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Fan-Forcing Your Intercooler, Part 1

Improving intercooler efficiency, especially in stop/start conditions

by Julian Edgar

Click on pics to view larger images

This article was first published in 2008.

If you’ve got an intercooler mounted in the front guard (fender) or under the bonnet, here’s a modification that can dramatically improve its cooling effectiveness. It’s a fan that draws ambient air through the core, keeping it cooler for when boost does hit. Especially effective at fighting low speed heat-soak, it avoids that build-up of intercooler temp that occurs so severely in urban stop-start conditions.

The installation can be customised to your application, costs very little, and uses a two-speed fan system to prolong the life of the fan motor.

It isn’t very long ago that we covered the installation of a fan-coooled (Underbonnet Intercoller) – so why are we doing it again? And on the same car? Wasn’t the first system any good?

In fact, the situation’s just the opposite. The small fan-forced intercooler and associated water spray worked so well that when a different – and potentially much better – fan became available, I decided to completely redo the system.

A new bonnet scoop was also going on, so this was a good opportunity to really improve ambient flow through the cooler.

The Maxima

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My 2-litre turbo V6 Maxima runs a fairly small Mazda factory intercooler that’s been mounted under the bonnet. In the initial installation, it was fed outside air by a small marine-type transom ventilator that was sealed to the intercooler core using foam rubber.

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In addition, a cut-down Daihatsu radiator fan was positioned under the intercooler in a custom-built shroud fabricated from sheet aluminium, with the fan triggered whenever engine revs dropped below about 1000 rpm (ie it turned on at idle).

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A high-pressure finely atomised water spray was also fitted; this was pulsed on for short bursts by a timer system whenever a dashboard switch was manually flicked. The water spray installation is covered at water spray installation.

As I’ve indicated above, the system has worked very well. There was a temporary glitch when a tyre alignment mechanic cable-tied up the normally drooping undertray beneath the engine (this stopped air from escaping the engine bay, so decreasing flow through the ‘cooler – you can read more about this at Driving Emotion - but otherwise for the last six months, the intake air temps have been kept well in check.

That’s with an engine power I’d estimate at about 140kW and a peak boost of about 0.6 Bar – sure, neither is a huge figure, but they’re both pretty much in keeping with lots of mildly modified older 2-litre turbos. (I could run higher boost but the turbo exhaust housing is small so that top-end power doesn’t really change with the increased charge pressure. As it is now with the stock turbo, this is an engine with fantastic mid-range torque – something I quite like with the auto trans. Compared with standard, the engine feels like it has perhaps 50 per cent more torque.)

The Fan

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It was the purchase of this cabin ventilation fan – or more correctly, centrifugal blower – that opened-up the possibility of better high pressure forced-air cooling of the intercooler. The blower cost 50 cents from a shop at a municipal tip. The fan is marked with 'VW' and 'Audi' brands and is made by AEG. It has two outlets (each of about 50mm) and a single central inlet (about 90 mm in diameter).

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The motor is also pretty sizable – a good clue to the power of the blower, which in the case of this ‘un, is BLOODY HUGE. Yes, attach this blower to a battery and firstly, you’ll need to hold onto the thing or the reaction of the fan rotation will cause the motor spin the other way and jump all over the floor, and secondly, the air coming out of the two outlets will feel like some kind of jet blast. Well, I exaggerate – but not much. You can certainly feel the air flow from the two outlets at 3-4 metres away. With a current draw of 15.5 amps at 12V (that’s 180 watts!), this is one powerful blower.

And it’s not alone. Sure, not every car ventilation fan is going to be this powerful but if you keep a lookout for those with big motor bodies and a decent fan diameter, you won’t go far wrong.

Incidentally, the fans in these designs aren’t shaped like radiator cooling fans; instead they’re centrifugal designs. Air is drawn into the ‘eye’ of the hurricane and then thrown out the edges. The housing in which the fan sits directs and constrains this flow – most designs have just the one outlet, rather than the two of the blower shown here. A centrifugal fan is capable of developing a much higher pressure than an axial (ie propeller-like) fan. That makes it idea for the intercooler application, where it has to move air through a resistance.

