsepower gains just by switching filters, these claims are simply unfounded and the sad fact is many aftermarket filters actually flow less air or even worse allow more particulate matter into the engine. This is downright dangerous and will shorten the life of your car. Choose your filter wisely.

An even worse theory in some tuner circles is the idea of running no filter. This is stupid. There is no way to sugar coat that. I’m sure these people who practice this idea have never opened an engine up before and have seen the extra pitting and scratches in the cylinder walls, rings, piston heads, etc. All those decrease compression, increase blow by, and ultimately decrease power and engine life.

This is not the only side effect of not running a filter. It’s also likely that not running a filter is causing you to lose power because of turbulence created at the inlet mouth. These turbulence not only decrease airflow but can also throw off air metering if the MAF or MAP is close to the inlet. A well designed air filter will not only filter out the air but have a slight taper inside that smoothly directs airflow into the intake piping and smooth airflow means more airflow.

At this time I wish I could provide you will a nice list of air filters that both filter well and flow well but without lots of money for test equipment and filters I just can’t do that. I have been trying to research to see if the flow numbers are out there but have come up empty so far. I will keep hunting.

The Airbox
air box
Keep in mind that it’s important to have a filter and one that does the job of protecting your engine first, and flowing air second. I say this because on most cars the real restriction at this point in the system is in the air box or silencers. Cars are manufactured to make little noise, not most horsepower. If you look at your air box it’s not hard to see why it’s so restrictive, your inlet is usually sealed in a box with a snorkel or silencer tube as it’s only access to outside air. Think of this as trying to breathe through a straw, which is essentially what your engine is forced to do with a stock air box. When you replace your intake either with a cold air or short ram system, most of the time the stock air box is replaced as well.


The restrictive air box is why cold air or short ram intakes are so popular in the car world. These products eliminate the stock air box and silencers to allow your engine to quit breathing through a straw. So at this point you are thinking great, I’ll go get one of those, but beware, not all intakes are made equally.

The design of the intake itself should have as few bends as possible, especially upstream of a MAF sensor and before the throttle body. This hopefully gives any turbulence created in the intake a chance to smooth out before reaching the MAF so that it gets an accurate read and before the throttle body so it gets a full charge of air when it opens. The intake piping itself should also be of a consistent diameter which is as close to the size of the throttle body bell mouth as possible. This again helps keep airflow smooth and dense.

intake and throttle body

Speaking of air density, we should segway into why short ram intakes should be avoided as much as possible. Heat is your engines enemy in every way shape and form and SRIs do nothing but suck hot air into the engine. It’s generally accepted that power decreases by 1% for every 13 degrees Fahrenheit temperature increase. On a 200hp engine that can easily equate to 10hp difference between a car with a cold air intake on a 50 degree night drawing in cool air from outside and a car with a short ram intake drawing in under the hood temperatures that can easily exceed 100 degrees. To put it another way, that 10hp is probably what you gained putting that new header on. This is why I never suggest an SRI to anyone.

Your MAF sensor

So now that we’ve decided eliminated the air box and chosen our intake tubing what else do we have to look at that may be robbing our power? Well another culprit may just be the air meters in our system. The MAF in particular can be restrictive depending on what type it is. Hot wire MAFs typically are not, but vane and vortex types both are. We have three options to deal with this, the first being to switch to a hot wire MAF which can be impossible depending on the type of engine management used. The second is to switch to a bigger MAF sensor which is not possible on a vortex MAF and finally we can drop the MAF all together and just use the MAP for our fueling. This is how most race cars handle the situation and in fact it is extremely rare to see a race car using a MAF.

Throttle Bodies

Following the intake system towards the engine our next stop is the throttle body. The throttle body is a natural restriction since it is the first control of how much air actually goes into the engine. The restrictions we have to look for here are in the actual size of the opening, the thickness of the throttle plate, the spindle, and the lips of the bell housing. At this point the decision has to be made to either stick with the stock throttle body and modify it or to switch to a completely new throttle body which presents it’s own set of challenges.

Switching throttle bodies

Let’s look at switching throttle bodies first. The advantages of doing this are quite clear, better throttle response from more airflow without having to do any machining or modifying of the stock throttle body. Sometimes costs can be recouped by selling your stock throttle body which is also a plus. The downside is that larger throttle bodies will often result in poor drivability for every day use due to the much larger airflow at lower throttle levels, this can be tuned out to a certain extent but often times not completely corrected. Of course a complete remapping of the ECU’s VE and MAF tables will have to be done to compensate for the new throttle body as well as several other miscellaneous tuning parameters. If you do not tune yourself this will not be cheap or quick as it can take a good amount of dyno time to dial in a new throttle body.

Modifying your existing throttle body

The other option we have is modifying the existing throttle body which in many cases is the best way to go for a multitude of reasons. Of course other modifications need to be considered to insure that even after you’ve modified the stock throttle body it will flow as much air as the other modifications you may do.

To modify the stock throttle body we need to look at three different areas the first is the lips or entrance of the bell housing. Often times on a stock throttle body the entrance may be quite bulky and to ensure better airflow you should taper them to smooth out airflow as it comes from the intake tubing into the throttle body keeping in mind that we want a gradual transition to create as little air flow slow down or turbulence as possible.

Our next area to modify is the walls of the throttle body themselves. Many throttle bodies have enough material in the walls that some can be safely removed to allow for more flow. Often times it’s possible to remove 1 to 2 mm of material from a stock throttle body which may not sound like much but does increase air flow substantially. It’s important to keep the transition as the air moves towards the throttle plate smooth and not to remove too much so that the throttle plate does not close properly.

Throttle plates are a necessary restriction in the system There are a couple areas to look at here, the first being the plate itself. Again some material may be able to be removed, but be cautious because removing too much here means it will not close properly which will make it nearly impossible to get the car to idle correctly. The spindle that holds the throttle plate together is probably the most restrictive place in the throttle body. Often times this can be shaved down significantly around the between and on either side of the screws to dramatically increase flow when the throttle is open. Of course it is important not to compromise the strength of the spindle by removing too much. if something were to break those pieces and your engine are not going to play well together.

Volvo Throttle Body

That will get you a good start on increasing the efficiency of your engines intake system. I’ll look at part two of this, which deals with the manifold and runners next week on Tech Tuesday. For now any questions, comments or additions are of course always welcome.

GTO Photo by: Skitz on Flickr
Vovlo throttle bodies by: crazyoctopus on flickr
Other Photos by Infinite-Garage.com