An Adjustable PCV Valve For The Street

Words And Photos: Jeff Smith

Sometimes it’s the little things that make a difference for street engines. While bench racing sessions will go on forever about hero horsepower and torque numbers and which engine combination would be the best for your current street car, often there are seemingly minor considerations that can produce positive effects.

Take the lowly PCV valve for example. Positive crank ventilation (PCV) was created in the early ‘60s based on General Motors research that revealed oil vapor dumped into the air (and on the road surface) from road draft tubes were a major source of pollution. To get a feel for that effect, all you have to do is look a few photos of early freeways where there are long black streaks down the center of each lane – the results of thousands of gallons of oil dumped on the highway. GM designed the PCV system to seal the engine and capture these crankcase vapors and introduce them back into the intake. Beyond reducing emissions, the PCV valve system also benefits engine durability by removing harmful contaminants from the crankcase.

One of the common by-products of combustion is water vapor. This vapor collects in the crankcase and will remain a liquid that mixes with acids F. If this acid-laced water is allowed to collect in the crankcase, eventually forms a nasty sludge inside the engine. A PCV valve helps pull these contaminants out of the sump before the sludge can form – assuming, of course, that the PCV valve is working properly.

Performance street engine builders sometimes eliminate the PCV, choosing instead to rely on breathers to vent the crankcase. These breathers do vent the crankcase, but rely on crankcase pressure to force the gasses out of the engine. A properly functioning PCV valve actually introduces vacuum into the crankcase that does a much more efficient job of removing these vapors.

Off-the–shelf PCV valves employ a tapered valve and spring arrangement for a staged flow between idle and cruise rpm. Unfortunately, most stock PCV valves underachieve on both levels, creating only a tiny volume of flow from the crankcase when the engine is running at part-throttle.

Production engines are tuned to use a specific PCV valve. But what happens when the engine has been modified? Until now, engine builders have had to rely on a flip-of-a-coin selection for a production style PCV valve and hope for the best. Since there is no simple way to accurately measure the flow through a PCV valve, users have to assume it was working as intended. Gene Wagner and his son Matt decided that this wasn’t good enough. Working out of their Bear Creek, Pennsylvania shop, they spent years designing and building a billet aluminum high performance PCV valve they call the M/E Wagner (ME/W) Dual Flow PCV valve. The Dual Flow valve is calibrated to produce one flow rate at idle and increase this volume during cruise situation. Because there are so many different street engine sizes and combinations, the valve is completely adjustable.

The valve operation is divided into two circuits. The first is a dedicated idle circuit. The flow rate is adjustable with the “Idle” screw. The second circuit opens when there is more vacuum present during cruise applications to pull additional volume. The idle circuit is pre-set from M/EW although it is completely adjustable. This sets the flow rate, which is much greater than most stock PCV valves. Flow rates are not easy to measure, but you can tell by the strength of the signal (compared to a standard valve) that the volume is increased. The second adjustment is for cruise speeds where engine is lightly loaded and the engine could benefit from bypassing an increased volume of crankcase vapors. There are well-written instructions included with the valve to show you how to adjust the valve.

Increasing the amount of vacuum used to pull additional vapors out of the engine may mean minor changes to the idle speed and mixture screws on a carbureted engine. This is because the new PCV is essentially bypassing a small amount of air around the carburetor. This may demand a slight idle speed adjustment. After the idle speed is adjusted, this may also require a small change to the idle mixture screws. If the Dual Flow valve is used with EFI, no adjustments should be necessary as the computer will automatically adjust the idle airspeed motor to compensate for any changes in idle speed and air-fuel ratio.

For engines with big cams and big Holley carburetors, the M/EW Dual Flow valve can also be used as an idle by-pass to increase the amount of air into the intake for idle purposes. This will allow the tuner to close up the throttle blades and perhaps not have to resort to drilling holes in the throttle blades in order to place the throttle blades I the correct relationship to the idle transfer slot.

So how effective is this greater crankcase flow volume? M/E Wagner used a sensitive pressure device to test the effectiveness of stock PCV valves on a mild small-block Ford street car. Of the four stock PCV valves tested, one pulled vacuum in the crankcase barely 3 percent of the time, while another was effective only 11 percent of the total driving cycle. A third was capable of vacuum barely over half the time. The Dual Flow valve on the other hand produced vacuum 99 percent of the time the engine was running. That last 1 percent was at wide-open throttle (WOT).

Generating a continuous amount of crankcase vacuum to pull vapors out of the engine is a good thing. If you’ve ever driven a street engine without a PCV valve that has breathers sticking out of the valve covers, then you’ve no doubt also smelled engine oil vapors while driving the car and also noticed a light film of oil on the valve covers. Even with a PCV valve, it’s common to see oil coating the area around the valve covers and the PCV valve. This could be because of a poor seal in the valve cover, but more likely its due to crankcase pressure pushing oil vapor out of the engine, which means the PCV valve is not doing its job.

If you currently have a PCV valve on your engine, you can test its effectiveness at idle with a vacuum gauge hooked to read manifold vacuum. Pull the PCV valve out of its grommet and plug the end with your finger. The engine vacuum reading should improve by 1 – 2 inches. If the vacuum reading doesn’t change or changes very little, the PCV valve is not doing its job.

Because the M/EW PCV valve flows additional crankcase vapors, this also means you will need to pay attention to the baffle placed below the PCV pickup point. With aftermarket valve covers, it’s important that engine oil not have a direct, line-of-sight contact with the pickup side of the valve. This will allow the PCV valve to pull both engine oil and vapor. Even just a flat sheet baffle with work well, as long as there is a minimum 3/4-inch clearance between the bottom of the PCV valve and the baffle itself. Otherwise the increased velocity past the baffle tends to pull oil into the valve, which is counterproductive.

With this increased volume of vapor, you may want to consider a vapor separator as an additional component of the system. Vapor separators are becoming increasingly popular, especially with the LS engine crowd. This device separates liquid oil from the vapor, reducing the amount of oil drawn into the intake manifold at part throttle while still reducing the pressure in the crankcase. If oil consumption increases, you might consider either reducing the volume of vapor passed by the Wagner unit by adjusting the idle portion of the valve, or by employing an oil separator.

There are several oil separators on the market. M/E Wagner prefers a Moroso aluminum unit that can be bolted near the engine and placed in between the engine and the Dual Flow PCV valve. As its name implies, the unit is designed to separate the liquid oil from the vapor. After a number of miles, the oil can then be drained from the separator. You can also use this collection point to measure the amount of oil pulled by the Dual Flow valve. If the amount of oil in the separator seems excessive, first determine if the PCV valve is sufficiently baffled. If it is, then you might consider reducing flow slightly by adjusting the idle port slightly.

A properly functioning PCV valve can have a big effect on engine performance and durability even if the improvements may not show up on a drag strip time slip. But the improvements demonstrated by the Dual Flow PCV valve are hard to ignore and may in fact improve the life of your favorite street engine.

Parts List

Description PN Source Price
ME/W Dual Flow PCV valve DF-17 M/E Wagner $129.00
ME/W Inline Adapter for Dual Flow PCV DF-17INL M/E Wagner $19.95
ME/W Vacuum gauge tuning kit DF-17VAC M/E Wagner $4.99
Moroso oil separator 85496 Summit Racing $134.97


M/E Wagner Performance

Moroso Performance Products

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