Venting a typical V-8 engine isn't a complex affair. A breather atop each valve cover is usually all that's needed. Of course, replacing one with a PCV valve to introduce a bit of vacuum into the system and to redistribute the unburned hydrocarbons back into the engine via the carburetor or throttle body yields a cleaner and much more environmentally friendly solution. Supercharged applications, however, can be a bit more finicky. Increased pressure in the crankcase can cause blow-by when using a traditional push-in style breather, covering that trick engine compartment with a fine mist of fuel-oil. Adding a PCV valve is a good idea, until a boost situation kicks in, at which time the internal check valve is forced shut, rendering the valve moot. At this point, instead of drawing fresh air into the breather and vanquishing the crankcase pressure through the PCV valve, the internal pressure is vented out the breather, possibly resulting in another oily blow-by situation. This usually happens when the engine is under load or at high rpm, which is when pressure builds up quickly and needs to be relieved the most.
The extreme solution to prevent all of this is to install a vacuum pump that continuously draws the pressure out of the crankcase. For most modest horsepower street engines, however, a vacuum pump is overkill, though it probably wouldn't hurt since drawing out the vapors and relieving any internal pressure is a good thing. What isn't a good thing is drawing too much out of the crankcase, which can be a problem in an engine that builds up significant crankcase pressure and is equipped with a vacuum pump. In this situation, the system can draw out more than just leftover hydrocarbons and vapor, but engine oil as well, requiring some sort of catch can to retrieve the gathered fluid.
It was with this in mind when it came time to design the crankcase ventilation system for our supercharged LS engine. I knew it was going to be important to let the engine breathe, but I also wanted to design a system that wouldn't fill the engine compartment with hydrocarboneous byproduct. And since proper ventilation is key to improved ring seal, oil scavenging, and windage, I wanted to be sure our supercharged LS had plenty of opportunity to breathe freely.
You'll recall a few months back when we dressed up our engine that we're using PML finned valve covers from Speedway Motors. These feature a 1-inch hole on the top of each designed to accept a push-in breather or PCV valve. Originally, my plan was to use a pair of PCV valves with the internal valve removed, one in each valve cover, plumbed to a Summit Racing breather tank on the firewall. With the valves removed, the PCV valves simply act as 90-degree elbows. This setup would be the "inlet" side of the crankcase venting system, while a traditional PCV valve mounted in the valley cover and connected to the intake would act as the "outlet" or recirculation side of the system. Fresh air would be drawn through the breather tank and down through each valve cover then out the valley of the engine, via the vacuum signal on the intake side of the PCV valve.
1. Here are the PML valve covers that we're using on the LS engine. Note the 1-inch hole at the top of each for a breather/PCV.
2. My initial idea of using a PCV, sans valve, would have worked perfectly. I just didn't like the possibility of blow-by occurring, especially since the valve covers and intake are as-cast pieces, which are especially hard to keep clean given their porous nature.
3. After whittling a small adapter out of aluminum and a little machine work, I came up with this nifty deal. The adapter mates to the valve cover via three #8 fasteners and accepts an Aeromotive ORB-06 AN fitting. A similarly sized and branded stainless braided line runs from each valve cover…
4. …to a "Y" fitting at the back of the engine.
5. From the "Y" fitting, a single AN-6 line runs to one side of a Summit Racing breather tank. This tank allows the engine to breathe freely while containing any oil that may be drawn up through the system.
6. The second part of the crankcase ventilation system is related to the valley cover. Later-model LS engines come equipped with a PCV valve grommet location, but our LS327 crate engine did not, so it was necessary to drill and tap one.
7. Once again, AN lines and fittings were used to connect the valley cover breather line…
8. … to a Moroso air/oil separator. LS engines are notorious for drawing oil into the crankcase ventilation system, especially from the valley area of the engine so we opted to run the fumes through an air/oil separator before mating it to the breather tank, allowing the pressure to evacuate the system, sans oil.
9. An overall shot of the system gives you a good idea how it all works. Basically, the valve covers vent through the breather tank on the left while the valley cover vent passes through the air/oil separator before venting out the breather tank as well.