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Into the Intake - Part 2

The complete guide to modifying intakes.

By Michael Knowling

Click on pics to view larger images

In Part 1 of Into the Intake we discussed the basic theory of intake mods as well as the pros and cons of aftermarket pod filters versus airbox changes. Here we'll take you through how to install a pod filter, and how to modify the airbox - properly.

Installation Methods...

How to Pick and Install a Pod Filter

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There are a few key factors when selecting an aftermarket pod filter.

From a performance standpoint it's best to buy a filter that has the largest element area - for a given filtering material, the greater the element area, the less airflow restriction. The shape of filter is largely irrelevant - so long as it fits in your engine bay, it's fine. There are whispers about a conical filter aiding flow though the centre of the intake opening, but we're yet to see anything proven.

The particular brand you run with comes down largely to personal preference, but it's a good idea to chat with various people who have used an assortment of filters. We've heard some ghastly tales of inadequate dust filtration when we've spoken to off-road racers, for example. (And we've also seen some of those guys running a quite different filter inside a sponsor's external sock filter!)

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Most filters are sold as a universal fitment product - that means you don't get the necessary adaptor to attach it to your intake, nor any filter support structure. The cost of these two easily overlooked items can quickly mount up - by the time you get a metal plate cut out to suit your airflow meter (if you have one), have a length of pipe welded on in addition to a couple of structural support rods and - finally - get it all powder coated, you can be up for $100 - $150. Pre-fabricated adapters can be bought to suit various cars for around $70.

The quick-and-easy bolt-on appeal of an aftermarket pod cops a further beating when you consider what's involved in a 'proper' installation...

Underbonnet heat is the enemy of any exposed under-bonnet air filter - remember, hot intake air can undo the potential benefits of reduced restriction. To get around this you need to fabricate (or get someone else to fabricate) a comprehensive heat shield. When we say 'comprehensive', it has to box the filter off, or - at least - enclose it with a perfect seal against the inner guard and bonnet. There can be nowhere left open for hot air to be drawn through.

A lot of people use sponge rubber or C-section automotive edge trimming (the kind you see being used around door apertures) to make the seal against the bodywork. It's a neat idea.

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While sealing the edge of the shield is ultra important, it is equally important that the shield is fabricated from a material that'll keep the under-bonnet heat on the 'other' side. Stainless steel is a traditional favourite here, but - really - the thicker and better insulator the divider is, the better.

A guaranteed way of reducing under-bonnet heat transfer is to make the shield out of blank sheets of 'double layer heat management material' as used on the exhaust manifold of current Ford Falcons. A section 760mm x 412mm can be bought for around $75 from various exhaust component wholesalers. Mount the heat shield to the vehicle body using rivets or bolts and nuts.

A Quick Word on Pod Intakes

Just because you're going to install a cold air duct to the filter, that doesn't get you out of making a heat shield.

When there isn't air being forced through the intake (ie when the car's not travelling at speed), the filter will be inhaling stagnant engine bay heat. And - even when the car is up to speed and there's cool air flowing through the intake duct - what makes you think the filter will selectively suck in the cool air, not the hot air also swirling around with it?

A heat shield is 100 percent mandatory!

Yet another consideration is the proximity of the filter to surrounding body panels - there's not a lot of point squeezing a filter up against the inner guard, because it won't flow to its fullest potential. An open area for the filter works best.

So there are a few essential points to remember:

  • Buy the filter with the largest element area
  • Speak to others about their filter's performance - particularly in regard to their experience re dust filtration
  • An intake adapter is needed, along with supports to hold the filter in position (some pod filters are sold as a complete package, but most aren't)
  • Underbonnet heat must be avoided at all costs - use an effective heat shield that thoroughly seals the filter area from under-bonnet heat
  • Mount the filter in 'clean air' under the bonnet - not jammed up against anything

The final - and one of the most important - considerations is running a cold air feed to the filter. Since this applies to any type of intake system, we'll get to this after we've discussed...

How to Modify Your Existing Pre-Filter Intake System

As mentioned in Part 1, much of the pre-filter restriction found in a standard system can be blamed on the intake 'snorkel' - the duct that joins the airbox to the pick-up point.

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Begin by discarding the factory box snorkel and examining the cross-sectional area of the intake to your airbox. On an engine producing - say - 200hp or more, you'll need to open it out to at least 3 inches in diameter (or the cross-sectional area equivalent). As a rule, though, the bigger the better.

It's often possible to file out the existing airbox feed to a more appropriate area. In other cases - where you expect a cold-air feed to join the airbox in a different location - you'll need to block up the existing opening (to prevent the sucking of under-bonnet hot air) and create a new one. A holesaw is the easiest way of achieving this. If the airbox has a complex shape, however, you may need to attack it with a combination of drill, jigsaw and file - with these you can create any shape feedhole you want.

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Depending on where you plan to route a feed through to the airbox or pod, you may need to cut away some of the under-bonnet metal work. This wheelarch cold-air feed into the bottom of a Commodore airbox, for example, required a fair chunk of metal to be removed. A four-inch circle to be exact. Be careful to ensure you're not overly weakening anything structural; adding a brace around the hole can help restore some of the lost structural integrity, if required.

Don't be scared to cut - just look before you leap.

Installing a Cold Air Intake

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Having now enlarged the airbox opening or fitted an aftermarket pod filter with a heat shield, you need to turn your attention to feeding a cool supply of induction air.

There are a variety of locations to pick up cold air:

  • Inside the wheel arch
  • A forward-facing opening next to the radiator
  • A forward facing scoop (perhaps in or below the bumper)
  • Over the top of the radiator near the bonnet latch
  • Through the bonnet via a forward-facing scoop
  • At the base of the windscreen through the cabin ventilation grille

Each of these locations has pros and cons.

