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Engine Volumetric Efficiency

December 18, 2017

This is a term that measure how efficiently your motor ingest air through the intake and expels the exhaust gases out the exhaust side. I put this first for a reason. This is a very important term to understand, because most engine modifications effectively change the VE of your motor, and hence why you decreases or increases in torque and power at various RPM's. Mathematically, VE is the percentage of air that the motor ingests and expels vs the total volume of air the motor could potentially ingest and expel. First the easy part. How much can your motor ingest. Lets use the 2.0L FS motor as an example. Its total displacement is 2.0L's, and it has four cylinders, so each cylinder displaces approximately 0.5L's. Now if the motor were 100% efficient at all RPM's it would ingest 0.5L's of fresh air each cycle then expel it. Unfortunately, a 100% efficient motor is impossible. How efficient your motor actually is unknown, but as a general rule of thumb most dual overhead cam engines have maximum VE's around 85-90%. This actual numbers are not important though. What is important is the VE curve, or a chart of VE vs RPM. You VE curve will have the same trend as your torque curve, as measured on an engine dyno or a wheel dyno. So your peak VE will occur at peak torque. Without changing the displacement of the motor you cannot ingest any more air, but you can change the VE of the existing motor. This takes on infinite degrees of modifications, from something as simply as changing the air filter or as complex as reworking the cylinder head with larger valves and ported runners. So we want to change the VE of the motor? Almost all performance modifications will increase high rpm VE while sacrificing some low rpm VE. Recalling earlier statements, this means that the high RPM torque will increase and low rpm torque will decrease, and where torque increase, so does power. Learn this and understand it, as you will hear this again and again. 

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