Boost PSI vs. VE (efficientcy...)
Thread Starter
TECH Veteran
iTrader: (6)
Joined: Feb 2002
Posts: 4,766
Likes: 0
From: Phoenix, AZ USA
Boost PSI vs. VE (efficientcy...) am I correct in understanding that at WOT, if you have "boost", your actually over 100% VE? (did I say that right...)
here is the way I understand it:
NA engines usually never get past 85% VE...
in order to get to 100% VE, some form of forced induction is used....
"boost" is actually back pressure in the system (more air than the engine can take in)....
SO....if a person were to read .5 lbs boost on their engine, they are actually a little over 100% VE....
is all this right?
thanks,
Chris
here is the way I understand it:
NA engines usually never get past 85% VE...
in order to get to 100% VE, some form of forced induction is used....
"boost" is actually back pressure in the system (more air than the engine can take in)....
SO....if a person were to read .5 lbs boost on their engine, they are actually a little over 100% VE....
is all this right?
thanks,
Chris
Re: Boost PSI vs. VE (efficientcy...) at some boost level depending on the engine, its safe to say yes you can get over 100.. but on an 85% engine you still have to get past the 15% between 85 and 100.. and that will take an x amount of boost itself to overcome.
TECH Fanatic
Joined: Mar 2002
Posts: 1,044
Likes: 0
Re: Boost PSI vs. VE (efficientcy...) The only thing I want to point out is that 100% VE means that you are moving the same amount of air through an engine as is theoretically possible at a given RPM. That typically happens at peak torque.
This is very possible for an NA engine. My 347 Henry moved about 5-10% more air at peak torque (453lbft at 5350) than is theoretically possible.
5350/2*347 = 537.2CFM Theoretical
I ran almost 600CFM at that point. This is the reason that larger carburators may be required than simply multiplying max rpm/2 times c.i.
SC-
This is very possible for an NA engine. My 347 Henry moved about 5-10% more air at peak torque (453lbft at 5350) than is theoretically possible.
5350/2*347 = 537.2CFM Theoretical
I ran almost 600CFM at that point. This is the reason that larger carburators may be required than simply multiplying max rpm/2 times c.i.
SC-
Re: Boost PSI vs. VE (efficientcy...) Exactly SS00Blue said.
If your engine is 100% volumetricly efficient, it will flow the same volume of air as the engines displacement. Since our engines are 4 stroke, if it displaces 346cid total, it will flow about .1001 cubic foot of air through the throttle body per revolution or about 600.7 CFM. If your power adder pushes more air than that through the throttle body, it is technically operating at volumetric efficiencies above 100%. But it's not as simple as that. On forced induction applications, volumetic efficiency will change because those things that used to be upgraded to support increases in V/E directly (cam, ported heads, etc), are now configured to work with the power adder. Now the important ratio becomes compressor efficiency and air density which one results in the other.
If your engine is 100% volumetricly efficient, it will flow the same volume of air as the engines displacement. Since our engines are 4 stroke, if it displaces 346cid total, it will flow about .1001 cubic foot of air through the throttle body per revolution or about 600.7 CFM. If your power adder pushes more air than that through the throttle body, it is technically operating at volumetric efficiencies above 100%. But it's not as simple as that. On forced induction applications, volumetic efficiency will change because those things that used to be upgraded to support increases in V/E directly (cam, ported heads, etc), are now configured to work with the power adder. Now the important ratio becomes compressor efficiency and air density which one results in the other.
