- Intercooling removes heat from the air coming out
of the turbo.
- The air is cooled and then enters the engine.
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
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.
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).
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.
-Same principal as a water radiator, except air
flows through instead of water.
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.
- The tube-and-fin used on production cars.
- The bar-and-plate used on customised rally and
- 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.
- Maximum flow with good heat transfer is the goal.
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
Coolers. - When space is tight, use twin intercoolers.
- Connect them in series is the flow is good through
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.
thick intercoolers can completely block air flow through it.
- The radiator and engine block must receive after
- 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.
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.
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
- Use the minimum diameter that will give the
- Avoid a lot of bends and minimise the severity of
- Keep the internal joints smooth and use mandrel
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..
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.
Ambient Air Flow.
- Keep the intercooler in front of all other
- Use ducting to direct air to the intercooler.
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
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 is used to flow over the outside of the
cooler instead of air.
- Usually requires another radiator to cool the
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
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
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).
- Keep the intercooler clean and straight.
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.