About Aftermarket Air Intake Systems

13 Oct 2018
by Damien Devillian
NOTE:
If you're the kind of user whose reason for getting an air intake is just the sound, this article isn't for you.
You honestly can just take a sewer pipe and wrap a piece of rag at the end to achieve that objective.

This detailed article explains the different aspects of an Aftermarket Air Intake System from materials to design, and warns how a cheap air intake system can lead to power losses.

Introduction - Myth and Benefits


Just read a few articles on a certain site. I was quite angered to see how oversimplification, and omission of facts can lead to a completely misleading article, without actually being a lie.


I will attempt to explain some common phrases uses to sell you on air intake systems (or not):



1) More Air = Better Combustion = Better acceleration.


While this is broadly true, it gives the impression that the volume of air is of importance and that as long as can get more air into your engine, the more power you gain - leading people to only consider aspects like the size of the filter and tube.


In reality, oxygen is the main component, and a key design consideration of any good air intake system is to keep the air as fresh, and as dense (cold) as possible, while increasing the air flow.


Another over-looked consideration is that performance gained from Air Intake Systems, have an upper limit - your engine cylinder volume. No matter how much air you force down the intake, your engine can only draw in so much in each cycle.



2) Air Intake Systems improve your Fuel Economy.


This seems to be a perpetuated myth that just keeps going year-on-year. I don't doubt this might have been true at some point in the past - but it is rarely true for any modern vehicles.


Modern vehicles have a combustion monitoring process - which constantly adjusts the fuel injection to ensure optimal combustion. This is driven by both stringent emission controls, and competition among manufacturers to improve engine efficiency. These tiny adjustments are made by sensors constantly monitoring both the inflow and exhaust gas levels, and also matches individual driving style.


Another consideration when it comes to Air Intakes and Fuel Economy - fuel generates the power. The combustion process is essentially a chemical reaction that is proportionately balanced. A perfect combustion is where all the fuel and air has interacted. It is impossible to conceive being able to get more power because you're getting more air, yet somehow using less fuel.



3) Power losses in low-end RPMs.


This seems to be a catch-all phrase but is a little more complicated with a lot of varying factors, but we will attempt to summarise as best we can. Also this assumes you have a well designed and properly manufactured air intake system to begin with.


There are a lot of explanations as to why some air intake systems can lead to a low-end power loss, with most varying between vehicle models.


One explanation is that with some air intakes, the amount of available air ready to be used when you press the accelerator is different. In shorter intakes, you have a lesser amount of air in your tubes before fresh ones needs to be pulled through the filters.


If moving off in stationery, another explanation used for loss of low-end torque, is that the air being drawn into your intake is hot air from the engine bay. So this hot air leads to a reduced power until your vehicle starts moving and fresh air is pushed through.


Another factor that seems to contribute to power losses in certain RPMs, is the change-in-pressure wave that resonates through the intake piping when your throttle body opens/closes. The best way to imagine this, is by using a highway traffic analogy - "imagine a line of cars driving on the highway, and the first car presses the brake to avoid a collision. The car behind then presses the brake, then the car behind that, and so on. This braking takes place through the entire chain of cars. By the time it reaches the end of the chain of cars, the first car is already back to travelling the normal speed". So the condition changes, but the change-of-condition information takes some time to travel.


Because these variables are so unique to each vehicle setup, it is not possible to confirm nor dispel this understanding, but we can raise a few other factors to consider.


The first is the power gained versus the power loss. If you gain 10% overall, but lose 3% in a specific RPM, it would still count as a performance enhancement.


The second thing, is that dyno charts don't convey the information of time. One of the benefits of a well-engineered Air Intake system is that response time. So even if you had a power loss at 2,000rpm, you need to factor in how much faster to reach that rpm compared to the stock intake, and also for how much less time you stay in that rpm.


And now that I've vented my frustration on those articles, I can move forward to explain how to choose an Air Intake System.



4) Heat


Having touched on power losses, it is only appropriate to talk about Heat before we get into the Air Intake Systems. Everyone knows that air temperature is a big factor when it comes to power - and is a consideration when it comes to choosing an Air Intake System. However, most don't seem to realise how big a difference it makes.


The above graph is charted specifically for Singapore - but can apply to any hot & humid climate. Showing how the density of air is increasingly less dense the hotter it gets, and this effect is multiplied the higher the humidity.


The density difference between getting fresh air from the outside of the engine and the air within the engine bay of a stationary vehicle is almost 30%. This means there's 30% less oxygen to burn, which is directly relational to making 30% less power.


Research & Development


The first consideration for any product, should be the company behind the brand. A company with outstanding products, is one that invests in skilled engineers, technology and research & development.


A properly designed Air Intake System should go through a thorough design process, that involves looking at the vehicle for hidden potential that can be utilized, and then testing prototypes for real world benefits.


This process is often repeated through multiple cycles, and only reaches the production stage when everyone is happy that the Air Intake System is the best version it can be.


