Why You Don’t Need a Front-Mount Intercooler on a WRX/STI
Do you really need that hulking Front-Mount Intercooler (FMIC) on your WRX/STI? It looks kind of cool, but does it really live up to expectations or are FMICs just for looks? Do they hurt the performance? Can you have a big turbo and not need a front-mount intercooler? Is Top-Mount Intercooler sufficient for cooling your intake air charge? Will it heat-soak on a hot day? Let’s discuss and then you decide.
In the Intercoolers 101 article we talked about the basics of intercoolers – what they do, what kinds of intercoolers are most common on the market, and common installation location. The two most common intercoolers you will see on WRX and STI cars are Top-Mount Intercoolers (TMICs) and Front-Mount Intercoolers (FMICs). Subaru uses air-to-air intercoolers versus air-to-water intercoolers, which are less efficient.
Intercooler efficiency is measured by intercooler’s ability to cool air passing through it. In case of air-to-water intercoolers, the efficiency is only about 55-60%, meaning that the air passing through is cooled by 60% in the best-designed air-to-water intercooler. Air-to-water intercooler efficiency can be greatly increased with a separate closed water system using ice chilled water. Not usually practical for a daily driver.
Air-to-air intercoolers are more efficient. 2002-2006 stock WRX intercoolers are about 70% efficient. An upgrade to a 2.2 times larger aftermarket TMIC will yield about 78% efficiency. The best designed air-to-air Front-Mount Intercoolers have an efficiency of about 80%.
Intercooler efficiency drops dramatically on hot days, especially as the intercooler becomes heat-soaked. This heat soak is apparent on a hot summer day in slow moving traffic.
An aftermarket intercooler upgrade is highly recommended when you modify your car. A larger aftermarket intercooler will be able to cool more air and cool it more efficiently than a stock intercooler. Also, better air flow through the larger intercooler tubes results in less air pressure drop within the intercooler. This offers an increase in performance potential. But which intercooler should you choose – Top-Mount Intercooler or a Front-Mount Intercooler? Let’s discuss which one is a better choice.
Front Mount Intercoolers
Both FMICs and TMICs have their advantages and disadvantages. Advantages of FMICs are:
- FMICs, due to their location in the front bumper, can have exceptionally large cores, 3-4 times larger than the stock intercooler core. Larger intercooler core provides for better cooling efficiency and for less pressure drop;
- FMICs look race car “fast”.
Disadvantages to FMICs are:
- increased turbo lag – although the FMIC core may be 3 times larger than a stock TMIC core, FMICs have much longer piping. Air has to travel the whole length of piping with more pressure loss. And, the much
greater volume of air to compress in the whole system takes more time. This is the additional turbo lag;
- FMICs make your turbo spool slower / about 500-1000 rpm later than if using a TMIC;
- most aftermarket FMICs are incompatible with stock fog lights – fog lights will have to be removed;
- added weight – FMICs weigh quite a bit more than the stock due to sheer size and length of piping. The weight difference is made up in part by removal/cutting of your front bumper beam – this is necessary to install a FMIC on a WRX/STI;
- greatly reduced driver/passenger safety – installation of a Front-Mount Intercooler on a WRX/STI requires complete removal or cutting of the front bumper beam. Outfits that market FMICs insist that the removal of the bumper beam does not reduce your safety, since those bumper beams “are rated for only 5mph anyway”. Yes, JDM lightweight bumper beams are available for some FMICs, but they don’t meet US DOT crash standards. Now using your best judgement, think about it for a moment… Would you rather have a factory-designed safety device, in this case, a steel bumper beam, be an extra protective barrier in an accident? Or, would you rather have a soft aluminum FMIC, with or without a non-DOT safety spec beam to protect you?
Bumper beam will protect your engine from the impact in case of an accident. It will also help protect the engine from going through the firewall and into your legs and possibly save your life (see example photos below). A front-mount intercooler, even in a minor frontal impact situation, will be disintegrated, along with your engine. Plus there is a greater chance of bodily injury without the front bumper beam.
It is probably OK for track-only cars to have FMICs. In daily driven Subarus we highly recommend to choose a large aftermarket Top-Mount Intercooler over the FMIC. Also, rotated turbo setups cannot be fitted to a TMIC in most cases, so they require a FMIC installation. However, for an ultra-high-performance daily driver WRX/STI you do not need to have a rotated turbo setup – you can have 500-600 whp with a stock-location, large turbo and a Top-Mount Intercooler. Or, you could go intercooler-less, but that’s a topic of another article.
Overall, FMICs potentially* offer better cooling than TMICs at the expense of weight increase and greatly reduced safety. Looking cool is not worth getting injured or losing your engine.
* potentially because with alcohol injection, TMICs perform the same or better than FMICs in cooling!
Disadvantage to TMICs is:
- TMICs are more prone to heat soak than FMICs. This is because TMIC is located right on top of the engine close to the turbo, and rising heat heats the TMIC up. TMICs are cooled by the air coming through the hood scoop. The large aftermarket TMIC pressure drop is not significantly higher over the FMIC, so that’s not considered to be a major disadvantage.
Advatanges to TMICs are:
- aftermarket TMICs can also have very large cores – up to 2.5-3 times larger than stock core, thus they provide much better cooling capacity than a stock intercooler;
- TMICs, even the largest aftermarket ones, are lighter than FMICs;
- much less turbo lag is experienced with TMICs over FMICs, your turbo spools much quicker;
- lesser cost than FMICs;
- no reduction is safety, no cutting or removing of the front bumper beam.
