Dec 03

Intake theory, the very basics. PART I

The intake system on a car is often one of the most restrictive parts of the system, yet one of the most overlooked.  The intake itself is usually replaced, but the rest of the system goes untouched.  The result is usually a minimal or zero gain in horsepower and money wasted.  What I hope to do here, in this discussion, is just touch on other areas where attention needs to be paid, in order to ensure you are getting the most out of your engine.

[B]The Air Filter[/B]

Logically, the first area 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 than the stock setup.

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

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. This turbulence not only decreases 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 relatively empty so far. Jeff Hartman does touch on the subject in his book “How to tune and modify Engine Management Systems”.  I will keep hunting for more info though.

[B]The Airbox[/B]

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 as little noise as possible, not as much horsepower as possible.  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 the only access to outside air.  Think of this like trying to breathe through a straw, which is essentially what your engine is forced to do with a stock air box.


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’re 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 that 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.

Speaking of air density, we should segue into why short ram intakes are to be avoided as much as possible.  Heat is your engine’s enemy in every way, shape and form, and SRIs do nothing but suck hot air into the engine.  It is 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; although in some cases it’s unavoidable.

So now that we’ve decided to eliminate the air box and we’ve 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 may 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 altogether 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.

[B]Throttle Bodies[/B]

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.

Let’s look at switching throttle bodies first.  The advantages to 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 depending on how your car meters air and fuel.  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.

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 ensure 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 bulk. To ensure better airflow, you should taper them to smooth out airflow as it comes from the intake tubing into the throttle body. Keep 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 smooth as the air moves towards the throttle plate and not to remove so much 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.
That takes us up to the intake manifold. We will cover that, and the rest of the engine in part two.