An effective intercooler can be a pivotal component in a turbocharged or supercharged application, at its most basic, it’s an air cooler for reducing the temperature of the boost charge before entering the inlet manifold. But there’s a lot more to it than that…..
Internal Flow Characteristics
Ideally you would want the boost air charge to flow as smoothly as possible through the intercooler with no restrictions. Anything that reduces the flow efficiency will create a loss of boost pressure on the cold side, and it stands to reason that the higher the pressure drop, the less power you make. So what we need here is a core which has no internal restrictions – this gives you 100% flow rate through the core and makes the most power. However, the core also needs to have enough thermal conductivity to get the heat out of the boost charge, creating inlet air which is nice and cool, giving you a higher air density and therefore, more efficient combustion. Is this all possible with no losses? No, it isn’t! The laws of physics will not allow you to get something out of nothing unfortunately.
The intercooler core must have enough surface contact to get the inlet temperature down, this occurs by having the air in contact with as much of the core internals as possible. If you look at the image here showing the open end of the intercooler tube, you will see the path of air has to be in contact with the internal ribs, which transfer heat to the core tube wall and from there to the core fins. This action of heat transfer is what gets the heat from the boost air onto the core surface, then the action of ambient air flowing across the core when driving is what dissipates this heat away from the core.
This action of surface contact must create resistance, if there were none then the surface contact would not be taking place and the boost air would not transfer enough heat. Inlet air that is too hot will cause engine management systems to back off boost and / or ignition timing, causing loss of power and a total waste of all the effort you have made building a high performance engine. A properly designed intercooler will allow your engine to make the power it’s been built for, there is no point building a high spec engine if you can’t cool it properly – You’ll get overtaken by a car with less power, if that car has the right cooling system in place.
The image above shows the cooling tube internal structure of a 90mm tube and fin core. The internal structure here is known as the turbulator and maximises the contact surface area whilst allowing efficient flow through each tube.
The charge air is forced through these tubes, transfering heat into the tube wall, and from there into the external fin structure, where it is dissipated via the incoming airflow.
Well designed end tank sections will help to steer the charge air effectively through the core, avoiding turbulence and maintaining boost pressure.
The distance between each row is determined by the height of the fin structure, in this case the fin height is 8mm. The fins can be formed for a tighter or looser pitch, and this is adjustable from 12 to 26 fpi (fins per inch).
In most applications we would recommend a fin pitch of 12-14 fpi for an intercooler. This gives a good amount of surface contact whilst allowing the incoming airflow to travel through the core efficiently. A dense fin pitch here is best avoided as it is easy to compromise the airflow through to your radiator.
Bar and Plate Cores
There are two types of intercooler core available, the bar and plate type, or tube and fin. Bar and plate cores are heavy duty units more suited for industrial applications, but they are also used in mass produced automotive intercoolers, as they are relatively inexpensive and simple to produce. This is what you will find in a bargain priced type intercooler and with these you can expect the following results:
Initial Power Runs from Cold, and Characteristics of Heat-Soak:
You will get decent results from cold with one of these, as the unit has a high thermal density. This means that it will act as a heat sink, soaking up heat into the body of the core. This action will promote a rather deceiving set of results if you are measuring inlet temps in a dyno environment; yes it looks like the inlet temps are low, but fact the intercooler isn’t getting rid of the heat, it is merely absorbing it. If the dyno run process lasts around an hour, then you won’t get to see the negative after-effects of this.
As the unit absorbs more heat, the inlet air temperature will start to rise, because the thermal density of the unit will not let the core cool efficiently enough when you are not at full boost. This is known as the recovery period, and it’s this process which drastically shows the disadvantages of the higher thermal mass. The unit will hold its heat because the unit is built from thick, heavy section materials, so consider the effects of this if you are driving on track in hot weather.
As the inlet temperature rises, so the power will start to drop as the air density falls. In extreme cases the higher temps could potentially cause other problems, such as detonation which can do massive engine damage. Using a water injection system to combat this is not really the answer, it might help to disguise the symptoms, but it is not the correct way to make up for an ineffective intercooler.
You will also find the higher thermal mass is an undesirable side effect of the mass of the unit itself. These units are generally 30-40% heavier than the equivalent tube and fin core, so how can you get better overall system performance and save weight at the same time?
Tube and Fin Intercoolers:
This is the construction type that we use for all our intercooler products, and we do this because we have a proven track record showing that it works. Lightweight, thermally efficient cores make a tremendous difference to the performance potential of your engine, they are more difficult to build but the result of the extra input means the level of performance is greater.
The cross-sectional design of our core tube keeps surface contact levels high, but is shaped in the most efficient manner, giving you the benefit of better thermal conductivity whilst minimising pressure drop across the core. A combination of these key factors is the best compromise, resulting in solid performance in terms of heat loss and retention of boost pressure.
The lighter mass benefits the system in two ways, light weight is obviously a bonus, but the lower thermal mass means quicker recovery because the core will not suffer from excess heat retention. These are the things that matter at competition level, the performance needs to be a constant or reliability will be under question – not ideal on a trackday, a disaster during a race!
All our fabricated intercoolers are made by hand, here in our own workshop. All those nicely shaped end tank sections are made by us, the elements of these design features are added to further enhance the performance. We have developed special tooling to create flared ports to intercooler pipes, enlarging port areas and creating smooth flowing designs to ensure reliable high performance.
Example showing custom fabricated end tank sections on a bespoke Nissan 200sx S13 intercooler.