The Intercooler Page

The cores



One of the drawbacks of the standard All Trac or GT-Four is the placement of the intercooler. While the stock intercooler might work well, it tends to soak up heat when the car is not in motion. The intercooler sits above the engine soaking up heat ("heat soak") through the surrounding air from the hot engine. This heat soak dramatically decreases intercooler efficiency due to higher than ambient air temperatures. We will start with a short description of the each intercooler section along with some upgrade suggestions.

The Core


This is the ST165 water-to-air intercooler

(WTA I/C). This little wonder actually works quite well for it's size. This intercooler is not

bad for everyday use but a upgraded

intercooler is recommended for a larger turbo because the intercooler air core is rather

small. The WTA intercooler does have a habit

of heat soak if left idling for too long. The

cause is when there is no throttle given for a period of time, the water pump that circulates the water through the system will shut down. When this happens the WTA I/C is no longer cooling and will absorb all the heat from the engine underneath.

This is the water core for the ST165. It measures about 10"x10" total mounted in the front left side of the air dam. Water is

circulated through this with a small water



This is the ST185 All Trac / GT-Four air-to-air intercooler (ATA I/C). Just like the ST165, it sits on top of the engine. There is a gray shield around the intercooler with a wire mesh screen across it. This protects the intercooler fins from damage by rocks and other debris that fly into it. Thought to be pretty efficient at highway speeds, the ST185 intercooler is rarely replaced. The flow is not quite top notch, it is based on an old Toyota design. It does have the advantage of a very short intake path, so the response is really nice.

This is a close up of the stock ST185 standard core. It measures 9" x 9", while it's pretty efficient for what it does, the very thin air tubes do not make for very good flow. An intercooler upgrade is a must for any type of racing, especially for those trying to break the 300 HP mark.


The intercooler system of an ST205/ST185RC is the holy grail among GT-Four

owners because it offers a water-to-air intercooler system with a core which is

more than enough for most situations. Also the radiator core for the water is

considerably larger than the ST165 water core by two times. Most ST205 owners

perform considerable upgrades without upgrading the intercooler. Part of that

is due to the efficiency and the other is due to the fact that it's rather hard to

fit a front mount into the ST205.


Considering the difficulty and time required to replace the original intercooler, let's discuss some things we can do to make it more efficient.

Water Injection

Basically by injecting water through a spray into the intake system, you achieve a colder air charge. We will further go into detail later on, but here is some information for now: Article

The All Trac responds well to this modification. It can be as simple as a few parts from Home Depot to a system that monitors engine conditions and adjusts itself to suit. Stay Tuned.

Intercooler pump run constant (ST165/ST185RC/ST205)

As mentioned above, the pump that circulates the water through intercooler cooling system is only on when you are driving and for a few seconds after. Once the computer has detected that you are no longer driving it will stop pumping. This is of course for a reason, to extend pump life. However, in a hot climate, the water that happens to above the engine will get really hot. Once you go to drive again, the intercooler will have become very hot and drivability suffers for the first few seconds until the water can be circulated and cooled again. By running the pump constantly, the water will always be cooled by the front water core, and rarely get very hot. Here are some links to articles on how to perform this modification:

Upgrade to large front water core (ST165)

The stock water core on the ST165 as you saw is very small. It works great for what it is, but the capacity of it is very small. If you will be running in a hot environment it will not cool fast enough. The rally spec water core is extremely large and runs the full length of the front of the car. This allows a higher water capacity which makes it harder for all the water to get hot at once. You can find a import radiator from a small compact car to use as a large water core. Here is a picture of the ST205 GT-four water core above, so you can appreciate the size. It is the silver aluminum core behind the bumper. It's not that difficult to install. Once you do you will notice that the water will rarely if ever get hot.

Cooling fan (ST165/ST185)

An I/C fan is one of the best overall modifications one should get for a stock intercooler. Since the ST185 intercooler is subjected to heat soak and poor cooling at low speeds, a high speed electric fan is mounted on top of the intercooler. This helps cool the I/C by sucking in cooler air from the outside through the hood scoop as if the car was moving at a high speed. This is not a high HP upgrade but gives back some intercooler efficiency and response from heat soak.

The ST165 can benefit from a fan on the front water core as well. Even if you upgrade the core, a fan is recommended.

For more information and pricing on intercooler fans, contact Aaron at ATS RACING.


For more information about the installation and availability of these items, visit the Intercooler FAQ.

Upgrade to Rally Spec Water-to-Air Intercooler

If you are lucky enough to find one, a very good upgrade is to use the rally spec ST185 intercooler. This was never brought to the USA but is found on the Rally ST185 in other countries. These are known as ST185 RC (Japan), ST185 Carlos Sainz (Europe), and ST185 Group A (Australia). You can also use an ST205 intercooler if you can find one which is even larger and flows more air. You will need to install not only the intercooler, but the water pump and water radiator that goes with it. A Water-to-Air intercooler has an advantage as it can still provide cool air even at slow speeds.

Click here to see our RC intercooler upgrade page with more detailed notes

Click here to see a rally spec Intercooler atop a standard ST165

Here's an article on the merits of WTA I/C

Upgrade to high flow Air-to-Air Intercooler (Top Mount)

This is less difficult then it seems. With a large high flow intercooler atop the engine you can still take advantage of the high speed, ram air cooling effect (ST185). However, you will need someone to fabricate this. You will basically need to buy a high flow intercooler core like a Spearco with the proper dimensions, 8" x 9" would suffice. You will also need to buy end tanks with 2.25" ports in the proper locations. Then you will need to have it fitted and welded. This type of setup will easily support 400HP with a proper turbo setup.

Estimated cost is about $700- $1200 USD

Upgrade with a front mount intercooler

  • MA70 Supra Turbo
  • Starion Turbo
  • Lancer EVO Front

MA70 Supra intercooler pictured and installed in ST204 Celica

You can increase the size of the core for only a few dollars by using a 87-92 Supra I/C core. As you can see it is the basically the same style core as the ST185 one but much larger. Also, this core will fit in the front bumper away from the top of the engine where heat soak happens at idle or low speeds. The fact that both outlets of the pipe are on the same side it makes installation less painful. You will need to then have custom pipes fabricated that would run from the turbo to the intercooler and then up to the throttle body. It would be good time to incorporate a blow off or bypass valve as well. Estimated cost $200-300 including fabrication and labor.

A very low cost solution.

Upgrade to high flow Air-to-Air Intercooler (Front Mount)

Here is an ST205 with a skyline GTR style intercooler. The inlet and outlet are on opposite sides. This is rather huge for a 4 cylinder, but we're sure that looks has to do with it as well. That is one mean intercooler.


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