Volumetric Efficiency – Calculate your engine’s VE (Volumetric Efficiency Here)
For a normally aspirated engine making 60bhp per litre use a figure of 70
For a normally aspirated engine make 100bhp/litre use a figure of 90
Turbo cars with 1 bar boost pressure, use a figure of 200
Turbo cars with 2 bar boost pressure, use a figure of 300
Using K&Ns recommendations from this page, for airflow of 12,000 litres per minute, the recommend filter area is 596 sq/cm
Bring those figures down to something more usable.
For 1000 litres per minute of airflow, filter area should be a minimum of 50 sq/cm
An air filter can never be too big, so lets aim for a figure of 100 sq/cm per 1000 litre/min of air flow.
The bigger the air filter, the better it filters the air and the less it needs to be cleaned. We are also adding some area to account for the area around the flanges and also to account for the use of different materials ie foam & paper.
We are also adding some headroom. We are also taking into consideration the performance of the air filter when it has been used for sometime ie when it has absorbed dust and is not flowing as much air as it did when it was new.
Finding the CONE air filter
So the next formula allows you to calculate the surface area of a filter when we knows its height, its base diameter and its top diameter. This is for round cone filters.
If you are using a panel filter you can use this formula
Air Filter Size Calculator – Why do I need to know?
- You are still using the factory airbox but you are using bigger cams/bigger turbo/more boost pressure. Is your factory air box big enough?
- Your factory air box is too small and you need to know what size of cone filter you need.
Air Filter Size Calculator – Other Methods
A vacuum gauge can help identify a filter that is too small.
Note the position of the vacuum gauge needle when the engine is switched off.
When you are at high/maximum RPM and full throttle, you want to see the needle in the exact same position.
If the needle is in the same position, it shows your intake system is not a restriction.
And if the needle is not in the same position….. to state the obvious, there is a restriction.
How to identify the restriction
The most obvious and easy way is to remove the air filter housing and see if it cures the problem.
if not, check if the air filter has a bell mount at the base, if not get a filter with a bell mouth, companies like AEM have bell mouths in all of their filters.
If using a bell mouth filter of the correct size still does not cure the problem then you need to start looking at the intake piping itself. Does it have sharp corners, are the hoses compressing under full throttle, what is the inside of the hose like (smooth/rough), can you shorten the intake path, is the filter able to pull in sufficient air (ie is the filter jammed up against the side of the engine bay)
In a perfect world you want a direct run into the intake manifold from the engine but for packaging purposes this may be impossible.
On cars with MAP sensor, you may also be limited in what you can do with the intake piping. To make an engine idle properly, the volume in the intake after the MAF sensor needs to be limited. If there is too much air volume after the MAF sensor, the air may not move smoothly across the MAF sensor causing idling problems.
In short, you need to be careful changing the intake pipework after the MAF sensor.