The production process can either be done in-house or by a third party fabricator. A company with an in-house production department tend to have an in-house QC process to monitor the parts being produced, and easily make changes if anything needs to be improved.


In comparison, a company that uses a third-party fabricator usually get the parts manufactured in batches, and any faults are only corrected in the next batch. Another consequence of using a third-party is that parts are often tried to fit multiple applications to reduce cost - which can lead to having to make compromises.


Off-The-Shelves & Universal Intakes


These types of Air Intakes, are basically the opposite of one that undergoes proper Research & Development. Little to no testing goes into these Air Intake Systems, and you are more likely to lose power or cause damage to your engine.


Air Intake Systems made of Off-The-Shelves parts, are easy to identify, because they come in multiple sections, which are mass produced for the cheapest manufacturing cost. The "design process" of these intakes is basically just grabbing different combination of sections of the shelves to see what fits your vehicle, and then sell it off as an Air Intake System. When they haven't got a properly sized section, they are replaced with silicone or flexible hosing.


If your intake also comes with an extendable flexible hose, this will lead to some of the poorest airflow. Each ring of the hose causes turbulence - going against being a performance intake, which requires a smoother airflow.


In the image above, the blue intake is also a clear example of how an off-the-shelf intake can be damaging. In the design, the Airflow sensor is positioned just after a bend. In air/fluid dynamics the flow just after a bend is turbulent, and never produces an accurate measurement of flow.


The Materials - Filter Element


Now that you know to avoid any manufacturers who don't use any form of R&D, let's start looking at some variables to consider when selecting your Air Intake System.



Paper Air Filters



Paper Air Filters are mostly an OE/OEM part that comes with your vehicle. They offer excellent filtration, and though historically may have been restrictive, they have come a long way to provide sufficient airflow for daily driving.


However, if paper air filters are included with an aftermarket Air Intake System, it is best to avoid them. Although the Original Equipment paper filters are built to international standards, aftermarket ones are usually chosen for the low cost. This could have less desirable specifications of porosity or fibre structure.



Foam Air Filters


Foam Air Filters are almost only found in cheaper aftermarket Air Intake Systems. Though technically washable & reusable, foam air filters are never meant to be long-lasting. With the conditions under the hood, they will surely degrade, and are more likely to start degrading from the side closest to your air intake pipe. So by the time you've noticed that your filter is falling apart, it is almost certain that large parts have already been sucked into your engine.



Cotton Air Filters


Cotton Air Filters are the choice of almost all aftermarket Air Intake Systems, because it works the best for providing filtration and improving air flow. Unfortunately, this popularity has also led to cheap variants being sold as a "good" air filter.


As with anything, the material of the cotton used and also the design plays a big part. A good quality cotton air filter, will be made of multiple layers of cotton, with each layer having progressively smaller particle filtration. They should also have a fine mesh that helps to stop large or sharp particles from causing damage or ripping the material.


Poorly made cotton air filters can be made with non-automotive grade cotton, be of a single layer, or not have sufficient filtration for small particles.



Metal Air Filters


Metal Air Filters may not be the most common, and although most people swear behind them being of equal or better quality to cotton ones, I personally have my doubts - due to limited information being available to their manufacture process.


One of the concerns I have, is that industrially, metal air filters seem to be used exclusively for separation of air from liquid particles - like grease. This could be an indication of insufficient filtration for small particles.


Another concern I have relating to the manufacturing process, is that unlike paper and cotton filters that are pleated at the edges, metal air filters are curved - which makes the metal wires stretch and increase the pore size at the edges.


So with little information, the best I can do is speculate and as a personal opinion, I wouldn't feel confident in metal air filters. If anyone has more information on the process used to manufacture metal air filter, do write to me and I'll update this article with more information.


The Materials - Intake Housing and Tube



Metal


Polished

Wrinkle-Black


Metal Air Intakes are often the cheapest option. With metal being easy to work with, it is usually the first Air Intake Systems to be made available when a new vehicle model is launched. Being easy to work with also reduces the production time and cost.


The biggest downside to a metal intake, is heat soak, due to their high heat conductivity, Within a short time of being in a hot engine bay, the intake pipes will get hot, and heat up any air flowing through it.


Some Air Intake Systems are offered with a black colouring, which provides an amount of heat resistance. While a darker coloured tube may take a longer time to heat up than a polished tube, it also retains this heat for longer compared to a polished finished. A material that is good at absorbing heat, is just as good at dissipating it.


An extra step to reduce this heat soak, is the wrinkle-black finish, which gives the metal a "bumpy" feel. These bumps increase the surface area of the metal, making heat dissipation quicker.


Another downside to metal, is its expansion, which can lead to stresses at the connection points.


A poor/cheaply made metal intake will use a rusting variety of metal, and not take into account expansion of the metal under heat. A poor/cheaply made metal intake will also use a spray painted wrinkle black finish, instead of a powder-coated wrinkle black finish. Spray painted finishes are very likely to flake, and scratch off.