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Any of the forward-facing type intakes have the potential to generate a ram-air effect when driving at moderate to high speed; this is good for aiding intake flow, but depending on your local driving environment, there are some important considerations to be weighed up.

In a dusty environment, for example, a forward-facing duct will suck up a lot of dust, not to mention bugs and all sorts of other air-borne trash. Frequent air cleaner replacement (or cleaning) is a must under these circumstances. Note - car manufacturers often install pick-ups inside the engine bay; sure, they might seem like fools for picking up hot induction air but - for one - it helps keep the foreign matter out of the intake.

Furthermore, in areas prone to flooding, it's best to site your intake as high as possible - this will reduce the likelihood of sucking water into the engine when you're driving through deeply submerged spoon drains.

Now we get into construction.

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No matter where you chose to draw cool air from, the pipe leading to the airbox or pod filter should be as large as practical. If you're running it into a modified airbox, make sure the duct's cross-sectional area is equally as large as you've made the feedhole. Also, aim to have a minimum number of bends in your cold air feed. The more turns and twists the induction air has to take, the more restriction.

The most common materials used to form a cold air intake are:

  • PVC plumbing pipes
  • Convoluted tube

Either is viable; the flexible convolute is easiest to route, but - ultimately - it doesn't flow as well as a well pieced together PVC system. The problem arrises from the inner wall of the convolute - the zig-zag roughness offers greater restriction to flow versus a smooth, straight pipe wall. If you really want to take the easy approach, convolute should be fine so long as its internal diameter is plenty big.

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If you want to construct your intake duct using PCV pipe, things are made easier thanks to the availability of pre-formed bends from your local hardware store. These come in 45 and 90-degree angles, but - if you want to get really tricky - you can make small angular changes by warming the duct with a heat gun and forcing it into the desired position. A makeshift mandrel can be used to maintain the internal diameter while you're attempting this.

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A heat gun can also be used to create a flare at the mouth of a plastic intake duct. Simply heat the end of the pipe and force it down over - say - the base of a tapered clay pot. The bell-mouth that's formed serves to aid airflow into the duct. An alternative is to buy speaker enclosure ports that have a large bell-mouth incorporated on the end. These come in various sizes from most car hi-fi and some electronic stores such as Jaycar.

Oh, you can also use the heat gun to make contoured over-the-radiator plastic intakes; simply heat the duct so it's nice and soft, hold it in your desired position and slowly close the bonnet down onto it. If you've slipped another makeshift compressible spacer (eg a cushion) inside the pipe before you started, you'll end up with an intake that makes use of all the available space. Very neat.

Mounting the intake duct is straightforward; aluminium or metal strips are ideal to form the necessary brackets. Use cable-ties to secure convoluted ducting to the brackets, but use bolts to secure PVC ducts.

Note - when using the bolt approach - ensure you slide the bolt through from the inside and secure it on the outer side with a nut and locknut (or a dab of glue on the exposed thread). This prevents the bolt coming undone and potentially making its way through your engine. Oh, and try to use bolts with a fairly flat head - this will minimise in-tube turbulence. Once your plastic duct is assembled, a quick hit with black spray paint will get rid of the plumber-pipe look.

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When connecting a cold air feed to an airbox, ensure there's a nice seal between the pipe and the box; this ensures no hot air can get in and no ram-air build-up can escape. So the benefit is two-fold.

The junction into the airbox can be formed using a gutter adaptor (available in various shapes and sizes) but, if the airbox entrance has a complex shape, you can again use a heat gun to manipulate the desired shape. Ah, thermal plastics. Any gaps in the join can be easily accommodated if you use a foam rubber seal around the end of the new feedpipe.

On the other hand, when running a cold air feed up to a pod filter, it's best not to have the end of the duct positioned immediately alongside the filter - all this does is aim in-coming bugs and stones directly at the filter. If you've got effective cold air sealing there's simply no need to have the duct outlet hard up against the filter.

So, have we got all that?

  • First - decide where you want your pick-up to be located
  • C-a-r-e-f-u-l-l-y consider hassles with water or dust ingestion for each possible pick-up location
  • Work out the most direct route from the pick-up to the airbox or filter
  • Use either PVC pipe or convoluted pipe to form the duct - no less than 3-inches diameter for engines over 200hp
  • Mount the duct using aluminium or metal strips
  • A bell-mouth aids flow into the duct
  • A heat gun is very handy for forming the desired shapes and profiles
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Finally - as the name implies - the primary purpose of a cold air duct is to supply a cool stream of intake air.

The cheapest and easiest way to measure 'before' and 'after' intake air temperature is with a LCD temperature meter. Simply insert the meter's remote probe into the base of your airbox to take the temperature samples. If you've installed a pod filter, it's easiest to cable-tie the probe across the front of the element to take measurements.

Where Can You Buy a LCD Temp Meter?

Electronic stores (such as Jaycar) and major retail stores (such as K-Mart) sell electronic temperature modules. These modules usually consist of a display and a couple of metres of wire connecting a remote temperature sensor. The AutoSpeed shop sells the Jaycar one - Jaycar Thermometer Module with Alarm.

Any sort of LCD temperature module can be used to probe intake temperatures. The only things to ensure are that the connection wire is long enough for you to locate the probe along the intake and have the meter located inside the cabin, and that the module can reach up to 70+ degrees Celsius. A fast sampling rate is also beneficial.

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S-o, having now removed a heap of restriction at the start of the intake system - not to mention having lowered the intake temperature - we next move on to examining the airflow meter and everything down-stream that connects to the throttle body or turbo compressor.

That's Part 3...

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