The wonderful thing about science is that it does not depend on you understanding it to work.  It just does. Take gravity for instance.  You don’t have to understand gravity for it to work on you.  Understanding is only really required if you want to try to manipulate science to work for you.  If your mindset is to simply ask how something is done within a social media environment without understanding the principles and laws that govern the answer, then you kind of get what you get… opinions from other people who don’t understand the answers they are bout to give you.  However, by simply understanding the physics that makes one answer right and another answer wrong will help you understand why certain choices are good and some are bad.

Daniel Bernoulli was a mathematician and physicist who specialized in fluid mechanics… the study of the effects of forces and energy on liquids and gasses.  Why should this matter to you?  Because air is a gas and the mechanical principles on how air affects how your car perform is directly related to Bernoulli’s Principle.

What is Bernoulli’s Principle?

Bernoulli’s principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid’s potential energy.  That sounds all sciency doesn’t it?  But what does it mean?  Bernoulli’s Principle can be rewritten to say that as the speed of a fluid or gas increases, the pressure that is exerted against surfaces decreases.  This may sound a bit counter-intuitive because when we think of the speed of something… say… water coming out of a hose, as we increase the speed the water passes through the hose, the further the water sprays out from its tip.  This doesn’t sound like a decrease in pressure; rather, it seems like an increase… but it’s not.

There is another scientific principle that states that “energy is neither created nor destroyed”.  That is not to say that it cannot be redirected.  Pressure is energy… when we think of something like an open hose, the energy is directed out of the end of the hose to push the water further out… but what is the effect of the energy of the water inside of the hose?

The answer is that pressure inside the hose decreases when the water is moving and increases when the water stops moving (like when we kink the hose to stop the water from flowing out).  Now… I know what you are thinking… you are thinking that the pressure inside the hose goes up when you close the nozzle because there is pressure on the other side of the hose pushing it.  But what if you close the faucet on the other end?  Does the pressure go down or does it stay the same?  By closing the faucet valve you are isolating the water that is already in the hose, so there is no longer anything “pushing” the water.  So, now you are thinking, the pressure is already there, so turning off the faucet will not relieve it, it will just stay in the hose.  But why?  Is there more water in the hose with the water sitting static than if it is moving?  While it is true that the pressure pushing the water into the hose came from another source, when the water is static in the hose, the pressure acts against the hose itself since it has nothing else to push against.  When the water exits from the nozzle, the pressure is redirected to push the water out thereby decreasing the pressure against the hose walls.  In the case of Bernoulli’s Principle we are not concerned about how forceful the water comes out from the end of the hose; rather, we are focused on how much pressure is exerted on the inside of the hose by the water that is in it.

Okay, so what?  How does Bernoulli’s Principle have anything to do with my car?

Managing airflow is an obsession for race car designers who are often limited to how much power their engines can generate.  When all other things are equal, airflow management becomes an important deciding factor (other than driver skill) in determining how fast a car can go and how fast it can take a turn.  Bernoulli’s Principle lies at the very heart of most aerodynamic principles that deal with lift and drag.

Think of drag as areas of low airflow.  When airflow slows, air pressure increases.  This is why the front of your car is a drag surface… the air hitting the front of the car must change direction, and in doing so, slows down which creates a high-pressure zone in front that the car must overcome to move forward.  The faster the car moves, the higher the relative pressure in front of the car as compared to the rest of the car.

Lift is created when the relative air pressure above the car drops while the air pressure under the car increases (or stays the same).  As you go faster, the air moving over the top of the car moves faster, creating a low-pressure zone on top… meanwhile all the stuff hanging under your car and the various crevices under there slow the airflow down, which maintains a high-pressure zone under the car.  When you have a differential in air pressure from the top to bottom, you have lift.  If you are flying an aircraft, lift is good.  If you are driving a car, lift is not desirable.

The last significant area of drag is actually behind the car.  The air moving around the car is fairly laminar… that is that the air flows fairly easily around it; however, once the air separates from the car, it becomes turbulent.  This turbulent air just behind the car creates drag which is almost like suction since the car is essentially cutting through the air and leaving a low-pressure zone behind the car (from the fast moving air on top, bottom, and sides) that actually pulls the car back.

So, how can I use this information to improve the airflow around my car?

