The WRX/STI/LGT/Forester XT Power Mod Path, Part 5
Hopefully, our previous engine management discussion didn’t overload your brain and just kept you coming back for more with this latest installment. We pick up where we left off moving to the “Stage 3,” which typically involves an upgraded intercooler. Just like with almost every modification we’ve discussed thus far, intercoolers carry with them some myths that many people believe to be true without ever having any personal experience to back up their claims. But, before we get into those myths, it’s important to know what exactly the intercooler does in the first place.
The intercooler is essential to making power and maintaining efficiency on a turbocharged motor. There are two main types of intercooler: air-to-air and air-to-water. Most factory turbocharged cars use an air-to-air intercooler, since it’s much less complicated mechanically (no need for a water reservoir for heat exchange), not as heavy, and typically not as expensive either. Air-to-air isn’t quite as efficient as a good air-to-water intercooler, but the cost/benefit involved tends to make the air-to-air intercooler the top choice is most situations. Nevertheless, both types of intercoolers do the same thing: they cool down the charged (compressed) air from the turbo so that it’s more dense when going into the motor. As air is compressed, it generates heat, and heat also expands. With more heat involved, there are less oxygen molecules, and with less oxygen going into your motor, the less power you can make. The intercooler helps this by pushing this heated air to the intercooler core, which looks and acts similarly to a radiator. As cool air passes over the intercooler fins, it helps cool down this air and makes it more dense as it then gets pushed into the motor. Cooler, dense air contains more oxygen molecules, and thus you’re able to make more power.
Since the intercooler core acts as a heat exchanger, the size of the core determines how well or efficiently it can cool down the compressed air. The location of the intercooler core also matters as well, since it can easily be affected by ambient temperatures and needs proper airflow to do its job. This is where a lot of the debate and myths start to come into play. Subarus come stock with a top-mount intercooler (TMIC). The size of this TMIC depends on the model and year of your car, with the STI TMICs typically being the largest offered from Subaru. Given the size of the stock turbo (depending on the vehicle), the stock TMIC is usually perfectly fine for cooling and efficiency. You can put on a larger intercooler and it won’t hurt anything, but you will need to get tuned properly for it and in the end, it might not yield much of a significant difference over stock. Seeing gains of maybe 10 whp maximum typically isn’t worth the $900+ you’d have to pay to reach that much of a boost in power.
The location of the intercooler is the next matter of contention. While the stock intercooler is located on top of the motor (hence the name top-mount intercooler), it really isn’t the most efficient place for the intercooler, especially when upgrading to a larger one. The main reason for this is the simple fact that it sits not just on top of the motor, but also on top of the turbo, which of course radiates quite a bit of heat. While the stock heatshield helps with this, when upgrading to a larger turbo, you can’t reuse the stock heatshield (or really any other heatshield for that matter) and thus the TMIC suffers greatly from heatsoak. This is much more apparent when the car is stagnant such as in stop-and-go traffic or in line at the drag strip, rather than when the car is actually traveling at speed on the road. In the case of the car actually moving, the stock hood scoop, combined with the stock air splitter, actually does a good job of helping provide proper airflow to the TMIC as well as shielding it from the ambient heat from the motor. However, the problem then is with a larger TMIC, because the stock air splitter is not typically compatible with a larger TMIC, and in the case of an aftermarket hood, there simply isn’t any air splitter whatsoever. This isn’t to say that a larger TMIC won’t do a better job than the stock one, but simply that there are definitely drawbacks to keeping the intercooler on top of the motor.
By far the best location for the intercooler is at the front of the vehicle. Front-mount intercoolers (FMICs) draw air directly from the front of the car, which helps not just when the car is moving with air flowing over it, but also at a stop since it has direct contact with the outside air. However, some people have insisted that by putting the intercooler at the front of the vehicle, there’s too much of a drop in pressure because of the added piping, and thus you will suffer additional turbo lag. With proper tuning, this is completely false, and is normally a complaint from people who install a FMIC and do not get properly tuned or even tuned at all. Owning two Subarus, both with FMICs, I can say with full confidence that there’s no difference in turbo lag whatsoever thanks to proper tuning. However, there is indeed a pressure drop nonetheless, and thus it’s important to choose the right size core.
There’s always been the idea that bigger equals better, but when it comes to intercoolers, this isn’t necessarily always the case. With Subarus specifically, there are some intercoolers that are just too large for the turbo its supporting, which causes too much of a pressure drop and thus a car isn’t able to make as much power as it could have with a smaller intercooler. For example, the APS DR725 you see to the left here on Bailey’s car is gigantic, so it really wouldn’t be the right choice for a smaller upgraded turbo such as an 18G or 20G, and definitely a bad choice for the stock turbo. Plus, given the fact that smaller intercooler cores such as the APS DR525, TurboXS, and Perrin are more than capable of handling well over 600 whp, you pretty much have to get a matching gigantic turbo to take full advantage of the DR725′s size. Along with the size of the intercooler core comes the change in weight distribution, since you’re essentially adding 35 or more pounds to the front of the car. Many would argue that this alone is reason enough to keep the intercooler in the top mount location, but generally speaking, this is more of a problem for those who do lower-speed racing such as auto-x, rather than types of racing that require the full power output, such as time attack and drag racing. Finally, the other problem with a FMIC is purely aesthetic, since in most cases you have to do quite a bit of cutting of your front bumper cover to fit the intercooler there, not to mention you lose your factory bumper beam as well. Most kits will include a replacement bumper beam of some sort, but it’s still not the factory, government-approved, and crash-tested bumper beam, so more frontal damage than expected is likely to occur should you get into some sort of accident.
Despite its drawbacks, we normally recommend FMICs to anyone who wants to bring out the full potential of the car. It’s just important to choose the right size and design to suit your needs. Of course, since a bigger intercooler doesn’t really benefit you without a bigger turbo, we’ll be talking about the different aftermarket turbos out there in our next blog entry.