The safety advantage makes the Top-Mount Intercoolers more preferable to FMICs for daily driven Subarus.
Solution to TMIC’s only disadvantage
TMICs can be almost as efficient at cooling as FMICs, and even more efficient with the addition of alcohol injection. Since alcohol injection nozzles are placed in the TMIC itself, right by the throttle body, the pressurized hot air coming into the intake manifold at boost gets tremendous cooling from the alcohol. Here is how it works:
- at normal, no-load driving, the outside air temperature is almost equal to the air temperature that is coming out of the turbo, and gets some cooling from the intercooler;
- at throttle, at load, the turbo spools, building pressure and heating up the air coming out of the turbo. A real-life temperature readings using a 4-channel temperature sensor, taken on a 2.0L WRX with a 16G turbo and a 2.2 times larger-than-stock intercooler show that at 70 degrees F outside air temperature, at 19 psi boost, the temperature of air coming out of the turbo was 225 degrees F. So the turbo heated up the air by 155 degrees F;
- the hot air goes through the Top-Mount Intercooler, receiving about 78% cooling. In the above WRX example, temperature readings were taken at the intake manifold without alcohol spraying. Temperatures in the intake were about 120 degrees F;
- the alcohol/water mix sprays pretty much right before the throttle body, where there is good air turbulence;
- alcohol and water evaporate rapidly in the hot air, thus enormously cooling the air;
- cold air goes into the intake manifold. In fact, alcohol cools the air so well, that the total cooling efficiency of the TMIC rises to over 100%! Temperature reading taken at the intake manifold on the above WRX indicated that with alcohol injection, the air in the intake was only about 63 degrees F. That’s over 100% cooling efficiency! YES, the air going into the engine was actually colder than the air outside.
It is possible to achieve even more cooling. In the example above, 10 gal/hr alcohol spray nozzle was used. If you are injecting more alcohol, the cooling efficiency will increase due to the evaporation of larger amounts of alcohol and water. Read about how much alcohol you can inject.
In fact, the hotter the turbo outlet temperature and the hotter your intercooler is, the faster will alcohol and water evaporate, thus cooling the air even more than at colder temperatures.
Quickly pop your hood after a few runs with alcohol injection and put your hand on your TMIC. If you inject enough alcohol, your TMIC will be cold and wet! It literally freezes over when alcohol and water quickly evaporate inside.
In addition to the alcohol injection, you can prevent the heat-soak of your TMIC by heat-shielding everything you can. For example, put a heat shield over the downpipe and turbo hot side, so the heat doesn’t rise up from it but goes under the car:
- Take 2″ thick fiberglass pipe insulation (the white stuff, it won’t burn or melt), approximately 1 foot wide by 2 feet long, put the heavy-duty aluminum foil around it and attach the foil with aluminum tape;
- With aluminum tape facing up, put the insulation over the downpipe and turbo hot side, all the way up to the turbo. The insulation should be thick enough to stay in place and not slide down the downpipe. You may have to trim to fit;
- Then put a second layer of pipe insulation, prepared in the same way, over the first layer, covering any remaining space where the heat can rise up from the downpipe or turbo hot side.
- Air flow through the hood scoop on GD Subarus can also be modified to direct more air between the turbo and the TMIC.
What did you just do? You put an excellent heat barrier that will keep the heat from the downpipe and the turbo hot side from coming up. Instead, since the bottom part of the downpipe is exposed to air, the heat will dissipate under the car. You just done your intercooler a great favor.
It would be pretty hard to fully protect your TMIC from the engine heat, and you do not need the full heat protection. The intercooler will be doing its own cooling just fine during normal driving and at boost the alcohol will help your intercooler to be efficient beyond 100%.
To sum it all up, on Subarus:
- Front-Mount Intercooler is absolutely NOT necessary unless your rotated turbo setup requires it;
- FMICs for daily driven Subarus are not cost efficient;
- FMICs add turbo lag;
- FMICs are not safe, if the US DOT safety spec front bumper beam has to be removed or cut to install an FMIC;
- FMIC supports marketed by FMIC manufacturers are not a replacement for the stock bumper beam and are not safe;
- Large aftermarket TMIC with alcohol injection is more efficient at cooling than a largest FMIC without alcohol injection;
- FMIC with alcohol injection is not more efficient than a TMIC with alcohol injection;
- Stay with a large aftermarket Top-Mount Intercooler and a stock location turbo – other than looks and turbo setup requirements, there is no sane reason to install a Front-Mount intercooler on your car.
Keep in mind what you’ve read in this article when selecting an intercooler, don’t get sold on FMICs unless you absolutely need one, use common sense. Yes, you can get a big stock location turbo and have lots and lots of power while still staying with the TMIC.
Here is a technical article on Intercoolers for your further research.
TIP: to show you what happens in an accident and why the front bumper beam is important, view the gallery below. This 2005 STI ran into a guardrail – spun out on black ice at about 55 mph and slowed down a little before hitting the guardrail. Guardrail post was totalled. So was the STI. The STI had to be stripped and parted out. The engine was saved by the front bumper beam – the driver is thanking God he didn’t remove the bumper beam and install a Front-Mount intercooler – he would have lost a LOT of money and may have suffered serious injuries.
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