Volumetric Efficiency - WTF? (Stuff you should know)
Posted: Tue Aug 31, 2004 3:39 pm
If I said "volumetric efficiency" I'm guessing that a few people would be asking "WTF is it?" 
After doing a bit of reading and some calculations today I came across some interesting information which may be helpful to some of you.
In simple terms "volumetric efficiency" (VE) is the measure of the capacity of your engine vs. the actual amount of air that gets delivered to it. So why is this important? Because you're engine is probably not running as well as it could and there's things you can do to improve it.
From my reading it seems that most normally aspirated (NA) engines have a VE of 80% or in other words they are only getting the equivilant amount of air of an engine that is 80% of their size - e.g. a 2L engine which is only getting 80% of the air it can handle is like a 1.6L engine which is getting all the air it needs. Why on earth would you run your FTO with the equivilant of a 1.6L engine in it when you could be taking advantage of the whole 2L?
So why don't NA engines achieve a VE of 100%? The main reason for this is that NA engines rely on air being "sucked" through the intake rather than being forced in to it like turbos. Due to inefficiencies and restrictions not all the air that the engine wants is able to be delivered to it and as such there is a difference between its capacity and what it receives. In the case of a turbo where air is being forced into the engine, the VE is much higher giving it the equivilant amount of air that would be used in an engine of a much larger size - e.g. a 2L engine which received 1.5 times the amount of air would be like a 3L engine with a 100% VE or a 3.8L with a VE of 80%.
The first step to improving VE is to increase the availability of air and decrease any air flow impediments. The two obvious things to do here are to increase the surface area of the air filter and lower the restrictiveness of the piping that connects the air filter to the engine. A pod filter is a great option as it offers a larger surface area over panel filters.
Another factor to consider is the density of the air that is being sucked into the engine. Even if we could supply a 2L engine with 2L of air, if that air was only half as dense then it would be like only supplying 1L of air (make sense?). The density of air is related to its temperature, the colder the air the denser it is. From tests that others have done, the ambient air temperature in the engine bay of the FTO, as it is being driven, can get up around 65 degrees, working on the basis of the outside air temperature being 25 degrees, the difference in density of the air is around 14%. A difference of 14% in air density can equate to around 1-2 PSI or around 5-10HP!
Based on this, it makes a lot of sense to replace the stock air intake, panel filter and air box with a larger and less restrictive air intake pipe, a pod filter which has a larger surface area, an improved air box and a CAI to provide lower temperature air at a higher density.
By doing these mods you will be able to get your engine closer to 100% VE, and improve its power output. However, if you have an air box which is able to be pressurised and if you have a CAI which is facing forward, then air will be entering the air box above normal atmospheric pressure providing air of an even higher density which in turn could mean that you are able to achieve greater than 100% VE, in turn making your engine operate as if it had a larger capacity.
Using two (or more) CAI feeds would further this as you'd have double the volume of air entering the air box.
If after reading this, you're still saying "WTF?" don't worry as you're probably not alone.
After doing a bit of reading and some calculations today I came across some interesting information which may be helpful to some of you.
In simple terms "volumetric efficiency" (VE) is the measure of the capacity of your engine vs. the actual amount of air that gets delivered to it. So why is this important? Because you're engine is probably not running as well as it could and there's things you can do to improve it.
From my reading it seems that most normally aspirated (NA) engines have a VE of 80% or in other words they are only getting the equivilant amount of air of an engine that is 80% of their size - e.g. a 2L engine which is only getting 80% of the air it can handle is like a 1.6L engine which is getting all the air it needs. Why on earth would you run your FTO with the equivilant of a 1.6L engine in it when you could be taking advantage of the whole 2L?
So why don't NA engines achieve a VE of 100%? The main reason for this is that NA engines rely on air being "sucked" through the intake rather than being forced in to it like turbos. Due to inefficiencies and restrictions not all the air that the engine wants is able to be delivered to it and as such there is a difference between its capacity and what it receives. In the case of a turbo where air is being forced into the engine, the VE is much higher giving it the equivilant amount of air that would be used in an engine of a much larger size - e.g. a 2L engine which received 1.5 times the amount of air would be like a 3L engine with a 100% VE or a 3.8L with a VE of 80%.
The first step to improving VE is to increase the availability of air and decrease any air flow impediments. The two obvious things to do here are to increase the surface area of the air filter and lower the restrictiveness of the piping that connects the air filter to the engine. A pod filter is a great option as it offers a larger surface area over panel filters.
Another factor to consider is the density of the air that is being sucked into the engine. Even if we could supply a 2L engine with 2L of air, if that air was only half as dense then it would be like only supplying 1L of air (make sense?
). The density of air is related to its temperature, the colder the air the denser it is. From tests that others have done, the ambient air temperature in the engine bay of the FTO, as it is being driven, can get up around 65 degrees, working on the basis of the outside air temperature being 25 degrees, the difference in density of the air is around 14%. A difference of 14% in air density can equate to around 1-2 PSI or around 5-10HP! Based on this, it makes a lot of sense to replace the stock air intake, panel filter and air box with a larger and less restrictive air intake pipe, a pod filter which has a larger surface area, an improved air box and a CAI to provide lower temperature air at a higher density.
By doing these mods you will be able to get your engine closer to 100% VE, and improve its power output. However, if you have an air box which is able to be pressurised and if you have a CAI which is facing forward, then air will be entering the air box above normal atmospheric pressure providing air of an even higher density which in turn could mean that you are able to achieve greater than 100% VE, in turn making your engine operate as if it had a larger capacity.
Using two (or more) CAI feeds would further this as you'd have double the volume of air entering the air box.If after reading this, you're still saying "WTF?" don't worry as you're probably not alone.