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Picking a Used Intercooler

What to look out for in intercooler design

By Julian Edgar, most of the pics by Michael Knowling

Click on pics to view larger images

At a glance...

  • Design
  • End tanks
  • Cooling volume
  • Making choices
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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?

Design

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 longer’ approach.

Not clear? OK, well here’re some examples.

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This Mazda RX7 Series 6 intercooler uses lots of short tubes...

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...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.

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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.

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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.

End Tanks

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!)

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This Japanese market Mitsubishi Galant VR4 clearly shows this tapered end tank design.

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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.

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Note that intercoolers that use this type of tubular end tank construction never have tapered tanks.

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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 worthwhile.

Cooling Volume

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 depth.

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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...

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...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

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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 good.

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 radiator...

Making Choices

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.

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