Re: Can someone explain how Compression works?
Originally posted by psYcoSix
[B]Say you got a nice set of new cams for your head, and you already have a CAI/H/E setup, along with a High Flow Cat.
Once the cams are installed you should now be able to intake and exhuast more air, right?[/B]
Yes, you are now bringing in more air because aftermarket cams have larger lobes (means valves open further) and longer duration (means the valves are open longer). Of course you are only able to pull in what the other parts of the engine will allow…size of ports, throttle body…all kinds of other stuff will affect how much you are pulling in -but i’m sure you knew that.
Originally posted by psYcoSix
So you are actually adding more air into the combustion chamber, then was in there before, how does that work? Is that where the compression ratio comes into play? Will that give you a higher CR? I understand that other things play a role in CR such as Head gasket thickness, etc…
You’re getting quite technical now, and i believe you’re sorta thinking yourself into a hole…its best not to think about it like that. yes, you are bringing in more air (volume and molecules), however when the piston is at its lowest point and its highest point the volume of the cylinder is still the same as before you added the cams. Since compression ratio is volume of cylinder/combustion chamber volume…you have not increased compression. However you have increased how many molecules are in that same amount of volume, which means they are closer together when compressed…which is what happens when you raise compression (this happens w/ just about any power adder. remember, your motor is a big air pump, all you’r trying to do is get more air in and out more efficiently)
So, again, compression ratio is simply:
(cylinder volume + cylinder head volume) / (cylinder head volume) Unless you change those dimensions you are not changing the compression ratio. Things that will affect CR: pistons, stroke, bore, size of cylinder head volume, thickness of head gasket)
Originally posted by psYcoSix
[B]Also after you add more air you will be running more lean than before right? So that would mean you would need a Fuel Systems upgrade with a Regulator, to compansate for the added air, right?
So in the end are just actually raising your CR? Or did I totally fu(k this thread up???[/B]
I’m not super sure about the leaning out thing. I’m pretty sure your ecu compensates for most of this, however yes, you do reach the point where you will need to upgrade the fuel system.
and like i said before, the cams don’t raise the compression ratio.
Something you should do some reading on is “volumetric efficiency” VE is the percent of the total possible volume that the motor actually utilizes. For example, say you have a 1.0 liter motor…in its factory state it of course is not perfect, so you may only be drawing in 0.75 liters of air, the rest is vacuum (since air is a gas it will just expand to fill the full 1.0 liter due to the drop in pressure created by the vaccum…but the number of molecules will be the same as the original 0.75 liters of air at the normal pressure). This motor is said to have a 75% volumetric efficiency. By opening up passage ways like ports, tb’s adding cams… you are allowing the incoming air to pass through smoother and faster, which allows you to draw in more air, thus bumping up your VE.
as a couple side notes:
-one way of getting power is trying to increase the volume of air that can flow into your motor (VE)
-another method is trying to bring in the same volume, but of denser air (trying to get cold air)
-both the above ways try to increase the actual number of oxygen molecules that get into the motor, they just do so in different ways. A third way to increase power is to increase the compression ratio. When you increase compression you don’t actually add more molecules (you’re not drawing in more air, and you’re not drawing in denser air). What is happening is that you are simply compressing the air/fuel mixture a lot more before it is ignited than you were originally. By compressing the air more it changes the energy state of the molecules, the pressure on them is increased. By using energy from the motor to compress them more, when you ignite the air/fuel you get more energy back out. More Energy In = More Energy Out. You’d have to understand some chemistry and physics to fully understand how it works. Its been awhile for me so i forget some of the details and specifics.
hope that helps
