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This is a little write-up I put together for my friend's start up racing team. I just thought I'd share it with you guys:
First, the four stroke engine process:
Induction: The piston is moved downward, creating a vacuum. At the same time, air/fuel is drawn into that vacuum by the intake valve(s) opening.
Compression: The intake valve(s) close, and the piston moves back up, compressing the air/fuel that was drawn into the engine. This is done to make everything denser and therefore the explosion becomes bigger.
Power: The spark plug fires, and pushes the piston and everything connected to it back down.
Exhaust: As the piston revolves and comes back up, the exhaust valve(s) are open, and the spent gases are pushed out.
There are plenty of different ways to approach this topic, since it incorporates two (technically three, since Power is directly affected by Induction and Compression) steps. It's not really easy to summarize half of the engine process! Keep in mind that this is just an overview, so you should research your own specific questions.
First things first, we need to talk about air. At sea level, normal air pressure is about 14.7 pounds per square inch (psi).
Based on the steps that I stated above, during Induction air is pulled through the filter, into the intake pipe, proceeding to the intake manifold where fuel is mixed with the air, and then finally into the intake manifold. In a perfect engine, the amount of air that enters the cylinder would be at around 14.7 psi, since that is what air is outside.
Sadly though, engines are not perfect. The amount of air a cylinder gets is called Volumetric Efficiency (VE). In a perfect engine, the air entering would be 14.7 psi, or 100% VE.
Most stock 4 cylinder engines are about 90% VE (more cylinders could mean lower VE's). Basically, that means that whenever the engine tries to draw in air, there is actually a vacuum created. This has two affects: 1) The engine isn't getting the most air it can get. 2) The engine has to work harder to pull air in. Taking into account that most 4 cylinders are around 90% VE (newer cars like the rsx will probably be more efficient), that means that most engines are only getting around 13 psi worth of air during induction and working harder to get it!
Now, why does this happen? Well, stock airboxes are not made with performance in mind. They are made to be quiet, durable, and reliable. Example: Put a scarf around your face and try breathing through it. It doesn't feel like your breathing as much as you could if you were just open to the air right? Your engine is in the same predicament.
This is why most people get cold air intakes, or more economically, just replace their filters with something like a K&N drop-in replacement. These new pipes and filters help flow air better, so it improves an engines VE. Of course, it won't flow as well as having no filter at all, but GOOD filters will come very close. Unfortunately, filters are needed or else all the crap (dust, dirt, squirrels you just ran over) might get sucked into the engine and destroy everything.
Of course, that is just one of many things that affect volumetric efficiency. Other things include throttle bodies that are too small, intake manifolds that are made poorly, valves that are too small to draw enough air into the cylinder before it closes again, stock camshafts with poor durations, etc etc. That is why people get larget throttle bodies, polish (or replace) their intake manifolds, port and polish engines heads and get new camshafts with more aggressive grinds that increase lift and duration.
After air is drawn into the engine, the next step is Compression. As stated above in the steps, this is done to increase the density of the air. When this happens, a large amount of air and fuel occupies a small area. When lit, this creates a much larger and more powerful explosion that pushes the cylinder down with more force, thus turning the crank, which goes into the transmission and then finally to the wheels.
So the engine itself knows that denser air leads to bigger bangs. Why not help it? This leads to why running during a cold night makes the car feel faster, and why cold air intakes are such a big deal. Let me explain.
Air is around 14.7 psi, but remember, this is just how much air is pushing onto you (it's hard to realise this since we're so used to it, but jump into a real deep swimming pool and you can feel the pressure). By itself, pressure is not a great number. Air is only about 23 percent oxygen, and oxygen is what we really care about. Based on that we need to figure out how *dense* air is at any given point. The density of air will let us know the concentrations of oxygen within the air.
Without going into too much detail and making this article on the physics of gases, just bear with me and know that two main things affect density of air: Tempurature and Humidity.
Tempurature: The higher the tempurature, the less dense air is.
Humidity: The water content in the air is taking up space that should be occupied by more oxygen rich air.
We can't do anything about Humidity, so if you live someplace like the amazon, sorry, you'll just have to move or deal with it.
We can do alot about Tempurature though. Besides trying to race on very hot days, the other major culprit that robs air of density is the engine bay itself. Exhaust gases can reach 1400 degrees Fahreinheit (sometimes higher depending on things like turbos and bad tuning). This warms up the engine bay extremely fast, where the car's filter is. The filter is then drawing that heated air into the engine. General rule of thumb: about every 30 degrees F increase in air temperature, you lose about 1% potential horsepower your engine could be making. As you can see, that adds up really quick.
Solutions? Cold air intake for one. It will draw air from a spot that's away from the hot engine bay. This keeps the air charge cooler. There are also ways of preventing the hot exhaust gases from entering the engine bay. Ceramic headers and heat insulating wrap would work. Reflective heat wrap on intake piping could also be used.
This cooler air also has a secondary affect that is not immediately noticable. Cooler air going into the engine cylinders will help cool down the pistons and block, preventing detonation and overheating which could melt engine parts.
Well, that's it for now. Remember this is only a quick summary and overview.
******
Let me know what you guys think, or if any of your gurus out there see any errors or can clarify something better.
Tia for your input!  
16.1 @ 87.11
http://www.virtuallyinfamous.com
ARD 57mm tb and p&p manifold, Neuspeed STB, Pro-kits, Illuminas, Injen CAI with K&N, 2.5" pacesetter catback,
AF/X UDP, Rear STB, Powerslot rotors, RRE SS Brakelines.
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