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The Zen of Used Parts: WRX STi Intercooler Swap

We find more planets in a used-parts universe

Cars that are used like cars (ie: driven) and not detailed all the time (or parked in a garage and polished relentlessly) tend to get dirty. It's a real-car thing that trailer queens and poseurs wouldn't understand. The stuff under the hood intermingles with the petrochemical miasma all about and gets slimy, and sticks to everything. Spend any hours wrenching and you know it—wash your hands 10 times and they're still spider webbed with black lines of grime that won't go away short of a belt sander. So my clean hands are still dirty.

Mission to the Aftercooler

Subaru builds the WRX and its freak-speed sibling, the WRX STi. The Sti, as if the WRX wasn't fast enough (it usually is), comes off the showroom floor with more boost, more displacement, more technology, more power, and more handling prowess, etc. Many of the hi-po goodies found on the STi, such as the turbo, hoses, transmission, drivetrain parts, brakes, suspension, can be bolted right on to the WRX and so then when folks with STis upgrade their parts, these excellent STi parts become available for plane-Jane WRXs. OEM quality, OEM fit & finish, OEM appearance (for the smog-conscious among us) and used. Used good. Hulk smash.

Digging into our list of things to mod (and still chopping away at the shelf of uninstalled performance parts in the garage), we came across a set of STi tumble-generator valves we'd ported for the WRX. Ready for duty and sexy as all, we'd discovered that the STi TGVs didn't match the WRX as well as we needed—the injector boss on the STi's side-feed hosers was slightly wider than the WRX's, and without retuning the computer we couldn't use the STi injectors. Nobody named Justin felt like ponying up for a dyno run and reflash, but we had plenty of friends who'd modded their Stis before. Who needed a set of ported & polished TGVs? Turned out Military Mike did. He still had his stock STi aftercooler (having adapted to a front-mount), and I wanted that. Time to shake hands and give that lonely aftercooler a new home.

Let's get one thing straight—the air-to-air or air-to-water devices that supercharged and turbocharged automotive engines use to cool the air charge is an "aftercooler," not an "intercooler." An intercooler is a device that is placed between two compression devices, such as after a turbocharger and before a supercharger. It's old World War II fighter plane technology, still relevant but not currently employed in automotive applications. What your Cobra, your WRX, your Evolution, and even your Mercedes use is an aftercooler, which does its job after the air is as compressed, before going on its way to the engine. Some day I'll tell you why the terms "turbocharger" and "supercharger" are technically incorrect too.

Some Parts are Simpler

With time already invested in the underhood life of Subarus, we've had the WRX aftercooler off before. Five bolts loosened, a few hoses disco'd, some hose clamps expanded and the original WRX aftercooler comes right off. It's best to yank the 'cooler first If you work on the firewall or the turbo, or if you needed access to the manifold or throttle-body. I'll give you a secret—once the unit is removed, take out the driver's side bracket too, that will make reinstallation quicker.

You'll need to swap the blow-off valve from the stock aftercooler to the STi device (it's only a bypass valve if it routes the surge back to the intake charge), and the factory gasket does not typically like to come off. It's not cardboard, there's metal in it, so prying it loose can be a pain so use some RTV or buy a new gasket. RTV will need a day to cure properly, so don't go throwing danger-map boost at it until a day has passed.

Before installing the new (to us) STi part, we took the time to groom the thing. With a small set of 90-degree needle-nosed pliers, you can straighten the vanes on both top and bottom of the unit for idealized air-flow, in light of the damage caused when using an aftercooler as a tool shelf. We fashioned an old lock-pick into a hook for snagging gravel out of the voids between vanes.

Be cautious when working with aluminum parts like aftercoolers. Vulnerability to crossed threads is the real hazard because when working with steel screws, they'll chew up the bolt-hole before you can spit out a bad word. While going over the new and old parts, you'll notice some oil sludge inside. A little is okay—turbocharged cars tend to blow oil occasionally—but slicks and puddles of crude in your as-removed aftercooler indicate you've struck oil somewhere expensive within the motor.

This is an easy-off, easy-on job, so don't fret if things don't line up for the first stab. The factory STi turbo-to-aftercooler hose was probably trimmed poorly by whomever assembled it in Japan. This is a problem common to STis. With some patience, you'll get that piece of silicone to line up. Aftermarket silicone hose kits fit better. It's best to get the turbo discharge hose tightened first as with that on everything else falls more nicely into place. If you use an aftermarket strut-tower brace, make sure it fits the STi aftercooler.

When the turbo is cinched up, address the aftercooler-to-throttle-body hose. This is more easily done if you tighten the aftercooler side of the hose while it's off the car. Be sure that none of the silicone hoses are crossed, twisted, or bunched up. The clamp needs to lie flat on the hose, and the hose flush with the turbo and throttle-body.

Is Crooked Okay for Used Parts?

Looking at the finished piece, it's a charmer until you notice that the unit doesn't sit quite straight. Damn. We had to bend the passenger-side bracket so the 'cooler could move enough for the turbo hose to work. It's only two or three degrees out of whack. With the STi aftercooler in place though, the gap in the WRX engine bay over the turbocharger is filled visually. We've also added a larger aftercooler, and with a 160.5cm2 greater face area we'll probably pick up a few horses at high boost. Of course, now we need to modify the WRX hood duct to fit it, but that shouldn't be too rough, if we can find the parts used.