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 - Intercooling removes heat from the air coming out of the turbo.
 - The air is cooled and then enters the engine.

The more correct name for intercooling is ‘Charge Air Cooling’. Some tuners also refer to it as ‘Aftercooling’ but the term Intercooling is the most widely accepted terminology.

Heat is added to the ambient air due to the boosting process (turbos or blowers). The job of the intercooler is to remove as mush of this heat as possible and return it to ambient temperatures if possible (100% efficiency) or even lower it more than the ambient air (more than 100% efficiency). We are aiming for a high charge density in the cylinders. The colder the air, the higher the charge density.
Apart from the primary benefit of increased charge density, there are a number of related reasons to maximise the intercooling of your forced induction setup. Lower charge temperature reduces the risk of detonation and pre-ignition. Road cars that are upgraded from naturally aspirated to turbo setups can take upto 8psi of boost without an ignition remap or a cam change if good intercooling is used(of course a fuel remap is always necessary).
There is also less heat during the exhaust cycle, so the exhaust valve and piston crown run cooler. This leads to less heat being added to the fresh inlet charge coming in on the next inlet cycle. Also, any residual exhaust gas remaining in the cylinder is cooler. On turbos, there is less heat stress on the exhaust manifold, turbine wheel and turbine housing.

Air-to-Air Intercooling.
 -Same principal as a water radiator, except air flows through instead of water.

An air-to-air intercooler is similar to an ordinary water radiator, except that charged air flows through it instead of engine coolant. The charged air flows through the heat exchanger tubes and gives up its heat to the outside air passing across all the fins laid between the tubes.

Design Considerations.

 - The tube-and-fin used on production cars.
 - The bar-and-plate used on customised rally and race cars.
 - The bigger the better.

There are a number of considerations when designing an intercooler. Firstly, there are two common construction types, the tube and fin and the bar and plate. The tube and fin type is usually fitted to production cars because of its ease of construction and because it is lighter. The outside of the tubes have fins attached similar to a regular radiator, however, the inside of the tubes also have fins attached so that they pick up the heat from the charged air. A big disadvantage is that there is a lot of flow resistance because of the way the tubes are joined at the end plates.

Another disadvantage is that the fins themselves impede flow and if we make them smaller so as to increase flow then insufficient heat is transferred. Also, because it is a mass
produced item, the exact size you are looking for may not be available.The bar and plate design is usually a custom made item and is therefore more common on rally and race cars rather than road cars. It is also double the cost of the tube and fin type. The tubes are square and flat.
The advantage of the bar and plate is that it can be suited to your exact needs and it has much higher heat rejection qualities because the finning is more dense…. Generally speaking you should go for the tube and fin type is the size you are looking for is available. The next important design consideration is the size of the heat transfer area. It must be large enough to maximise heat extraction from the charge air. Also the air charge flow must be as smooth as possible while still allowing good heat transfer. In practice, choosing an intercooler is simply a matter of picking the biggest one that is available on the market and which fits easily into the space you have available under the bonnet which provides the best air flow to allow max cooling. If the intercooler you choose is from a true performance car like a 911 turbo or a Skyline GTR then  you know that the intercooler is as good as it gets for cooling capacity and flow efficiency. However, many car makers use intercoolers that are built to a price and they are often unsuitable for any kind of performance oriented setup.

Flow Vs Heat Rejection.
 - Maximum flow with good heat transfer is the goal.

As mentioned, intercooler tubes contain internal fins called turbulators which agitate the charge air so that all of it comes in contact with the tube walls and gives up its heat. This heat is then conducted to the outside fins and is cooled by the outside air-flow. All this agitation affects air flow and a balance must be struck between smooth air flow and max heat dissipation. To achieve this balance some intercoolers use  dense turbulators while others opt for low density turbulators to maximise flow through. Another factor is how long the tubes will be and how many tubes there are. The position of the intercooler and the complexity of the charge air tubing will also have an impact on the intercooler design bearing in mind that the tubing should be as short and as simple as possible and free from and acute bends. Also, intercoolers of equal size can have a lot of short tubes or fewer long tubes depending on the orientation of the tubes (horiz or vertical).

Twin Coolers.
 - When space is tight, use twin intercoolers.
Connect them in series is the flow is good through both.

When space is tight, like on an MR2 or an F40 then we might have to go for two air-to-air intercoolers or an air-to-air and a water-to-air cooler. When this arrangement is used the intercoolers are usually placed in front of the front two rear tyres, depending on engine location. However, there are many options such as a horizontally mounted water-to-air cooler combined with a vertical air-to-air cooler or one horizontal and one vertical air-to-air coolers (eg. ST185/205 and the STi). If we are dealing with factory intercoolers then the flow won’t be great so they shouldn’t be connected in series. If flow is good then an in-series setup is best because the air is uniformly spread across all cylinders.

Core Thickness.
 - Very thick intercoolers can completely block air flow through it.
 - The radiator and engine block must receive after the intercooler.
 - Use thickness of 2.5in as rule of thumb.

