Really complicated subject where the deeper you go into it, the more complicated it gets.
The standard crankcase breather/fire trap arrangement is a bit inadequate, as is the plumbing for the breathers.
But, the higher the combustion chamber, the higher the volume of combustion gasses which can pass through the gaps in the piston rings and even past the rings seal against the bore.
The piston ring to bore seal, is not made by the spring tension of the piston ring, but mostly through the difference between the pressure above the ring, and the pressure bellow the ring, spreading the piston ring out, and therefore applying more pressure to seal it against the bore.
If you produce more blow by gasses per second, than the standard breather system can handle, you end up with the pressure bellow the piston rings increasing, and therefore providing less pressure differential between above and bellow the piston ring, therefore less force to seal the ring. Which then provides more blow by gasses, which in turn cause more crankcase pressure, which then cascades into an ever worsening situation, even over the course of a few split seconds.
The standard breather also is routed into the intake air stream, which has another hazard, which is not as simple as it sounds on first glance, which is that the blowby gasses contain oil vapour, hydrocarbons, carbon monoxide and carbon dioxide, which will be a volume of gas, which is not normal air, as assumed by the ECU, or measured by the MAF or airflow meter, which will mess with not only the mixture at a time when it needs to be spot on while under boost, but also will alter the dynamics of the air fuel mix in a number of ways.. some of which beneficial to knock reduction (detonation avoidance) as well as detrimental to knock reduction in ways which can not be measured or assumed.
As well as this, the oil vapour can condense in the inter-cooler, which is not good for its thermal efficiency, but also not good for combustion as those droplets of oil get as far as the intake manifold, reaching the cylinders in different quantities per cylinder due to mass differences.
The other problem, which can raise its head as an indicator that the car's crankcase breathing has become compromised, is that oil seals and gaskets can start leaking, as they will be experiencing pressures which they are not designed to handle.
So, what is the solution...?
The fire trap can be modified to improve its ability to seperate oil and gas.
The breather hoses can be routed to a catch can to collect the gasses, promote drop out of oil from the air stream, and then ventilated to either free air (not ideal but cheap and easy) or can be routed to another source of evacuation, which would ideally be at an air pressure lower than atmospheric... Once source is back to the intake pre turbo.. but not ideal for combustion as mentioned above, or to another source.. Some people use a vacuum pump for this, some people use something else.. In the past I have used an exhaust venturi, but it does take a lot of experience and development to know where to put it, and to make sure the exhaust is of suitable type.
You can litterally spend thousands on a breather system, or you can spend less than £100 on it... You spend your money and take your choices, but in the end, it depends on the budget and on your requirements.
Typically, somewhere past 340 BHP the original breather system no longer copes well, but to be honest, all 944's could do with an improved system and it also depends on the engine and its condition.
One additional variable is the state of tune... The more suitable the state of ignition timing and fuel quantity, the less blowby gasses you will experience.. If the tune is bad, the volume of blowby will be greater, this has a lot to do with flame propagation speed, which can bring on an entirely new set of variables and dynamics to the mix.
Really, whatever solution you need is going to be unique to your car, its state of tune (performance level) and how it has been tuned.