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Designs to look for include cabin ventilation fans fitted to people movers having rear vent outlets, those fitted to trucks and buses (but check that the fans are 12V not 24V), and blowers on prestige European cars. If you have a chance to try before buying, place your hand over the inlet and see how much suction it can draw – the more the better. You’re after suction because the fan will be located so that it draws air through the restriction that is formed by the intercooler fins.

As you can see from the above two photos, this blower is quite a sizable unit – so how the hell do you fit it under your intercooler? In fact, in the Maxima covered here there was just sufficient room to fit it under the ‘cooler, but in many situations the fan will need to be mounted remotely. That’s likely to be not anywhere near as hard as this install was – the fan can be placed where there’s plenty of space (up to a few metres away from the intercooler) and still draw lots of air through a duct.


Heat exchangers like radiators and intercoolers need a lot of surface area to carry out their heat exchange function - that’s why radiators are always large and skinny. But it also means that fans working on radiators need to be shrouded so that all the air that they are moving is drawn through the radiator core. (Seen a square fan lately?) In terms of the area it covers, an unshrouded fan might be ‘missing’ over half the radiator.

(There’s also another reason for a shroud – an unshrouded fan will also have a lower flow efficiency... it will be whizzing air around its blade tips, rather than pushing it forwards or backwards.)

The centrifugal blower type fans being used here already have an effective shroud – it’s the housing in which the fan sits. However, to make sure that all air being moved by the fan is being drawn through the intercooler, another shroud is needed at the intercooler end of the system.

In the case of the install shown here (where the blower is mounted against the intercooler), the blower and the intercooler shroud are basically one assembly – but it doesn’t have to be like that. As mentioned, the blower and the shroud can be connected by a long flexible tube, allowing the blower to be mounted remotely.

But we’re getting ahead of ourselves – let’s go back to the intercooler shroud.

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An intercooler shroud can be organised in a few different ways. In the approach shown in this diagram – called a full shroud – all the air that passes through the intercooler is gathered by the shroud and fed through the one outlet, which is connected to the suction side of the blower. There are advantages and disadvantages in this approach:

  • Advantage - high cooling efficiency when blower is operating as the air is being pulled through the intercooler across its full area.
  • Disadvantage - lower cooler efficiency when blower isn’t operating because the shroud and blower will restrict flow when normal forward car movement is pushing air through the core.

One way of overcoming the disadvantage (incidentally, this disadvantage is one that applies to all fan-forced heat exchangers, including electric fan radiators) is to keep the fan running all of the time, perhaps only at slow speed when at low loads. In fact, that’s the approach we took on the Maxima. But there’s another way of doing it too – a partial shroud.

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In this approach half (or whatever proportion you want) of the intercooler core keeps functioning as it did before, while the other half operates as a forced-air design.

In a mostly urban driven car, the highest intake air temps you’re likely to record are in stop-start conditions, especially when you cane it away from a set of traffic lights after being stationary for a while. In this case, we’d suggest the full shroud. OTOH, a car mostly driven on freeways or country roads could go with the partial shroud.

Making the Shroud (and Blower Bracket)

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The easiest way that we’ve found of making an intercooler fan shroud is to use sheet aluminium. If you buy from a scrap metal dealer you’ll find that aluminium is extremely cheap – perhaps (including enough materials for mistakes!) about AUD$5-10 for the amount shown here. Aluminium is also easy to work with hand tools (cutting, filing, bending, drilling), doesn’t rust, can be polished, and is light in weight. This photo shows the design of the previous shroud – the one that used the cut-down small radiator fan.

With the new blower, the fan opening could be much smaller (ie to match the diameter of the ‘eye’ of the blower) so it was decided to discard this design and start again – though keeping the concept much the same.

