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Into the Intake - Part 1

The complete guide to modifying intakes.

By Michael Knowling

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There are two critical aspects to a modified air intake system - the volume of airflow and the temperature of the intake air. Combine these two factors and we know the mass of intake airflow; it's this weight of air being inhaled by the engine that's the most important factor in determining the power that can be developed.

Our aim - from a maximum power point of view - is to maximise intake airflow (by reducing restriction) and to maintain the lowest possible intake temperature.

How It Works...

Increasing intake airflow and reducing intake temperatures achieve a similar end result - greater combustion pressure and, therefore, torque.

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By enhancing intake airflow, you allow each descending piston to inhale a larger mass of air into its combustion chamber. The greater the mass of inhaled air, the more fuel that can be mixed with it. And the more air and fuel that you get to burn, the greater the combustion pressure.

Lowering intake air temperature - without changing the intake flow volume - also results in a greater charge-air mass. This is due to cooler air being denser and, therefore, heavier.

In addition to increasing combustion pressure, maintaining a cool intake air supply serves to reduce the chance of detonation. Detonation is an unstable and rapidly spiking combustion process that can cause major engine damage; anything that suppresses detonation is very welcome.

Other Factors

Many modern cars use a complex engine management system, which can affect the end result of intake modifications.

If - for example - intake air temperature is increased, the ECU may load an excessive amount of fuel into the combustion chamber. This results in a significant reduction of torque.

Furthermore, if intake temperature goes up substantially, those vehicles equipped with a knock sensor will retard their ignition timing in large steps. This results in a further reduction of torque.

Different Approaches to Modification...

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On factory intake systems, the path to the air filter is usually quite restrictive; in our experience, it's typically 25 percent of the total restriction before the throttle body (or turbo on those engines).

So what can we do about it?

There are two common solutions:

  • Install an aftermarket pod air filter, replacing the entire airbox and snorkel assembly
  • Revise the existing pre-filter hardware

Let's take a look at the pros and cons of each approach...

Installing a Pod Filter With No Other Mods

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Discarding the factory airbox, filter and snorkel and installing an aftermarket pod filter is a guaranteed way to massively improve intake airflow. The only remaining restriction (at this part of the system) is the filter element itself - and that's usually negligible.

There are some major downsides, however.

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Without any other mods, an aftermarket pod filter will be inhaling intense under-bonnet heat. Don't breeze over this fact because, quite often, the increased intake air temperature completely undoes the benefits the freer flowing intake might otherwise give. At the same time, the likelihood of detonation is also increased.

Other potential problems are:

  • Cost (a brand name filter can often cost a couple of hundred dollars)
  • A rougher idle (the flow through the airflow meter can get disrupted)
  • Airflow meter damage (the oil from the filter can be deposited on the hot wire)
  • Noise (induction roar can be excessive)
  • Effectiveness of cleaning (a washable air filter is often viewed as a plus, but getting it to maintain its filtration and flow performance after it's been cleaned is problematical, in fact look below...)
  • Unknown filtration quality
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We should point out the abovementioned filtering problem can be major. You can be certain car manufacturers extensively test the dust filtration effectiveness of their standard air filter; who knows how effective the filtration of an aftermarket product is? The chances are that if it flows better, it lets bigger particles through to the engine. For this reason, it's dangerous to pay attention to 'filter tests' that have been performed solely on a dyno - the filter that makes the best power is unlikely to be the best filter overall.

Don't be completely put off, though - pod filter installations can be quite effective when installed intelligently. We'll cover this in detail in Part 2...

Revising the Existing Path Into the Airbox

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Pre-filter restriction in a standard intake system is invariably caused by there being too many bends and insufficient duct cross-section. Some cars also have their pick-ups placed up close to panelwork (which hinders flow into the intake mouth), while others suck the intake air through resonant boxes (these can be simply tee'd into the intake duct, or integrated as part of the system so all of the intake air has to pass through - that latter is worse).

With so many causes of restriction, you have to be ruthless in your approach to modification.

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Depending on the specific mods you perform, a revised path into the airbox can give terrific gains - even more terrific when you take into account the cost. By enlarging the feedhole into your airbox and running a large forward-facing duct to it (which, again, we'll come to in Part 2) you can eliminate nearly all of the restriction prior to the filter.

In comparison, an aftermarket pod filter will out-flow a revised intake feed - that's because there'll still be a tiny pressure drop across the revised intake (though probably immeasurable), and there's the restriction of the standard filter element and the flow through the airbox. In any case, doing a good job revising the airbox feed should remove half to three-quarters of the restriction previously seen in that area.

While the revised path to the filter doesn't flow quite as well as a pod filter, there are numerous advantages that sway favour back towards it:

  • Revising the path into the filter is cheap
  • Idle quality won't be affected - or certainly nowhere near as much
  • There's no inherent possibility of damage to airflow meters
  • Maintains a high level of dust filtration
  • The engine inhales much cooler air (depending on where you site the pick-up)
  • Induction roar is minimised
  • Near factory under-bonnet appearance - or is that a bad thing?

Obviously, this approach to modification has relatively few problems - we'll cover 'how to' in the next instalment.

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Oh, you might also be contemplating dropping an aftermarket filter insert into your airbox. Chances are you won't gain a thing; based on our previous intake testing, the standard filter element in an un-touched intake system is responsible for only around 3 percent of the total restriction. A drop-in replacement won't make any noticeable difference.

Again, though, the 'advantage' of a washable aftermarket filter might be attractive to you - it depends on the cost of new OE filter replacements versus the initial cost of the aftermarket filter and any necessary washing chemicals. Keep in mind filtration performance as well.

Replacing the Airbox

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If the size, shape and inlet/outlet diameters of your airbox are causing excessive restriction, it's possible to replace it with a freer-flowing unit out of another car.

Without flow-bench testing, it's difficult to guess whether a given airbox offers less restriction than the one you've already got. As a rule, though, the larger the filter area and the smoother the box contours are, the better. Other factors to look for include a bellmouth'd exit (or the whole box shaped to flow into the exit), and really big diameter entrance and exit pipes.

After you've picked out the airbox of your (car's) dreams, make sure you check the price of new OE replacement filters - it's no good picking an airbox if you have to pay an arm and leg for the filter inserts. Go for a common airbox to ensure you can get cheap replacement filters.

The biggest hassle with the new airbox approach is it's often difficult to find a box that fits in the engine bay cavity and has intake and outlet pipes in the right places; a lot of searching is likely to be required. Where possible, one trick is to look for boxes from a later model of the same car: sometimes the manufacturer have made up-dates and come up with an improved design - one that will fit into place in the older engine bay.

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In short, upgrading your airbox is a neat way of doing things - but, unfortunately, it's a case-by-case scenario.

Now you know the pros and cons of the two common approaches to intake mods. In Part 2 of Into the Intake we'll give you the how-to details of installing an aftermarket pod filter and revising the intake to the airbox...

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