no questions on horsepower? ok then. ill just move along then.....

VOLUMETRIC EFFICIENCY
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Ok so as far as Performance goes, volumetric efficiency (VE) is probably the most important thing IMHO.
your probably wondering what it is.. its a pretty simple thing really. its basically, its a measurement or even a
ratio of how much air is going into an engine, vs the size of the engine. for example...
say you have a four stroke 2.5 liter naturally aspirated engine. (sounds familiar eh?) we know that for all the cylinders to go thru all 4 cycles it has to rotate 2 times. so, when the valve of one cylinder opens and the piston travels down the air rushes in because the piston is making a low pressure area so the air flows in. (not rocket science right?) well in what we call 'static' conditions, or perfect world conditions
the amount of air going into the cylinder should equal the "swept" volume. (swept volume is more or less the displacement of that cylinder).
so in a perfect world your engine would take in 2.5 liters ever 2 RPMs at wide open throttle. this would be %100 volumetric efficiency.

BUT.

we dont live in a static/perfect world do we? no.
it used to be that if u had an old V8 like say a small block chevy, you would get %90 if you were lucky, the best race teams could get %110.
now days thanks to computers, DOHC, 4 valve per cylinder heads etc. most cars are about %115. how is this possible? alot of things. such as altitude, air velocity etc. which im not going to get into at this point in time. this also goes hand in hand with static and effective compression ratios. for example. the EJ25D heads flow much better than the EJ22E heads. so lets say you put EJ22 heads on a EJ25D block. your static compression would be about 12:1 but the effective compression ratio is going to go down, because the efficiency of those head are about %95 where as the EJ25D are about %110. i believe (im hoping to find dyno numbers to back this) that the power gains would be very minimal.
thus far, all the dyno numbers i have seen have confirmed my belief. but not definitively because of altitude and dynometer differences...

ANYWAYS, BACK ON TRACK..

so you maybe thinking 'thats great but what does that have to do with performance'
well, any car guy will tell you more air is more fuel, more fuel is more power.
if you were to get a graph that plots your VE and lay it over a graph that has your engine torque,
in most cases they are directly related. why? because in a way, torque is a way of measuring how efficient your engine is using the air it inducts.

now you maybe thinking 'well thats great but im turbocharged/supercharged so i dont need to worry about it.'
and you would be wrong. (this is where things get a little more complicated.)
say you have your little 2.5 and you want to make more power, to do this you will need to increase the air going into the engine.
there is 2 ways of doing this.
flow and pressure.

flow comes down to port design, valve size, combustion chamber shape, basically anything that effects air going into the engine.

pressure is increased by going down in altitude, or adding some kind of forced induction.

unfortunatly the subaru community seems to only grasp the latter of the 2. all the wrx guys have the attitude of 'ill just turn up the boost'
which isnt the best way to do it. consider this.. say you have a small garden hose, and a large garden hose, the larger hose will flow much more water than your small hose. so you have to increase the pressure of the smaller hose. realistically the pressure would have to be about 3.1 times greater to flow the same amount of water. now in terms of cars, there is a law of physics called the 'ideal gas law'. in this law, it says that gas such as oxygen (air) when compressed gets hotter, or more difficult to cool. so by increasing the pressure at a certain point you will start making more heat than power. this is where thermal efficiency comes in (which i wont be covering today) so lets say you have a 2.5 with a turbocharger on it, and you want to make more power, by increasing the flow, the power will be more useful, and made with lower boost levels.

im tired. if anyone has questions, let me know. that was a very basic VE in a nut shell.