How does it all work in my Subaru?

Engines basically work by mixing air and fuel together and then compressing them to combust. All this happens inside the engine so you don’t see any of it. What you can see is the air filter box and the piping that goes to the turbocharger and then the intercooler and then into the engine. This type of engine setup is referred to as “forced induction” or “turbocharged”. Turbocharged engines need to draw in clean and cool air in order to work safely and efficiently.

The turbocharger uses exhaust gasses to spool up when you accelerate and draws in air through the air filter. It then compresses and forces this air into the intercooler and then into the engine. The compressed air that the turbocharger is feeding into the intercooler is usually much warmer than atmospheric air so the purpose of the intercooler is to cool this compressed air down by using cooler outside air that goes into the bonnet scoop, through the intercooler and effectively cools down the warm compressed air. The engine then uses this extra cool air to create a bigger bang which produces more power. This entire process is referred to as “boosting”. Boost pressure is measured typically in Pounds per Square Inch (PSI).

“The turbocharger uses exhaust gasses to spool up when you accelerate and draws in air through the air filter. It then compresses and forces this air into the intercooler and then into the engine.”

Once the turbocharger has reached a target boost pressure, the ECU then controls a solenoid that opens a wastegate inside the turbocharger to vent any excess compression in order to maintain that boost pressure. This goes into your exhaust system, hence why the sound of your exhaust changes when your car is “boosting”. The turbocharger will continue to provide the engine with an endless supply of boost as long as you are accelerating. The moment you release the accelerator, the turbocharger then spools down.

To explain how a wastegate works in simple terms, think of a balloon. If you were to keep pumping air into the balloon, it would keep getting bigger and bigger. Now, if you wanted to keep the balloon at a certain size without stopping the supply of air being pumped in, you need vent out some air. This is done by slightly loosening the balloon opening. Just like a wastegate, it opens to vent any excess boost to keep boost pressure in the engine steady.

Would my car benefit from upgrading the air filter system?

After carrying out back-to-back testing with different types of air filter systems, I found that the standard air filter system works most efficiently in terms of how well it flowed, intake temperature, drivability and its power limits. The standard air filter system outperformed all other air filter systems for any engine producing less than 240kW at the wheels. Any engine producing more than 240kW would begin to see the true benefits of a pod type filter when mated with a smooth walled pipe that draws in air from inside the guard.

Is it worth upgrading the turbocharger?

Depending on how much more power you are wanting to gain, upgrading the turbocharger will definitely put you in the right direction. However, this doesn’t mean that you should go and buy the biggest turbocharger possible and bolt it to your engine. There are a few things that you should take into account to help determine which turbo is best for you.

Turbochargers are measured by physical size and how much air can flow through it which is measured in cubic feet per minute (CFM). Smaller sized turbochargers offer quicker spool up making for a very responsive engine but do not flow as well. This means that they will reach target boost pressure very quickly but they will also run out of puff very quickly meaning less top end power.

Medium sized turbochargers offer a compromise between spool up time and flow, usually offering the best of both worlds depending on what size engine the turbocharger is fitted to and what kind of modifications will be fitted to the engine. This turbocharger offers the best between drivability and top end power and is ideal for those that want a power upgrade without taking away too much turbo and engine response.

Larger turbochargers take longer to spool up but once they do, they flow extremely well and offer large amounts of boost without running out of puff. This kind of turbocharger is ideal for those that want brutal amounts of power, are chasing the final power figures and aren’t too worried about how it drives.

There are also different types of the same turbochargers available. Typically, you can choose between a journal-bearing type turbocharger or a ball-bearing type turbocharger. Ball-bearing type turbochargers offer slightly quicker spool up of around 500rpm and respond quicker to sudden changes in boost pressures in turn making an engine more responsive.