It has to remembered that the front of an intercooler is much better at heat transfer than the back because as the outside goes through the intercooler it picks up heat as it goes. It gets hotter as it goes through the intercooler so that at the back of the intercooler a lot less heat is extracted. On top of this, the fins tend to stifle the air movement to the extent that a very thick intercooler can completely block the air flow. This is unacceptable, especially when there is a radiator and possibly a air-conditioning condenser at the back of the intercooler. To test this you can place a dyno fan in front of one intercooler and put your hand on the other side and see how much air gets through. Now put three intercoolers in front of the fan and you will soon realise how much air is stifled by a thick intercooler core. In general, core thickness should be limited to 2.5 inches when there are other coolers behind it. If the intercooler is on its own then thicknesses of 3-4 in can be used. Anything beyond this (for tube and fin) is pointless and counter productive unless you have some kind of trick water spray or water-to-air arrangement.

Intercooler Front Area.
 - The more front area that can be exposed to incoming air the better.

Once the core thickness has been decided on, we must calculate how much tube area (or frontal area) is required while maintaining a suitable flow path for the charged air. For maximum efficiency, the maximum area that can fit under the bonnet should be used, but this has to be curtailed if the size is so big that it overly impedes flow through the inside of the intercooler. The only way to really test this is to use a flow bench, but there are a few rules of thumb that can be followed. Based on a road car that has 400bhp, and an intercooler that is 2.5in thick (typical values for a tuned jap sports car), we would need 32sqin or tube area. If we can use 3.5in core then the area required would be 20insq.

Improving Flow and Efficiency.
 - Use free flowing end-tanks that equally distribute the air across the cooler.

It is not only the size and number of tubes that determines an intercoolers flow capabilities. Using radiused inlets on the start of each tube can improve flow by 3-5%. The design of the end tanks on either side of the intercoolers is essential to keep flow going. Its important to equalise the flow down each tube. Flow losses increase greatly with increased air flow down each tube, that’s why equalising the flow is important. The skyline GTR intercooler is exceptionally good in this regard. If you are tuning any turbo car upto 400bhp then always try to use the GTR intercooler is you plan a front mounted design and if space permits.

Pipework and Joints.
 - Use the minimum diameter that will give the required air-flow.
 - Avoid a lot of bends and minimise the severity of bends.
 - Keep the internal joints smooth and use mandrel bent pipes.

It is a common mistake for tuners to use diameter tubing that is too big and heavy for a particular installation. The inlet duct diameter for a 250bhp motor should be around 2in diameter, for a 700bhp motor it should be 3.25in in diameter. Oversized bore ducting means slightly more turbo lag without any extra air flow advantages. Obviously, sharp or deformed bends should be avoided like the plague. Flexible hose is also to be avoided as much as possible. Smooth, large radius mandrel bends are the way to go. If its necessary to step up or step down in diameter then it shouldn’t be changed by any more than 1/4in at a time and a hydraulic expander should be used to change the diameter. Larger steps must be done using conical sections with angles not exceeding 7deg. As a rule of thumb, you will lose 5% of horsepower for every 90deg bend – this means that nine 10deg bends or three 30deg bends will also cause 5% decrease etc..

Another area to consider is the use of hosing. Hosing should only be use as a means of joining two solid pipes. Rubber hosing should be generally avoided in any high temp, high boost situations, otherwise it should be replaced regularly. Don’t use silicone sealant, only use permatex or some other non-hardening compound and use strong jubilee clips or better still use motorsport seals. Silicone are preferable to rubber hoses and anywhere that hose has to be used instead of hard pipe then silicone hosing is the only option. Don’t even think about using rubber. If the boost is approaching 2 bar then use double jubilees or use a tie bar as the ultimate solution.

Good Ambient Air Flow.
 - Keep the intercooler in front of all other coolers.
 - Use ducting to direct air to the intercooler.

The intercooler should be placed at the front of the car ahead of all other coolers. Anything that can be done to direct more air into the intercooler should be done so long as its practical and doesn’t overly affect aerodynamics. The cooler shouldn’t stand alone in the air stream. Otherwise the air will simply duct around the edges of the cooler. To avoid this ducting must be used to direct the air to the intercooler and should be sealed onto the front of the intercooler is possible (sometimes this is not practical on a road car). Also, light ducting should be used incase of a frontal impact. The ducting should be weak enough to collapse before it damages the intercooler. A long strip of rubber can be used at the end on the ducting to connect it to the intercooler. The duct opening should also be slightly smaller than the face of the intercooler to ensure that the air doesn’t just escape around the edges of the face. If the intercooler is smaller than the radiator face then a separate duct should be used to make sure that the intercooler gets an adequate  supply of air. That is, don’t just house the intercooler inside the radiator ducting and expect enough air to go in through it. The next challenge is to make sure that a pressure differential exists between the front and back of the intercooler so that air is encouraged to pass freely through it. This means that we have to allow plenty of space in the engine bay for air to escape  out and we have to limit flow in the engine bay except to where it is needed (eg. Onto the wastegate, oil filter, oil coolers, distributor, alternator, battery, fuel lines, injectors, turbo, harness). However, a big bonnet scoop can over supply the engine with air and cause a positive pressure against the back of the intercooler and not allow air through. One or two smaller boot vents, directed in the right direction are better than a big scoop dumping large amounts blindly into the engine bay. A big powerful dyno fan should be used to check this. Use the fan to spray water or smoke into the engine and note its flow path.