What you should come to understand is that making your car more “slick”, that is, reducing drag wherever you can, is not necessarily your objective.  What you are more interested in from a handling standpoint is the balance.  You want to be able to balance the top and bottom airflows to reduce the tendency of the car to lift at high speed.  There are a number of ways to do this…

1.  You can try to improve the airflow under the car by reducing the amount of air infiltration coming in from the front and sides while simultaneously speeding the air up around the sides of the car.
   • This is most effectively accomplished using air dams that redirect the air in the front to the side of the car and by using side skirt
   • A front vortex generator (most commonly in the form of a front bumper canard wing) will create a vortex against the skin of the car that the air traveling around the car can travel on.  The vortex essentially becomes a layer of padding which will improve the laminar flow of the air on the sides of the car which will prevent areas of high pressure along the sides of the car which will create drag.
   • The combination of redirecting the air around the car and creating a vortex pad will reduce the amount of air under the car and create suction as the speed of the car increases; thus reducing lift.

1. You can increase the speed of the air under the car.
   • A front splitter is used to “split” the air… redirecting the air on top of the splitter into the radiator or around the sides of the car while allowing the air under the car to travel unimpeded to the back.
   • In order to keep the air moving, the bottom of the car must be as smooth as possible… this means installing underbody panels to streamline any protruding parts and isolating any pockets which can produce drag.
   • A rear diffuser is used to speed up the air exiting the rear.  This is typically done by channeling the air in such a way that the airflow is increased just before exiting the rear.

1. You can increase the amount of drag on the top of the car.
   • This is usually accomplished by installing accessories that will impede airflow going over the car.
   • A front splitter can be adjusted to a downward position to create a very high-pressure zone in the front of the car which will increase drag in the front.
   • A spoiler or a wing on the back of the car will slow the airflow in the rear causing a high-pressure zone in the back of the car.  The more exaggerated the angle of attack of the spoiler or wing, the more it will interfere with the airflow and the more drag it will create.

While the concepts of balancing airflow expressed above are very simplistic, the governing principles are very basic.  There are essentially two ways to create a low-pressure zone under the car… one is to control the air going under the car in the front and speeding up the air going around the sides of the car which will create a reduced pressure zone… the other is to speed the air up going under the car which will have the same effect.  To create a higher pressure zone going over the car, you simply need to add appliances that will interfere with the laminar airflow traveling over the top.  Slowing the air down means that the negative air pressure is reduced, thereby reducing the amount of lift.

Note:  Increasing drag (through the use of wings, splitters, and other appliances) WILL slow the car down.  More engine power will be required to overcome the effect of drag which in turn will impact how fast the car can accelerate and the top speed.  Additionally, increasing the drag will also have a negative impact on your highway gas mileage.  Ideally, balancing the airflow is most effectively accomplished by leaving the appliances off of the top of your car and focusing on improving the airflow under your car.

Your engine compartment and airflow

What we have pretty much established so far is that air will have a tendency to move from a high-pressure zone to a low-pressure zone.  Your engine is cooled by air passing through the radiator.  If you have a turbo with a front mount intercooler, this too relies on air passing through it.  You can take advantage of the high air pressure zone that inevitably forms in front of your car to rapidly pass air through these coolers… so, it goes without saying that if we can use the Bernoulli’s Principle to create a greater pressure differential from the high-pressure zone in front of the car with an even lower pressure zone in the engine bay, we can improve the effectiveness of the stock coolers.

The OEM hood of your car serves two purposes:  to protect the components inside the engine compartment from the elements and to provide a laminar surface for improved airflow over the car (reduce drag).  The negative effect that the hood has on cooling is that it forces all the hot air inside of the engine compartment to exit underneath the car since it cannot exit from the top.  The obvious solution to this problem is to create an airflow path through the hood to extract the hot air quicker, which will also reduce the amount of air being forced under the car; however, how you create this airflow path may have the opposite effect than you wanted.

Forward facing vs. reverse facing hood scoops

A forward facing scoop will create an additional high-pressure zone on your hood, and depending on how much protrusion that scoop has, can create a turbulence wake that will create a high-pressure zone behind it which in turn will increase drag.  Additionally, ramming air into your engine compartment from the hood will artificially increase the air pressure under the hood and reduce the amount of air going through your coolers as a result of reducing the pressure difference that draws cool air through the coolers.  All in all, unless that forward ram scoop is either pulling air straight into your intake or forcing it through a top mounted intercooler, it is probably reducing the cooling capacity of your front mounted coolers.

