This article was first published in 2001.
Japanese-import intercoolers have helped make turbocar modification extremely affordable. Just a few years ago, the only readily available approach to charge-air cooling was a fabricated intercooler - often at a cost of well over $1000. Today, however, imported secondhand cores are commonly available for less than $300 - an expensive custom or aftermarket intercooler is no longer a requirement unless you're aiming for a helluva lot of power.
As you may be aware, there are many different secondhand intercoolers for sale at Japanese import wreckers - in fact, it isn't uncommon to see 30-or-so unidentified Japanese intercoolers bundled into a single pile. Indeed, there's plenty of choice - but it's very difficult to identify which ones are the best. To help answer this, we went along to Adelaide's Japanese Motorsport and compared fifteen off-the-shelf secondhand 'coolers.
Methods of Comparison
There are two main considerations when selecting an intercooler - airflow capacity and cooling capacity.
To obtain comparative airflow figures, we employed an inclined manometer and, as a source of air, a vacuum cleaner (the most powerful that we had access to). The nozzle of the vacuum cleaner was taped into one of the intercooler pipes, while the other intercooler pipe was left open to atmosphere. Our manometer was then tee'd into the vacuum pipe to measure the pressure drop occurring. The higher the pressure drop, the more restrictive to flow the intercooler core is. Relative airflow measurements were taken by reading off markings on the inclined manometer scale.
Now - as you may have gathered - this technique doesn't give cfm figures like a proper flowbench (and a flowbench's absolute cfm figures are often meaningless anyway, given that they are achieved at frequently less than 1 psi pressure differential! - Ed). The manometer measurements do, however, give an excellent comparative airflow guide - meaning that we canaccurately rank the 'coolers from best to worst in flow. And how sensitive was our measuring system to flow variation? Put it this way, we could easily see the difference in the flow restriction of two intercoolers otherwise identical except that one had 48/48mm plumbing, and the other had 48/40mm connections!
The other important quality of an intercooler - cooling capacity - is virtually impossible to measure. (Even on a dyno the test conditions do not represent real world driving, especially in relation to airflow and engine load variations. Don't believe all you read!) However, a guide to intercooler cooling capacity can be found by simply weighing each core. This identifies the mass of material that charge-air heat can be transferred to. In most road car applications, it's the heatsink ability of the intercooler that is at least as important as the ability of the core to exchange heat with the atmosphere. Therefore, increased mass generally means a greater heatsink ability - it's no accident that most aftermarket hi-po cores are heavy, even when they're no bigger than the factory 'un.
Here's how our fifteen typical Japanese-import used intercoolers compared...
Toyota 4A-GZE Supercharged
This was the only intercooler out of the test group that is factory teamed to a supercharged engine - a 1.6 litre four cylinder rated at up to 123kW. The core has a 2.7 kilogram mass and displayed quite a lot of airflow restriction - up to manometer 900 units. The ID of its plastic piping is 44 and 48mm. $100 will buy you one of these cores, but why would you bother?
Subaru Impreza WRX STi (EJ20)
Ahh, the 206kW Super Sube intercooler. We were impressed by the airflow through the late-model STi core - which we tested with the factory (aluminium) Y-pipe still in place. Its big 65mm outlet and 48mm inlet enabled it to stop our manometer dye at a low restriction of 750 units. At 4.3 kilograms (again, including the Y-pipe) it also had a high mass. Due to high demand, the price of a secondhand genuine STi intercooler is around $595. Those dollars make it an expensive proposition - but if you can find one available cheaply, grab it!
Toyota Supra Twin-Turbo Air/Air (1G-GTE)
The all-aluminium Supra twin-turbo intercooler is probably the most popular of all Japanese-import 'coolers. Tied to an engine rated at 157kW, this relatively long (but not very thick) core flowed relatively poorly at 840 units of restriction on our manometer scale. With a 2.7 kilogram mass, its potential heatsink abilities were also in the lower half of the field.
Around $200 will buy you one of these, but it's one to steer clear of.
Toyota Supra Twin-Turbo Water-to-Air (1G-GTE)
Mass: =4th without water
This is the only water-to-air intercooler of all fifteen tested. Note that this core was used on the same twin turbo'd 1G engine as the above intercooler - but it flows a bit better than its air/air cousin. Weighing 3.1 kilos (obviously, including the core's outer sleeve but not the all-important water) this core breathes through twin 29mm inlets and a 56mm outlet and drew our blue die up to the 800 units of restriction. When filled with water, the heatsink ability of this core (as with all water/air heat exchangers) would be very good. $100 will buy you one of these in the photographed condition (but you will also need a pump and a heat exchanger to complete the water-to-air system).
Daihatsu Mira TR-XX Turbo
The diminutive Daihatsu intercooler is designed to flow enough air to generate a factory power output of only 47kW - and it showed. With 34 and 36mm inlet and outlet pipes, the aluminium end-tanked little intercooler pushed the manometer to the highest restriction reading of the day - 1300 manometer units! This device weighs 1.1kg and will lighten your pocket by around $50, but we can't see why you'd want this on any car.
