Volume vs. Mass: AKA Clearing up a misconception.....
09-04-2002, 11:40 PM
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Volume vs. Mass: AKA Clearing up a misconception.....
I apologize for trying to give new life to a locked thread, but I feel that the subject matter is appropriate here even if the posting members are not. Here is a partial quote followed by what I feel is an accurate response:
The real misconception here is volume vs. mass, or simply put, charge air quantity vs charge air quality. Let's skip back to High School chemistry for a moment and look at the ideal gas law: PV=nRT. Using this equation, it is possible for us to see that a given number of O2 molecules can be happily contained in 300 cubic feet or 1500 cubic feet depending pressure and temperature.
Now, most engine components (intake manifolds, throttle bodies, heads, etc...) are rated in CFM. With proper selection of these components you can maximize the VOLUMETRIC efficiency of your ride. So what. Volume is nothing when compared to density.
Turbochargers are rated in Lb/min. What that means is this: At 14psi, a turbocharger rated at 60lbs/min will cram twice as many O2 molecules into 300 cubic feet than one rated at 30lbs/min. If your engine can only flow 300 CFM, that becomes important because you now have 2x the Oxygen available to burn fuel in that 300 cu.ft. This usually leads to something near twice the power (at the same boost level) if you actually add the fuel with it.
I'll take two helpings of O2 please.
[Modified by B20C5 Turbo, 1:50 AM 9/5/2002]
now i dont know what the figures are, but lets just say that a stock wrx flows 300 cfm at full boost at 5000 rpm. with the T66 it will flow the same on the wrx, but with the T66 on the V10, at 14 psi (i dont even think this is possible, but bear with me) you would be flowing more like 1200 cfm at 5000 rpm. follow me here? so ya the turbo is capable of flowing more at the same psi, but its only going to flow more on a bigger engine (or with bigger cams...). so the only real reason the little SI's engine makes so much power out of so little boost is because of the high compression ratio.
Now, most engine components (intake manifolds, throttle bodies, heads, etc...) are rated in CFM. With proper selection of these components you can maximize the VOLUMETRIC efficiency of your ride. So what. Volume is nothing when compared to density.
Turbochargers are rated in Lb/min. What that means is this: At 14psi, a turbocharger rated at 60lbs/min will cram twice as many O2 molecules into 300 cubic feet than one rated at 30lbs/min. If your engine can only flow 300 CFM, that becomes important because you now have 2x the Oxygen available to burn fuel in that 300 cu.ft. This usually leads to something near twice the power (at the same boost level) if you actually add the fuel with it.
I'll take two helpings of O2 please.
[Modified by B20C5 Turbo, 1:50 AM 9/5/2002]
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