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Originally Posted by Chevota
It's not quite that simple... Even a carb/EFI mfg will tell you need more than the calculated # at 1.5" vacuum or you're screwing yourself. (We are talking 1.5 right?) A good 50% larger is a nice start  |
Greetings Chevota,
I have to disagree regarding theoretical airflow calculations: it really is a simple matter of physics. That formula is a longtime staple of internal combustion engine physics taught from Stanford to Stuttgart. I think it is mentioned in every ICE design text on my bookshelf, and one of the first taught to automotive engineering students.
If you have an open mind to learning about this subject, please continue reading. The theoretical airflow calculation results in the most *optimistic* value possible under normal atmospheric conditions, based on physical dimensions of the cylinders. I would be glad to recommend engineering texts to anyone who wants to take this subject seriously.
I presented a simplified version of a more accurate equation for determining intake requirements. Here is the original formula version, using the 119.974 for reference:
bore * stroke * pi = cylinder volume
3.79in * 3.35in * 3.141... = 39.88 cubic inches
cylinder volume * # of cylinders = total displacement
39.88 cubic inches * 8 cylinders = 319.04 in^3 (cid)
cubic inches / 1728 = cubic feet (we are calculating cfm)
319.04 / 1728 = .18463 ft^3
displacement (cf) * engine speed (rpm) = airflow (cfm)
.18463 * 5700 = 1052.39 cfm
Looks like the old DangerMouse goofed, right? Wrong.
We still need to divide that value in half because any beginning student knows that a four-cycle engine breathes air every other revolution:
1052.39 / 2 = 526.19 cfm at 5700rpm
Remember the message earlier about volumetric efficiency. Everything about tuning a forced induction setup has to do with increasing volumetric efficiency of the end-to-end system.
Forced induction systems function by increasing the intake charge pressure above atmospheric ('xx psi boost'). The same thing would happen if your naturally aspirated car exceeds some absurd vehicle speed.. air molecules would compress, increasing your effective atm. Back to physics 101, heat is generated by friction whenever air molecules are compressed.
Theoretical volumetric efficiency thus exceeds 100% as engine displacement has not changed, but is brought back to reality because of heat-induced efficiency loss at the intake manifold and supercharger housing. Note that this is the basis for all thermal management via intercoolers... decrease the pressurized intake charge temperature before it reaches the cylinder.
Anyhow, the well-designed street engine (na) realizes 80-85% volumetric efficiency at maximum engine speed.
This means that the 119.974 on a good day is sucking in 526.19 * 0.85 = 447.26 cfm at 5700rpm.
447 cfm is still far below the capacity of a K&N 33-2678 panel filter; any reputable paper element (Hengst, Knecht, etc.) can handle those flow requirements without breaking a sweat. My last car with a six-inch K&N cone filter flowed 405 cfm in good conditions under maximum boost pressure; much lower than the filter rating and stock throttle body limitations with room to spare.
If someone wants to select a free-flow / low-restriction filter for reasons other than performance gains (K&N marketing spin), more power to you. I think you will run the risk of introducing particulate compounds (esp silica) that can eventually damage the cylinder walls.
I also think you run a higher risk of damaging the delicate MAF element with the K&N filter oil. I had to replace mine twice in three years because the wire element was destroyed by filter oil blowby. Just something to consider.
Regards,
-DM