Blow-By 101: How To Prevent It From Ruining Your Engine

  • Droplets of oil and fuel pushing past the piston rings and into the crankcase are known as blow-by.
  • PCV systems vent blow-by from the crankcase but introduce oil and fuel into the air intake.
  • Oil catch cans provide a solution to catching rogue fuel and oil vapors without subjecting your intake to them.

Welcome, ladies and gentlemen, to the eighth wonder of the world: the internal combustion engine! They’re pretty magical beings when you analyze them. Controlled bombs that combine and combust air and fuel to create motion. When you think of how many moving parts there are inside an engine, it makes it even more impressive that they crank right over and perform after sitting sedentary for decades. It’s not all rosy, though.

The power they create in their combustion cycle is not without its byproducts. High pressure on the top side of the piston pushes combustion gases, along with droplets of oil and fuel, past the piston rings into the crankcase. This dreaded mixture is known as blow-by, the subject of today’s discussion. Thanks to some keen research and product development by our friends at Mishimoto, we’re here to help spread the knowledge of why blow-by isn’t good and what measures you can take to limit it.


Where older engines might have vented harmful gases into the atmosphere, modern engines come equipped with Positive Crankcase Ventilation (or PCV) systems to vent blow-by.

Pressurizing your crankcase is not suitable for many reasons, but most of all, it will cause issues with oil sealing and robbing your engine of precious power. Instead, blow-by is pulled from the crankcase via the PCV system and routed back into the intake via a vacuum. Typically, oil and fuel are not what you want in your intake system. Modern vehicles implement air-oil separator systems to minimize how much of the blow-by mixture makes its way back into your engine, but most are not entirely effective.


Sludge, carbon buildup, and knock. Over time, blow-by and its accompanying byproducts reduce engine efficiency (and even power) as oil and fuel coat the intake. It doesn’t stop there. If you have a forced induction setup, then the intricate pathways of your intercooler can become infected as well. With older engines, worn piston rings and cylinder walls allow more fuel and oil to pass into the crankcase.

Furthermore, past the intake systems themselves, the effects of recirculated blow-by can hinder the performance of intake valves and engine internals, including inside the cylinders (yes, the very same engine area working to vent the stuff out).

Both port and direct-injected engines can suffer from sludge buildup on the backs of valves. While a regular port engine’s mixture of fuel typically cleans the backside of the valve, it is not immune to sludge buildup, should the blow-by condense enough. Direct-injected engines, however, never stood a chance because the fuel never reaches the back of the valves for any cleansing. Instead, it’s injected directly into the cylinder (downstream from the intake valves), which, as you can imagine, will cause an issue with the valves if the fuel isn’t cleaning off the blow-by. In return, blow-by and sludge compound itself, which impedes flow into the cylinder and can cause more significant engine issues.

After the valves, blow-by that makes it down into the cylinder can build up enough to effectively increase the compression ratio by taking up space in the cylinder. It can also lower the octane rating of the injected air-fuel mixture. Anyone familiar with knock (or pre-ignition) knows what happens next. The foreign blow-by lowers the octane rating enough to ignite the mix before the spark plug fires. This process, known as knock, causes very high cylinder pressures, ruins timing, and can (and will) kill your engine.

Not to mention, any buildup coating the cylinders will foul the spark plugs, causing misfires as well.


Okay, blow-by sucks, so let’s just do something about it, right? At its core, a catch can just catches and condenses the fuel and oil vapors associated with blow-by before they reenter the intake system. For this, we turned to Mishimoto and its wide array of baffled catch can solutions. The Delaware-based brand has a team of dedicated engineers who took all of the guesswork out of oil catch cans for you by creating application-specific direct-fit kits for popular models and universal catch cans for any custom application under the sun.

Our can of choice today is a pretty choice can (ha!), the carbon fiber baffled oil catch can. This solution has a quality 3K twill weave carbon fiber construction that cuts down on the overall weight added and gives true motorsport feel to any engine bay. Its 7.4 fluid ounce capacity is more than enough to spread out our service intervals, and if it’s not, it has a simple drain incorporated into the bottom. Otherwise, a quarter-turn lid makes checking levels easy while still providing the necessary tight seal when assembled. Inside the can, a high-flow filter protects without affecting PCV pressures, and the specially-designed baffle improves air-oil separation. On the outside, a sleek CNC billet bracket securely mounts the can in my engine bay and Mishimoto’s Lifetime Warranty ensures long-lasting protection.


There are two things a catch can must do well, allow blow-by gasses to be vented from the crankcase efficiently, and give the oil and fuel vapors somewhere to condense. By completing these two functions, it will prevent the harmful elements from reentering the intake system. Now, of course, there’s more than one way to do this.

One way is to vent the crankcase to a can equipped only with a small breather on top. This method vents pressure out of the top of the can and will eliminate any possibility of recirculating blow-by, as it has no way of completing the cycle. This is a sure-fire way to protect your engine, but in doing so, it harms our environment, and there may even be legal issues associated with it. In most states, a vent-to-atmosphere (VTA) catch can will not pass inspection, and venting a PCV system that is not factory vented is prohibited at a federal level!

The other method is recirculating into the intake, which auto manufacturers generally practice, especially those with forced induction models. Not only will this method be friendlier to our environment, but it utilizes the vacuum of the engine to clear oil and fuel vapors from the crankcase faster. In a VTA setup, when that vacuum is removed from the equation, vapors can stay in the crankcase and condense, which we learned earlier is a no-no.


Yes, the catch can and its associated vacuum lines removed the harmful vapors from the crankcase, but what’s keeping them from being sucked right back into the intake? Well, to explain that, let’s establish a little science behind the whole thing.

When blow-by exits the crankcase, it’s so hot that it’s in a gaseous state. It doesn’t become liquid again until it reaches the intake, the area of the engine designed for condensing air. Unless, of course, it enters a baffled chamber to slow down the gas, cool, and condense the vapors, then it changes to a heavier liquid beforehand. Aiding this process is an increased surface area on the baffle itself and Mishimoto’s filter we discussed earlier. Colliding the vapors with the filter material will collect large particles while the vacuum pulls the rest of the gases back into the intake. All of this culminates in air-oil separation and keeps harmful vapors trapped while the engine retains its precious vacuum.


After a speedy (and easy) install, I removed my factory PCV hoses and incorporated Mishimoto’s carbon fiber catch can with new lines. My setup is now ready to continue evacuating harmful oil and fuel vapors without hurting the environment or integrating them back into the engine. It’s a win-win. I also have a little more motorsport aesthetics in the engine bay, with the clean carbon fiber weave making itself seen within my bay, so Michael Scott would say it’s a win-win-win — we all win. If you want to know more about oil blow-by, visit Mishimoto’s engineering blog. You can also browse the company’s full line of catch can solutions on its website.