Toyota Celica GT4 Turbo (3S-GTE)
Like the Supra air-to-air intercooler, this unit is quite thin - but that's no surprise as it is designed as a top-mount to fit under the bonnet of the Celica GT4. Rated for use on a 153kW engine, the GT4 core weighed in at 3.1 kilograms and drew only 800 units of restriction. It connects into 45mm ID aluminium pipes. Expect to pay around $200 for this pretty good intercooler.
Nissan R31 Skyline (RB20DET)
The R31 Skyline GTS uses a 133/141kW version of the RB20 twin-cam turbo. The intercooler that's used is tucked into the guard - hence the compact, boxy dimensions of this core. With plastic tanks and 45mm piping, this device caused a high 870 unit pressure drop and its weight is 2.1 kilograms - towards the bottom of the field in both aspects. Expect to pay around $150 for an intercooler such as this, but don't get too enthusiastic about buying it when you find one.
Toyota Soarer Twin-Turbo (1JZ-GTE)
The intercooler fitted to the 206kW 2.5 litre twin-turbo Soarer is an interesting bit of gear. Very thick but with relatively little frontal area, it has fat 63mm pipes leading in and out. Extra finely spaced turbulator fins (clogged with oil on our example) were clearly visible inside the core. The oil probably contributed to this device's relatively poor airflow rating (given its volume) - 820 units of restriction were seen on the manometer. It weighs 3.1kg. $250 will buy you one of these, which, when cleaned out, looks to have good potential.
Nissan CA18DET Turbo
The ultimate guise of the CA-series motor was the 130kW DOHC, 16-valve CA18 turbo - and this is its intercooler. Minus its heavy gauge plumbing (45mm diameter in and out) this core tipped the scales at only 2.5kg and in terms of airflow, it pushed a high 850 units of restriction on our inclined manometer. This poor 'cooler retails for around $120.
Kei Class Intercooler - Long and Skinny
The origin of this long and skinny intercooler is unknown - but it's quite possibly from another 660cc Kei-classer. With 45mm internal diameter inlet and outlet pipes it flowed quite poorly, displacing 910 units of restriction. Its weight was approximately 1.5kg. Around $100 will buy you this cutie - but you'd never want to use it except as a paperweight.
Nissan Pulsar GTiR (SR20DET)
The ultimate SR20 - the 164kW version as fitted to the Pulsar GTiR - uses a large area top-mount intercooler. Made completely from aluminium (including the end-tanks) it tips the scales at 3.4 kilograms. The 52 and 55mm ID plumbing also helps it to flow satisfactorily - drawing up to 830 units of restriction on our test. A GTiR core costs around $500 - overpriced considering its performance in this company.
Mazda Series 6 RX7 Twin-Turbo (13B)
The 190kW Series 6 RX7 uses a single air-to-air intercooler to cool the charge-air from its sequential snails. This core uses a 63mm inlet and outlet pipe on its plastic end-tanks and was enough to keep the blue dye down to only 730 units of restriction. Very high-flowing indeed, although its heat sink capacity was further down the field at around 3.0kg.
Two hundred dollars will get you a S6 intercooler - a good buy.
Mazda Series 5 RX7 (13B)
The 150kW Series 5 RX7's intercooler is also another capable performer. With 48mm ID plumbing and nice flowing end-tanks, this 'cooler moved the manometer fluid up to only 760 units of restriction. It weighed 2.7 kilograms. The cost for a S5 intercooler is 200 dollars - another good buy.
Mazda Series 4 RX7 Turbo (13B)
The Series 4 RX7 intercooler is very similar to the S5's. The biggest difference is one significantly smaller diameter pipe. Where the Series 5 used 48mm plumbing on both sides of the core, this version used a 48 and 40mm piping. Other than that it looks identical. The S4 core was measured as having 800 units of restriction and weighed the same as the S5 intercooler (2.7kg). Again expect to pay around $200, but pick the above intercooler in preference.
Trust Series 4 RX7 Turbo (13B)
We threw in this secondhand aftermarket core just out of curiosity - and, boy, did it flow. Despite having modest 51 and 53mm plumbing, this monster displaced only 650 units of restriction. Its mass was also quite good for heatsinking at 4 kilograms.
Being Japanese brand name, though, expect to pay up to $695 for this used core - ouch.
Assessing the Results
In terms of outright airflow, the aftermarket Trust core convincingly proved to be the best in this group of fifteen. The best heatsink award goes to the Subaru STi intercooler (even without the added weight of its aluminium Y-pipe). But - most importantly - the overall winner must surely be the Mazda Series 4 and 6 RX7 intercoolers. Costing a mere $200 each, these flowed very well and had adequate heat-sinking abilities. On the same value-for-money basis, other honourable mentions are the Celica GT4 and the RX7 Series 5 intercoolers.