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Air Filter Calculator – Calculate air filter size for BHP/Power

Featured image of an air filter on a page that contains a calculator that gives the required air filter area for a certain horspower

This air filter calculator will tell you the approximate filter size you need for your engine’s power.

What we are doing

This calculator has two steps

• Find out the filter area you need
• Enter the dimensions of your current air filter to see if it is bigger or smaller than the area you need (it will also help you choose a filter of the correct size)

• RPM of Peak Power
• Redline RPM
• Horsepower of the engine
• Engine Capacity in CC

Now we know the air filter size the engine needs we can see if the filter you are using is big enough and if it isn’t, what size of air filter you need.

Cone Filter

If you have a cone filter, enter it’s dimensions here

Panel Filter

If you have a flat air filter eg you are using the standard air box, enter the filter dimensions here

Examples

How do the numbers from the calculator compare to the actual filter sizes?

 Car Engine CC RPM (Peak Power) BHP ActualFilter Size CalculatedSize 911 Turbo 3.8 2016 3800 6400 533 120sqIN 228sqIN Audi S3 2018 2000 5500 310 80sqIN 154sqIN Mini JCW 2011 1600 6000 208 79sqIN 95sqIN

For the Audi and the Porsche the calculated required filter size is around double the actual filter size. But the Mini filter calculation is close.

Why the difference?

The formula used in the calculation says there should be one square inch of filter area for every 2.5CFM of airflow. The goal of this 2.5 number is to have the absolute minimum restriction for a long period of time. It could be too conservative.

In our calculator we have already reduced the 2.5 number by 20% but perhaps this is not enough.

Factory Tolerances

From the three examples above it seems that the tolerances between manufacturers can vary by a lot. BMW/Mini seem to want to fit a large filter, perhaps to extend service life of the filter, whereas for VAG, service life could be a lower priority.

The size of the factory filter could even come down to filter placement and the ducting. If the factory airbox receives a lot of dirty air, then the filter size maybe larger to cope with the increased dirt it will be exposed to. For a filter with average or restrictive ducting the filter will be exposed to less dust so can be smaller.

Peak VE – Peak RPM

Another reason for a difference between the calculated required filter size for a stock engine and the actual filter size is that the formula is assuming we are getting peak VE (Volumetric Efficiency) at redline.

We do this to build in some headroom for modified engines.

If you do not want the calculator to do this, set the RPM of peak power and the redline RPM as the same.

Interpreting the calculated filter size.

If you see that there is a big difference between the stock air filter size and the calculated filter size for the stock bhp, I repeat, for the stock BHP, note down the difference and use this in your calculations for the modified engine.

For example

The Audi S3 above. The calculated filter size is around 200% the size of the actual filter. When you use the calculator to find the size of filter required for 400bhp, for example, multiply the calculated filter size for 400bhp by 0.4 or 0.3 to get a more accurate number.

About air filter sizing in general

You always want to use the biggest air filter possible.

The benefits of a bigger air filter are legion

• More airflow
• Longer service life ie does not need replacing so often.
• Better filtration. The bigger the surface area, the slower the dirt hits the filter helping filtration

There are no negatives to fitting a huge air filter except maybe weight

If you are using the car’s standard airbox, there is nothing you can do about filter size but if you are choosing an aftermarket cone filter you can choose the biggest filter that will fit in your engine bay.

This calculator takes into consideration the service life of the filter, when it gives the filter size.

When an air filter is new it flows more air than when the filter has been used for some months.

We want to be sure that even if the filter has been used (ie it has been slightly clogged) the area is still large enough to flow the air the engine needs.

It is less than ideal to have an airbox that is big enough for the air flow requirements but only when we are using a clean, brand new filter.

This air filter size calculator builds in some headroom to take into consideration the reduced flow capacity of an air filter that has been used for some time.

The numbers are approximate.

Different manufacturers will use different paper filter material, they will use pleats of different thicknesses and different manufacturers will use different pleat densities.

Ultimately, the figures this calculator puts out are approximate.

Why use this calculator?

You have increased the power of your engine and you are still using the car’s standard air box. Find out if the standard air filter is big enough.

You are looking for a performance cone filter for your engine and you want to know what size you need for your engine power.

Sources

To calculate the size of the filter required we have used data from The State of Louisiana, Department of Natural Resources. They have given a number of 2.5 cfm per square inch of filter for a HVAC application.

To calculate and engine’s volumetric efficiency, we have used this website

A fantastic air filter test was carried out by a Duramax enthusiast under lab conditions. You see read the full test results here.