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Because the blower was to be mounted right up against the shroud, I built both the blower mount and the shroud pretty much at the same time. For the blower mount, a piece of aluminium sheet was cut out with an electric jigsaw and then folded into shape using a homebuilt sheet metal folder. (You could just as easily use a couple of bits of timber and a vice.) A holesaw was then used to make the openings opposite each of the air outlets – you don’t want these obstructed if the blower is to work as well as it should.

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The blower was attached to the bracket using the already existing mounting screws that mount the plastic part of the fan to the motor.

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Next up, the shroud was folded-up out of sheet aluminium. A hole was cut for the ‘eye’ of the blower to project into and the two assemblies – the blower support and the shroud – were bolted together.

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To smooth the transition from the shroud into the blower, a cut-down flared plastic speaker port was used. (I had some of these ports left over after a subwoofer project – they’re Jaycar Electronics cat no CX2690. If you don’t have any, you can easily get some by cutting up some plastic cake dishes – see the end of Driving Emotion for more on this effective approach). If you are mounting the blower remote to the intercooler, the speaker port also makes an ideal way of providing a mounting for the hose that will link the shroud to the blower.

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All this work resulted in the following assembly – which looks nothing like it should be associated with an intercooler!

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Here’s how it all looks when it’s mounted on the underside of the intercooler. Note that his intercooler had enough mounting lugs and brackets to allow the shroud to be bolted to the intercooler – if this isn’t the case, you may have to fold lips that allow the shroud to clip into place. At the sheet metal folding stage don’t worry too much about getting an airtight seal between the shroud and the intercooler because...

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...they can be easily formed from strips of foam rubber, painted black with a spray can and held in place with contact adhesive. It’s VERY important that the shroud is sealed to the intercooler – air will flow to the fan by the easiest means and if it can avoid having to pass through the restriction of the intercooler, that’s just what it will do.

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Even if you have a top-mount intercooler with very restricted space beneath it, you can still use a full or partial shroud. The trick is to make the shroud a different shape. For example, the shroud can be relatively thin with the suction hose joining the shroud at one end and exiting past the top surface of the intercooler. Or you could make the exit hose parallel with the intercooler – there’s a heap of ways of doing it.


The simplest test is to connect power to the blower motor and see what happens. However, a word of warning. Make sure that you try the power connections both ways around – these fans are directional, although they will blow air whichever way the fan is spinning. However, one direction of rotation should flow a lot more air than the other – normally the fan speed is slower in the ‘right’ direction.

Placing a sheet of paper over the top of the intercooler is a good way of seeing how much air is flowing – the paper sheet should be immediately sucked flat against the top of the core. If the airflow is less than expected, check that the air outlet(s) of the blower are unrestricted and that the all the sealing between the shroud and the intercooler is airtight.

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In the case of the design shown here, I was initially disappointed by the airflow – until I swapped the polarity of the fan leads...

Then, with the fan rotating in the correct direction, a massive amount of air was drawn through the intercooler – perhaps 4-6 times as much as with the cut-down Daihatsu axial radiator fan. In fact, the fan at full speed was working too hard for continuous use – after running for 10 or 15 minutes, the motor was getting quite hot (not really surprising when you look at its power) and the noise of the blower working at full power was quite loud. That’s no problem though – it’s very easy to slow down a brush-type DC motor so that you have high and low speeds available – something we’ll cover Part 2.

With the bonnet shut (and the intercooler feed scoop sealed to the intercooler) the amount of air being drawn in through the open mouth of the scoop with the fan speed on high is amazing. Just as eye-opening is the speed with which the intake air temp drops after a boost event – any heat dumped into the ‘cooler is dissipated very quickly indeed.


Using a high-powered centrifugal cabin ventilation blower and a shroud over part or all of the rear of the intercooler allows you to fan-force your intercooler, dramatically improving efficiency, especially at low road speeds. And if space is a problem, you can easily mount the blower remote to the intercooler, joining to two with large diameter convoluted tube.

Reader John Howard has told us that the blower used in this story is a 1974-79 Volkswagen Kombi cabin blower, as fitted to cars with the Type 4 1700, 1800 or 2000 engines. It is found in the engine bay above the motor.

Next week: The two-speed control

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