Intercooler Water Spray.
 - Works very well on high boost engines.
 - Can sometimes be used instead of changing the stock intercooler.

 Good water spray onto the front of an air-to-air intercooler will increase its heat rejection efficiency by 8%. This isn’t that significant in low boost, small intercooler setups, but when boost goes up and intercooler size increases, it becomes more and more difficult to get good efficiency figures. An intecooler that is 70% efficient would require 25% more core area to get the extra 8% increase in efficiency that a water spray would give. However, in order to achieve this, the water spray must be setup up to cover the whole intercooler and must spray large amounts of water onto the intercooler.
This means multiple nozzles and a large water tank reservoir( typically 30 litres). You must make sure that the 30kg penalty in weight is overcome by the 8% increase in efficiency. All means should be used to ensure that the minimum of water is used so that the minimum of water has to be carried on board. The spray nozzles should spray evenly and should only output the min required spray. The system should not be triggered until the charge air reaches 10-15% above the  desired charge air temp. Triggering waterspray in response to boost levels is not the way to go because intercooling temps can be lower when the car speeds are higher and the ambient temp is lower. Water flow rate/nozzle position/spray pattern should be initially setup very carefully using input from several temp and pressure sensors while the car is being run on the dyno with the dyno fans running at the correct air speed to mimic car road speed air flow.

The spray pattern should be adjusted by looking at the temp sensors and by simply observing the water spray while the car is running on the dyno. Lastly, make sure that you employ a double map strategy, one map for water spray and another map that automatically cuts in when water has run ou t(in case the user forgets to top up the water tank).

Water-to-Air Intercooler.
 - Water is used to flow over the outside of the cooler instead of air.
 - Usually requires another radiator to cool the water.

Water-to-air intercoolers are more complex than air-to-air arrangements. They are used where space for an air-to-air intercooler is not available, or is too expensive for a manufacturer to implement. Because so many water intercoolers get replaced in 400+bhp conversions, they can be got for a fraction of the cost of an equiv air-to-air intercooler.
With a water-to-air intercooler, water passed over the outside of the intercooler instead of air. A water-to-air unit can be thought of as an ordinary air-to-air intercooler encased in a box containing water being pumped through it (and sometimes water intercoolers are fabricated in just this fashion, by taking an air-to-air unit and welding an aluminium box around it). At the front of the car you need another radiator to take the heat out of the water that’s exiting the intercooler.
The size of this rad is based around the flow rate of the water, the volume of water, the amount of on-boost time vs off-boost. This is because the water can be brought back to ambient temperatures after a short on boost blast. A road car may be on boost for 10-20 seconds and then spend the next minute or two off boost. This means the radiator only needs to be large enough to handle the heat during the last 5 seconds of on boost operation, and it can slowly return the water to ambient temperature after the short blast of on boost activity. The next time boost is applied, the water is at ambient and the rad doesn’t have to start cooling until a few seconds into the next on boost situation.

However, in competition and hard road use, a bigger rad is required because the car will only be off boost for 2-3 seconds. In this case an air-con condenser rad can be used. If there isn't space up front for a large radiator, then a larger volume of water can be used in the intercooler but this will add weight. The coolant is usually pumped by using an electric pump to move the water. ¾ in hose should be used to minimise pressure on the pump. You need a pump that is capable of moving 20 litres per minute continuously without overheating. Pumps made for crop spraying are ideal so long as they are mounted on rubber mounts to minimise noise. In rally cars the pump can be left on continuously but on a road car its better to toggle the pump based on coolant temperature and boost activity( to anticipate temp increase).

Intercooler Maintenance.
 - Keep the intercooler clean and straight.

Air-to-air intercoolers require regular maintenance (water-to-air require less). You should keep it clear of any items blocking flow like leaves, grime etc. If you are fitting a second hand unit, make sure the inside doesn’t contain any old compressor impeller bits from the previous installation. Oil sludge may also find its way into the tubes and this should also be cleaned away. Any bent fins on the outside of the intercooler should be straightened. One of the hardest things to clean from the intercoolers is rubber thrown up from other cars – especially after any track days. If the power hose doesn’t get rid of these then they must be picked off by hand, or else remove the intercooler and soak it in solvent for a few hours and then use compressed air to blow out the rubber bits (from back to front). A pressure test should also be carried out every 20000 miles on a road car and every 1000miles on a rally car.