Why airfilters are important

I am often asked what air filter to I recommend and I usually refuse to give an answer as there are so many on the market which may be suitable. However sometimes I see cars with airfilters on which are obviously too small. When I delicately mention this to the owners I usually get a response like:

"Its rated to 200bhp so it'll be fine"

"Its from a XXX (Insert performance petrol car here) so it'll be fine"

Well sometimes it will be but sometimes it won't because contrary to most peoples thoughts diesels require MORE air than a petrol engine to make the same power. Even though a petrol engine revs higher diesel turbos still often require more air than their petrol equivilent.

Lets do a quick example to prove a point.

First a few assumptions we have to make
1. Each engine is 100% volumetric efficient. (This isn't true but it makes the maths easier - diesels and turbo diesels in particular are usually more volumetrically efficient than petrols).
2. Both engines idle at 800rpm.
3. Zero boost at idle
4. Petrol engine revs to 6500rpm
5. Diesel engine revs to 4500rpm

A 2.0l diesel engine at idle:
1 litre per engine revolution (as it is a 4 stroke engine and effectively at wide open throttle all the time).
Therefore 800 litres per minute or 13.33litres per second.

A 2.0l petrol engine at idle:
Lets say the inlet plenum is at 10inches of mercury (more usually a lower pressure) which equates to 0.3 atmospheres roughly. Therefore 1 litre per revolution but at 0.3 atmospheres equates to 240 litres per minute or 4 litres per second.

So at idle the diesel engine consumes 3 times as much air as the petrol engine.

Ok now lets take the maximum rated speed and wide open throttle and do the same again.
Assumptions as before

A 2.0l diesel engine at full revs/throttle:
Again 1 litre per engine revolution
However now at 1.2 atmospheres (17psi) above atmospheric pressure therefore 2.2 atmospheres absolute, i.e. 2.2 litres of air per engine revolution.
Therefore 9900 litres per minute or 165 litres per second.

A 2.0l petrol engine at full revs/throttle:
Again 1 litre per engine revolution. No turbo so therefore air is at atmospheric pressure at best. Therefore 1 litre per engine revolution.
Therefore 6500litres per minute or 108.3 litres per second.

Therefore the diesel engine requires about 1.5 times the air that the petrol engine does at full throttle.

So as you can see a turbo diesel engine of the same size as a NASP petrol engine needs a LOT more air.

This is why diesel air intakes and exhaust are usually bigger than the petrol equivilent and a petrol filter rated at for example 150bhp may not be sufficent for a tuned diesel.

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