Do Induction Kits or High Flow Air Filters really increase engine power.
To understand the principle of filters we can compare an intake system on a normally aspirated 4 stroke engine to our own air intake system. Humans have very High Flow Air intake system with minimal restrictions slowing the air intake. We have hairs in our nostrils and we have mucus lining our air nasal system to trap or capture dirt and dust, this dirt and dust is removed by Cilia (tiny hairs in the lining of the nasal cavity and trachea) which move these “contaminants” back up to throat, which is then coughed up or mechanically blown out through the nose. Still our lungs are susceptible to dust ingress which long term could damage the lungs efficiency.
In exactly the same way an engines intake system must have some method of “filtering” the air intake to remove dirt and dust. This is even more critical to an automotive engine as it has no way of removing any damaging dirt particles that may reach the combustion chamber and damage the internal parts of the engine and cause premature wear. If the air is not filtered to a high level before the air charge enters the combustion chamber there could be the opportunity for these particles to damage, pistons, cylinder bores, combustion chamber, spark plug electrodes, injector nozzles, piston rings and so on. This could all lead to premature wear or imminent failure of the engine.
Typical “element” air filter
Shown above is a typical “element” style air filter used in most cars. These are adequate at filtering out dirt and dust but by no means the most efficient when it come down to air flow and performance.
What are we trying to achieve with an after market Induction Kit ?
Many claim to improve air flow, many claim to improve BHP or engine power or torque or all of these.
Mmmmm lets think for a moment !
A bit of science first. Cold air is more dense that Hot air so common sense tells us that cold air will have a higher concentration of oxygen particles in the same volume of air when compared to hot air ! FACT.
Therefore forgetting about Performance Filters and spending wads of dosh – consider getting a cold air supply to your intake. If you want to get more “BANG FOR YOUR BUCKS” you need to be feeding COLD AIR into your intake system. If your intake system draws air from under your bonnet you may well be sucking hot air (engine heat, manifold heat, turbo heat etc etc) into your intake a BIG NO NO straight away. Now we have a cold air feed or an intake pipe that is drawing cooler air from outside of the engine bay we can consider filters, filter boxes, etc.
There are many differing types of after market filters available, do a bit of research and take time to make a “judged” decision. Panel, cone, paper element, wire gauze, cotton, sponge are all types that could be found to fit your car or bike or truck or whatever vehicle you are looking for ! So now we have considered air supply, lets look at the filters available – 
Above are a selection of filters you might come across when searching for intake filters By methods of research and testing FLASH CUSTOMS can say from experience that lightly oiled cotton filters offer the best flow rates of any. This is ok but many applications are not covered by these types of specialist filter.
We would recommend for average FAST ROAD use that a modern gauze type filter or sponge filter (panel, cone etc) be used with impregnated dirt re tentative spray, both far better than a standard paper element filter. Only a very slight coating of oil spray will aid filtration. If excess oil is coating the filter this may actually reduce the flow and even worse clog up your MAF sensor (Mass Air Flow) and confuse your ECU, simply making matters worse. Make sure you clean the MAF sensor regularly if this is the sort of intake system you have.
Whatever after market filter you fit, remember one rule of thumb:- the larger the surface area of the filter, the better the air flow.Most after market filters allow for complete removal of the original air box, good and bad arises from this.
- You will generally have far more induction “ROAR”, boy racers may love this !
- Better AIR FLOW can generally be achieved with after market filters.
- Positioning of “open” filters is critical (due to hot engine bay temps)
- A 20 degree rise in temperature can reduce BHP by as much as 3%
- Smaller engines sometimes suffer from a POWER LOSS when fitting “free flowing” after market filters
- Sometimes it is best to fit a replacement “free flowing” after market panel filter in your standard air box particularly for smaller engine sizes.
So as you can see there are positives and negatives to after market filter kits, getting the right one for your car, keeping intake charges “cool”, fitting in a designated “air box” or open is all things that need to be considered.
Filter with cold air feed pipe
Filter positioned at back of engine where lots of heat will accumulate
A shielded Filter set up, reducing the impact of under bonnet temperatures
Simple aluminium ducting that can be used to direct cool air into the filter
At FLASH CUSTOMS we still recommend a specially built induction box to fully surround the after market high flow filter with a cold air feed into the box. This must be sealed to stop hot under bonnet air entering the box and in some instances I have seen customers insulate this box as well!
I have personally seen many young drivers add a simple after market induction kit mounted totally in the wrong place, open to hot air temperatures with no cool air supply and wonder why it makes no difference what so ever to the performance of their car. But, it makes lots of induction roar so it must be going faster ?
With a little bit of thought and not always a lot of money these sometimes quite expensive induction kits can be made to work a lot better and actually give your car some extra performance.
Another “myth” that some kits promote is “RAM AIR” where simply a feed pipe is mounted in the front grill or under the bumper in the flow of air hitting the front of the car. Ram air systems really don’t work up until you reach approx 100mph (so absolutely no use what so ever in normal road driving conditions. Secondly many performance filters tend only to really work at higher engine revs and will in fact be detrimental to the lower end power band.
So personal conclusions to this blog on after market filters is that:- if you have a smaller engined car go with a high flow panel filter to simply replace the original and make sure that you have a nice cool air feed to the intake. For larger engines if you can go with a cotton gauze filter with a cold air feed and an enclosure to prevent hot air getting into the intake.
A final take on this subject, no matter what you do to your intake system to improve it if the intake manifold is rough, and edges and lumps and bumps in it, has mis-matched joins that do not allow “smooth” air flow you will be wasting your time trying to improve your intake system. Get the manifold “gas flowed” and the make sure that every piece of pipework and join is smooth as can be to help air flow.
Here at FLASH CUSTOMS we tend to prefer smooth bore thin wall aluminium tube to make up an induction pipe run, this is much more efficient at allowing air to flow smoothly compared to other types of pipework such as concertina type aluminium or plastic air hosing.
Smooth bore mandrel bent aluminium tube
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Recommended Reading
This work is aimed at the enthusiast engine tuner and race engine builder. Founded on the author’s many years of experience in building, tuning and modifying high-performance engines, it sets out the principles involved in forced induction, supported by tables and numerous illustrations. From basic theory through to building a rugged engine, all the important aspects of supercharging and turbocharging are explained and analysed.
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