A rear facing or flush vent will pull air out from the engine compartment.  Due to that principle that Bernoulli guy came up with, air rushing over the vent will create a vacuum that will pull air from the engine compartment.  A reverse facing vent will increase the amount of air coming through the front mounted coolers because the air pressure under the hood will have decreased.  Additionally, a rear facing or flush vent will reduce the amount of air that infiltrates under the car through the engine compartment as air (just like water) will travel in the path of least resistance.

Cowl scoops/hood risers

There is another low-pressure zone located at the base of your windshield.  A cowl scoop places its opening right at the base of the windshield to take advantage of this low-pressure zone.  This particular scoop is very popular among muscle cars because the raised portion of the cowl also creates vertical room for a top-mounted supercharger.

For those of us who really don’t want to raise our seats to see over the cowl because we really don’t need the room under the hood, the Honda community has come up with a somewhat genius, albeit a bit ricey solution of adding spacers to raise the back of the hood to provide a gap in that low pressure zone at the bottom of the windshield.  The hood riser method is effective at pulling heat from under the hood and increasing airflow through the front mounted coolers; however, the hood will not look like it fits properly as the alignment will be off to accommodate the raised portion.

Bernoulli and your exhaust

Oh yes… I’m sure you thought that the article would end at the engine compartment.  After all, what other gas would we concern ourselves with?  The other gas that is pertinent to this discussion is exhaust gas.  Since exhaust has to move down a pipe, it’s important to know how and why proper sizing of that pipe is important to engine function.

Who has heard that the exhaust NEEDS back pressure in the exhaust to operate properly?  Well, put the idea out of your head, it’s a LIE.  No combustion engine ever created NEEDS backpressure… in fact, back pressure will reduce the efficiency of the engine leading to reduced power and poor gas mileage.  Back pressure means not all of the exhaust is being evacuated away from the engine.  The engine, in turn, is not able to fully vent (scavenge) all of the spent combustion gasses out of the cylinder and some of those gasses remain for the next combustion cycle.  Since the gasses are already spent, they prevent the engine from being able to fully fill the cylinder with stuff that will burn.  This means reduced bang = reduced power.

So, how does a myth like this persist in popular lore?  The answer is that those who pass on the myth as though it was fact do not understand Bernoulli’s Principle (see what I did there?).  It is a fact that if an exhaust on a normally aspirated engine is too large, it reduces the scavaging effect that the exhaust should provide to pull the exhaust gasses away from the engine.  A smaller exhaust pipe can be more effective than a very large one.  So, to the casual observer, smaller pipe = more back pressure… so the engine must need back pressure to work properly.

How Bernoulli’s Principle works in this case is that a smaller pipe = higher velocity… and we know from our discussions above, when velocity increases, pressure decreases.  The reason why a larger pipe causes back pressure is because the velocity of the escaping gasses is lower; thus, the pressure inside of the pipe is increased as a result.

This does not mean that you need to change out your exhaust for a 1/2 pipe to increase the exhaust flow velocity (although it will).  We also have to balance the fact that we are also contending with an ever changing system in that as RPMs increase, the amount of exhaust gas produced increases.  Once the velocity of the exhaust gasses passing through the exhaust pipe reaches its maximum flow capacity, shoving more gas in the same pipe will result in higher back pressure.  It is impossible for one exhaust system to be efficient at all RPMs (flow rates)… so you need to decide for yourself what flow rate you would like to be optimized with the understanding that RPMs outside of the sweet spot is going to result in higher back pressure due to slower flow.

Now… as for you turbo guys out there… the philosophy is a little bit different because you have a turbine in your exhaust flow.  Since the turbine is free wheeling and doesn’t stop just because you let off the gas pedal, the turbo does your scavaging for you.  What this means is that you are not as concerned with exhaust flow as the NA guys.  For you, the biggest pipe is usually the most effective… and shorter is better if you can get away with it without killing everyone inside of the car.  The turbo will handle evacuating the spent gasses away from the engine… your only concern is making sure that that pipe is big enough to prevent choking of the pathway.

The final word

Obviously, an article like this is not going to be able to cover all the different ways that Bernoulli’s Principle impacts the performance of your car, but hopefully, it provides enough information that you can apply to your buying choices to achieve the performance outcome that you desire.  There is more to efficient car design than simply bolting on something that looks like it may work… and in many cases, your bolt on performance enhancement will have an opposite effect on your performance than what you thought it should have.  This isn’t necessarily because the aftermarket part you bolted on is defective; rather, it can usually be attributed to the fact that you really didn’t know what it was supposed to do in the first place.