If you've been pushing any real power through a Gen III or Gen IV small block, you probably realized pretty quickly that the factory PCV system is a bit of a joke, which is why switching to ls valve covers with 10an fittings has become such a standard move. It's one of those modifications that looks like a purely aesthetic choice at first glance—those braided lines and black fittings definitely clean up an engine bay—but the reality is much more about keeping your engine from pushing out seals or spraying oil all over your headers.
The LS platform is legendary for its reliability and power potential, but it has a few "quirks" that become glaring issues once you add a turbo, a blower, or even just a high-compression heads-and-cam setup. One of those quirks is crankcase pressure. When you're making big power, there's a lot of air moving around inside that block, and the tiny 3/8" or 1/4" factory vent tubes just can't keep up.
Why the factory setup fails under pressure
Most stock LS valve covers have these tiny little nipple-style vents. They work fine for a daily driver Silverado or a stock C5 Corvette, but as soon as you start seeing increased blow-by from higher RPMs or boost, that pressure has to go somewhere. If it can't get out through the vents, it's going to find the path of least resistance. Usually, that means blowing out your front or rear main seal, or pushing the dipstick right out of the tube and painting your hood with 5W-30.
That's where ls valve covers with 10an fittings come into play. A -10AN fitting has an internal diameter of about 5/8 of an inch. When you compare that to the restrictive factory orifices, the difference in flow capacity is huge. By running two of these large fittings—one on each bank—you're giving that pressurized air a massive, clear highway to exit the engine and head toward a catch can.
Choosing the right material and style
When you start shopping for these, you'll see a few different types. You've got your cast aluminum versions, fabricated (welded) aluminum ones, and the high-end billet options.
Cast covers are usually the most popular because they look more "OE-plus." Brands like Holley or even some of the high-quality no-name brands offer cast covers that already have the -10AN bungs welded or machined in. These are great because they tend to seal really well and don't warp as easily as cheap thin-gauge aluminum covers.
Fabricated covers have that "race car" look with the sharp edges and visible welds. They're usually taller, which is a big plus if you're running aftermarket rockers that need extra clearance. However, you have to be careful with the cheaper fabricated ones, as they can sometimes have flange sealing issues if the welding caused any warping.
The importance of internal baffles
This is the one thing you absolutely cannot ignore: baffles. If you buy a set of ls valve covers with 10an fittings that are just empty shells with a hole and a fitting, you're going to have a bad time. Without a baffle, the oil being thrown off by the rockers is going to get sucked or pushed straight out of that -10AN line.
I've seen guys fill up a catch can in a single dyno pull or one trip down the drag strip because they didn't have baffled covers. You want a cover that has a plate or a maze-like structure inside that allows air to pass through but forces the heavy oil droplets to hit a surface and drain back down into the head. Most reputable manufacturers build these in, but it's always worth double-checking before you bolt them down.
Plumbing your new setup
Once you have the covers, you need to think about where those lines are actually going. You aren't just going to vent these to the atmosphere and let them drip oil on the ground (well, some people do, but your driveway won't thank you).
The most common setup is to run both -10AN lines to a dual-inlet breather tank or a high-quality catch can. If you're running a street car, you might want a sealed catch can that still utilizes some vacuum to help pull the vapors out. For a dedicated race car or a high-boost turbo build, many people just run a "vent to atmosphere" (VTA) can with a large filter on top.
Using -10AN lines gives you the flexibility to use braided nylon or stainless steel hoses. Not only are these durable, but they also use actual threaded connections. No more worrying about a rubber hose popping off a slippery metal nipple when the engine gets hot.
Clearance and coil mounting
Another thing to keep in mind is that the LS engine uses the valve covers to mount the ignition coils. When you switch to aftermarket ls valve covers with 10an fittings, you have to decide what you're doing with those coils.
Some covers come with bosses to bolt the coil brackets right back on. This is the easiest way to go, but sometimes the -10AN fitting is placed right where a coil would sit. In those cases, you might need a coil relocation kit. Relocating the coils to the firewall or hiding them under the intake manifold is a popular move anyway because it makes the engine look a lot cleaner, but it does add more work to the project in terms of custom spark plug wires.
If you want to keep the coils on the covers, look for "high clearance" versions that are specifically designed to fit around larger valvetrain components while still providing a clear path for the AN lines. It's a bit of a balancing act, but most of the modern designs have figured this out pretty well.
Is it worth the investment?
You might be wondering if you can just weld some -10AN bungs onto your stock aluminum covers. You totally can! If you're handy with a TIG welder or have a buddy who is, that's a very cost-effective way to get the same results. You just have to be careful about where you place them so they don't interfere with the internal baffles or the rockers.
However, for most people, buying a purpose-built set is the way to go. You get a fresh look, better clearance for aftermarket parts, and the peace of mind that the baffling is designed to handle the increased flow.
At the end of the day, running ls valve covers with 10an fittings is about protection. It's about making sure your gaskets stay where they're supposed to and that your engine stays healthy under load. Plus, let's be honest, those big AN lines look pretty "pro" when you pop the hood at a local meet. It's a functional upgrade that happens to have a nice aesthetic side effect, and in the world of car builds, that's usually a win-win.
If you're still on the fence, just go pull your oil fill cap off while the engine is running. If you see a bunch of "smoke" (blow-by) puffing out, or if you see oil residue around your gaskets, your engine is crying out for better ventilation. Switching to a larger fitting setup is probably the best gift you can give your LS short block this year.