Positive Crank Case Ventilation

[EdgeAutosport.com]

Positive Crank Case Ventilation

The purpose of this is to help further explain the PCV system on the Focus ST and the Mazdaspeed 3/6 and what it does, as well as how it can be improved upon. Modding these cars is tons of fun but can be quite confusing at times. I hope this helps to shed some light on some of the more complicated aspects of the cars you guys enjoy making faster and more powerful.

What is Positive Crank Case Ventilation?

Positive Crank Case Ventilation or PCV is the operation in an engine that involves relieving excess crank case pressure caused when combustion gasses push past the piston rings and put positive pressure on the crank case. This excess pressure is called Blow By and is made up of unburnt fuel and the byproducts of combustion. If the Blow By gasses are not relieved, they will condense with the oil and cause excess sludge build up as well as push their way past seals and gaskets causing leaks and sealing problems.

In order to combat this issue, auto-manufacturers began installing ventilation systems in the early 1960’s. These early systems vented the excess pressure to the atmosphere by running a tube from a high point on the valve cover down into the slip stream under the vehicle. The air moving past the tube would create a vacuum in the crank case and draw out the excess gasses while the car was moving. Now days, everything is kept internal. The valve cover is vented to the intake post MAF and in the case of turbo vehicles, there is a PCV valve that allows Blow By gasses to be vented into the intake manifold under normal operating conditions. This PCV valve is designed to shut as soon as it sees positive pressure from the intake manifold and this operation keeps boost from entering the crank case. The vent on the valve cover is the only vent that operates at wide open throttle (WOT).

The upside to the modern form of crank case ventilation is that it does function better and relieves more excess crank case pressure than the old system under all driving conditions. It is also more environmentally friendly. However, there are some negative side effects from recirculating the blow by gasses. Because the contents of the Blow By gasses contain unburnt fuel and oil vapors as well as other noncombustible gasses, they can reduce the overall effective octane rating of the fuel being injected into the cylinders as well as reduce the displacement of the engine itself as part of the Blow By gas is no longer combustible and takes up space in the cylinder that could otherwise be used for combustion. The reason the Blow By gas reduces the effective octane is because the oil/fuel vapor leftover in the gasses will ignite at lower energy levels and can cause detonation in the cylinders. Another problem with having these Blow By gasses recirculated back into the intake system is that it will essentially turn your turbo and intercooler into an air oil separator (AOS). The oil will then coat the inside of the turbo increasing drag and reducing its ability to perform as well as the intercooler decreasing its ability to efficiently cool the charge air.


How can this be made to work more efficiently?

To start, the best approach is to keep your engine from burning the Blow By gasses. There are several ways to go about doing this. The easiest method is to simply put a breather filter on your valve cover and block off the port on your intake. This will keep the gasses out of the system during WOT and will keep your intercooler contaminate free. You may actually see a power gain from this because you are now filling the cylinders completely with fresh fuel and air. You can also go “old school” and run a hose from the vent on the valve cover down to the slip stream for a little vacuum effect.

This still leaves the PCV valve that vents crank case pressure to the intake manifold during part throttle/idle operation. You may not care to address this as it is not in effect during WOT when you are needing every last scrap of power, however, on the Focus ST and the Mazdaspeed 3/6, you do run into the issue of oil and other contaminates coating your valves. Being Direct Injected, there is no fuel spraying over the backs of the intake valves to keep them clean. That is where a standard catch can comes in. It traps the contaminates and keeps the intake manifold as well as the backs of the intake valves nice and clean. However, it does nothing to improve on the ventilation of the crank case.

For stock turbo cars, the factory crank case ventilation system may not need to be improved upon other than simply venting the valve cover to keep the Blow By gasses out of the intake/turbo/intercooler. But when you are running a larger turbo and more boost you will inevitably get more blow by and thus higher crank case pressures. And here lies the need to improve the factory crank case ventilation system.

To completely isolate the crank case ventilation system you can run a vented catch can. The vented catch can is attached to both the valve cover breather and the PCV valve so that no Blow By gas is ever let back into the engine. This option is better than stock because it allows both vents to operate under WOT conditions. However, the port on the PCV box is small and may still not be enough for vehicles equipped with larger turbos pushing more Blow By into the crank case. To improve upon this PERM Tuning has come up with a solution. The PERM Tuning PCV Plate replaces the factory PCV box and opens it up to two relatively larger ports instead of just one. The best setup to go with this plate is to run a normal catch can between one of the ports and the intake manifold. A checkvalve acts like the PCV valve and keeps you from boosting the crank case or catch can at WOT and the one port only functions under part throttle conditions and helps pull out the excess pressure via the vacuum from the intake manifold. The second port is routed to the vented catch can along with the valve cover port. This setup gives you twice the ventilation at WOT and still maintains good ventilation during part throttle conditions.

In conclusion, there are many benefits of setting up your PCV system to vent crank case pressure more efficiently. It prolongs the life of your oil, keeps contaminates off the backs of your valves and out of your engine, keeps power robbing Blow By gasses out of your cylinders for maximum power potential, and it provides plenty of crank case ventilation to ensure that the only pressure build up in the engine is on the top of the pistons where it belongs.

TL;DR

Excess crank case pressure robs power. Proper ventilation of the crank case restores power and lets the car run more efficiently.

[swd120]

Pictures are worth 1000 words: Perm PCV plate theory thing

PCV Plate