This article was first published in 2005.
These days there are heaps of secondhand intercoolers around – on eBay, at
wreckers and in the classifieds. In fact for under $100 you can pick up
intercoolers that new cost over $1000. But what should you look for when
selecting a used intercooler?
Intercoolers consist of tubes running between end tanks. Stacked in between
the tubes are thin fins. The assembly as a whole is very good at transferring
the heat from the engine intake air rushing through the tubes to the outside air
being pushed though the fins.
Intercoolers are almost always rectangular in shape. Either there are lots of
short tubes connecting the end tanks, or less tubes are used that are longer.
You can picture the variation in design if you think of a wide, short
intercooler. If there are end-tanks on the top and bottom (and so the tubes go
from top to bottom) the intercooler will be of the ‘short, lots of them’ design.
On the other hand, if there end tanks at each side of the intercooler, and so
the tubes run sideways, the design will be of the ‘less tubes but the tubes are
Not clear? OK, well here’re some examples.
This Mazda RX7 Series 6 intercooler uses lots of short tubes...
...and this diesel truck intercooler uses less tubes but they’re longer.
If you look at the paths through which the intake air can flow, you’ll see
that if all other things are equal, the design with lots of short tubes will
flow better than the design that uses a smaller number of long tubes. However,
most short-tube designs have a smaller cooling area and so while they’ll flow
very well, their cooling performance won’t be so good. (Obviously, a huge cooler
with lots of long tubes will both flow very well and cool well too!)
The Mazda rotary engine intercooler shown above is typical of those used by
the factory on rotary engines – because rotaries use lots of intake flow per
horsepower being produced, these intercoolers are always high in flow. And the
diesel truck one is also typical of that breed – diesels don’t have particularly
high power outputs and so their required flow is generally lower. (Sure, you can
find exceptions to the rules – after all, turbo diesels are available up to huge
power outputs and so the intercoolers used in those designs flow very well
indeed. But typically, light delivery diesel truck and diesel car intercoolers
have less tubes.)
Flow testing of the two intercoolers shown above revealed the
short-but-lotsa-tubes Mazda ‘cooler flowed 310 cfm at 28 inches of water
pressure differential, and the diesel truck intercooler only 193 cfm.
Here’s another diesel engine core, this time from a BMW. Again you can see
the use of a smaller number of long tubes.
So it’s important when inspecting used intercoolers to see what pattern of
tube design is used. However, there’s also another design – the reverse flow
intercooler. These have the inlet and outlet tube connections on the one end
tank, which is internally divided in the middle. At the other end, the tank has
no external connections.
This Saab intercooler is a reverse flow design. If you follow the path of the
tubes you can see that the flow capacity of this core is the same as if the core
was sliced longways into two and joined end to end – making it a very narrow,
long design. While it also depends on core thickness, on the basis of
inspection, you wouldn’t expect this design to have a high flow – though it’s
likely to be pretty good at cooling the intake air!
Bar and Plate vs Tube and Fin?
How come we haven’t bothered mentioning this part of design, when there are
plenty who swear blind it’s the single most important aspect of selecting an
intercooler? Well, it’s because we don’t think so. When selecting an intercooler
there are plenty of other more important aspects to keep in mind – the ones
we’re mentioning here.
The purpose of the input end tank is to distribute the air evenly across the
mouths of all the tubes. At the other end, the output end tank needs in turn to
collect all the flows and channel them towards the outlet tube. Almost with
exception, the end tanks on factory intercoolers are tapered in proportion to
the amount of flow entering the tubes at each point. (Note: one exception is the
RX7 intercooler shown above!)
This Japanese market Mitsubishi Galant VR4 clearly shows this tapered end
The way in which the plumbing connections are made to the end tanks will also
influence flow and distribution. This Isuzu Piazza intercooler shows really
superb plumbing integration in its cast alloy end tanks – even though the
plumbing connection on the right is tight because of external space
considerations, it still manages to have gentle curves and a tapered shape.
Note that intercoolers that use this type of tubular end tank construction
never have tapered tanks.
This Porsche 930 intercooler shows both excellent tapered end tank design and
superbly integrated plumbing connections. While we don’t have figures showing
how the flow of parallel end tanks compared with tapered end tanks, the very
widespread use of tapered end tanks on well designed cars implies that it is
The heat exchange that occurs between the hot intake air and the atmosphere
happens in each cubic centre of tube and fin volume. (However, it gets
progressively less as you get towards the back of the core, and it gets
progressively less as you get towards the cool end of the core.) But leaving
aside these pedantries, the heat exchange performance will be dramatically
affected by the number that you get when you multiply core height x width x
That may seem kinda obvious but a lot of people forget that a deep boxy,
23.5 x 20.5 x 10cm core like this Toyota
Soarer twin turbo intercooler...
...may have a similar volume to this large but thin 45.0 x 41.0 x 3cm Volvo intercooler. In fact, the Volvo
intercooler has the greater volume, but only by 15 per cent.
However, for the Soarer core to be effective, it needs to have a high
pressure differential across it. That is, the pressure on the front face of the
core (created when the car is moving) must be much larger than the pressure at
the back of the core. On the other hand, the Volvo intercooler can work where
there is less of a pressure difference. The factory location of each of these
cores therefore makes sense – the Soarer core in the front wheel guard and the
Volvo core ahead of the radiator.
So don’t just look at the frontal area when assessing the cooling potential –
instead think about the cooling volume.
Other Constructional Aspects
Some intercoolers have plastic tanks crimped into place, others have crimped
alloy tanks. Still others use cast alloy tanks welded to the core assembly.
Unless you’re going to modify the core, it doesn’t matter a lot which type it
is. The exceptions are if you’re going to jacket the core to make it into a
water/air cooler (in that case, the design with cast alloy tanks welded into
place is best) or you need to weld on mounting fittings (again cast alloy end
tanks welded into place will best cope with the heat).
We haven’t touched on plumbing size (normally its commensurate with the size
and capacity of the core) or thermal mass – and the last is important. For reasons that we’ve
discussed in a lot in other articles (do a site search under ‘intercooler
thermal mass’), the intercooler core should be as heavy as possible. If that
weight is in a heat-storing material (eg aluminium), the heat absorption
capacity of the intercooler on boost spikes will be enhanced – and that’s
Finally, before looking at buying a secondhand ‘cooler, work out exactly
where the plumbing can go – and select an intercooler to suit! This last one is
a much overlooked but vital part of the selection process – you don’t want to
get a core home and find that the plumbing has to pass through the centre of the
When considering a secondhand intercooler purchase, here are some key
questions to answer:
- Is flow or cooling performance more important?
- What was the standard power of the engine that the intercooler came from?
- Are the end tanks tapered or parallel?
- How is the plumbing integrated with the end tanks?
- What is the total cooling volume?
- Is a thin or thick core required in the installation location?
- How heavy is the core?
- Do the plumbing angles and locations match the possible tube runs?
Think through the answers to those questions and you’ll be much better placed
to make a good decision.
We’ve covered intercooling in many articles, including testing the flow and
heatsinking characteristics of secondhand intercoolers, discussing placement
issues, intercooler water sprays, and other aspects. Do a site search under
‘intercooling’ to find these articles.