Silicone


Silicone has really good heat resistance and chemical resistance making it a good material to have as an Air Intake.


One of the downsides of using Silicone as a material, is it's lack of rigidity, meaning it's shape can change under pressure.


The other downside from making it the ideal material, is that part of the production process is hand-made, and requires a skilled worker. Lack of experience in this process could also mean inconsistencies between different batches.


A poor/cheaply made silicone intake, will not have an even layering of the silicone sheets when made by an unskilled worker. They also will not have different layers that have different properties.



Plastic (XLPE)


Since plastic comes in a lot of variety, for this comparison, I will only be talking about Cross-Linked Polyethylene (XLPE) which is the choice for automotive grade ducts.


With a high heat resistance, chemical resistance and rigidity, XLPE Air Intake Systems are the ideal material for an Air Intake.


Being rigid and non-expanding also means XLPE Air Intake Systems can be made larger.


One downside to XLPE, is that the production process requires a moulding stage, which makes it more expensive than metal or silicone.


A poor/cheaply made XLPE intake would not be made of proper thickness. It can also be poorly finished, leading to burrs on the inner tube wall which can easily break off and travel through the intake tube. There's also the possibility that a cheap made product made not even be real cross-linked polyethylene.



Carbon Fibre


Carbon Fibre is possibly the best material for an Air Intake System with excellent heat resistance, strength and being lightweight.


The only downside to a Carbon Fibre Air Intake System is the cost of the material, and the production process which makes it the most expensive option.


This high pricing for Carbon Fibre, has also led to a whole industry of poorly made or fake variations. There are so many ways that corners can be cut to produce these cheaper yet still sell at a high price.


The most deceptive way to produce these cheaply, is using fibreglass for all the underside layers, with only the last layer being made of Carbon Fibre. These types of air intakes are often sold fraudulently - by outright lying and calling it a Carbon Fibre Air Intake.


Other methods of poor/cheaply made Carbon Fibre is by either using poor quality Carbon Fibre fabric, or by using an inferior production process. A well made Carbon Fibre uses a vacuum-forming and baking to cure. Carbon Fibre that are not vacuum-formed or baked, can lead to layers peeling apart.


Design - Air Filter Shape


These calculations show that the smallest change in shape or dimensions of the Air Filter, can have a big difference in the surface area. It is clear that the best shape for maximum filtration surface area is the inverted top design.


These calculations only reflect the actual area of the filters, and do not include the top/bottom, since they do not add filter the air.


The inner cone of the Conical w/ Inverted Top filter here, is taken to be 2cm less than the outer cone.


Design - Open vs Sealed Housing


We are naturally drawn towards an open Air Intake System, because it gives this illusion of being capable of producing more power. Open Air Intake Systems also add to this illusion by being louder.


While this may be true when you first start driving, this open design could instead lose up to 30% of the power as you drive continuously and the engine bay heats up - as explained previously.


The only positive side to having an open Air Intake System, seems to be easy access to remove your filter when it comes to maintenance.


For performance, open Air Intake Systems would only be an improvement if you are using it in short bursts with time to cool in between - like drag racing. In a longer sporting event, or as a daily driver, this layout could instead be your downfall.


Design - Long vs Short Intake Tube


As explained, the short intake tube within the engine bay, is not ideal for a daily driven vehicle that wants a performance improvement.


As far as can be told, the only improvement a shorter intake tube will provide, is in the short period of time when the throttle body position changes - but this needs to be balanced against the constant power-robbing hot air that is being drawn into the engine.


In this comparison, the assumption is that the intake tube is made longer for the purpose of being positioned outside the engine bay. With this assumption, it is safe to say the a longer intake tube will be more capable of providing constant performance improvement.


Design - Airbox Position


The importance of supplying the engine with fresh and cool air has become quite evident throughout this article - making it the primary requirement of a good Air Intake System. As such, the position of the airbox becomes an important factor.


Most car manufacturers these days, recognise the importance of the air supply, and even family sedans are being designed to cater. Most of them have a built-in inlet that supplies the air to the air induction track from the outside. A good Aftermarket Air Intake System, will make use of these inlets as well - increasing their ability to provide clean air to the engine.


Another way an Aftermarket Air Intake system can improve their supply of air, is through the use of scoops, or directional vents. These should be taking the air from the front of the engine, allowing them to scoop fresh air that flows through as the vehicle is moving forward.


The airbox itself, is best positioned as far away from the exhaust manifold, as this becomes the hottest part of any engine. It should also be placed as forward as possible, to access fresh air.


Summary


Hopefully this article has helped improve your understanding of the Air Intake System. If anything, I hope it at least lets you understand that an Air Intake System - though simple, is far more complicated to get right and be an actual performance improvement.


If you have any suggestions or comments, please feel free to leave it below, or just write to us.

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