The power limitation of the original induction system varies throughout the model years. The original air filter is not necessarily the main issue, it is the restrictive resonator unit and number of 90-degree detours the air must take. There are a number of after market air filters/induction kits, which mount the filter off to the side of the radiator under the bonnet. This type of installation “may” be a slight improvement on the original due to the more direct path; however, it is subjected to the hot air circulating under the bonnet. Air density is inversely proportional to temperature i.e. as the air temperature rises the density drops this means there is less air available. In order to achieve the target boost level the turbo must work harder (spin faster). This higher turbo rpm takes more time to achieve increasing turbo lag. It also requires higher exhaust gas pressure. Higher egp has the effect of reducing power output for the same boost level.
Our preferred solution is to ensure a direct cold air feed to the turbo at all times. One of the most effective ways to do this is by mounting the air filter in the void between inner and outer wings. This area is fed with cold air from the front bumper and is isolated from the hot area behind the radiator.
Selection of the correct air filter type is very important as certain models can cause premature mass air flow meter (MAF) failure due to vibration, dust or oil contamination.
On models years 93-96 the 90-degree turbo inlet track is particularly restrictive. By converting the turbo to front entry, eliminating the resonator box and upgrading the induction route (still 90 degree at the turbo) to a larger bore system - substantial gains can be achieved.
On model year 97 onwards cars the induction route is more direct but at outputs over 330bhp will still benefit from replacing the MAF-turbo induction pipe with a larger bore smoother flowing unit such as the Samco or Prodrive units.
The original PCV (Positive Crankcase Ventilation)system does indeed do some good for the engine by ensuring the corrosive gasses are removed from the crankcase and replaced by fresh filtered air.
The Subaru system has two main phases depending on manifold pressure/vacuum.
Under manifold vacuum conditions (probably 99% of the engines life) there is a non return valve under the throttle body, which allows the 'blow by' vapours to be drawn into the manifold. The vapours are replaced from the vents on the cam covers, supplied from the inlet duct. This provides the circulation of air through the engine, prevents condensation, oil contamination and internal corrosion.
This is indeed a good thing and there is a strong case to retain this system if the car is a daily driver.
Under positive manifold boost conditions, the non return valve under the throttle body closes and the hot 'blow by' gasses (Note - 'blow by' tends to be a cool vapour at low power but a hot gas at high power) are re-routed via a 't' piece to the intake duct for reconsumption by the engine. The cam cover vents may also vent out during this phase.
This is where you can get oil suspended in the gasses finding its way into the induction system......this is a bad thing ! The oil coats the intercooler, reducing efficiency and causes premature det due to lowering the octane of the fuel.
It is possible to 'catch' the suspended oil in a 'catch can' This is a reservoir deigned to reduce the velocity of the gasses, allowing the oil to fall out of suspension. The oil collects in the bottom of the can and the oil free gasses carry on to be either reconsumed by the engine or vented to atmosphere. The catch can will ideally intercept all 3 vent lines although the main culprit on the Subaru would appear to be the central crankcase vent.
The return of gasses from the catch can is also the subject of some debate. If the gasses are reintroduced to the intake system then you are asking the engine to burn an oxygen free mix ! ie you are reducing slightly the amount of oxygen fed into the engine, this can only mean less power. I personally prefer to vent to atmosphere and block off the return to the intake system. There are concerns that this is not very 'green' however the vented gasses are just of the same constitution as your exhaust gasses (assuming you have a decat exhaust)