Tech: How External Vacuum Pumps Free Up Horsepower

External crankcase vacuum pumps will provide significant power gains on both racing and high-performance street engines, primarily by promoting superior ring sealing. It’s an engine-performance trick practiced since the late ‘60s, first with passive crankcase evacuation methods or so-called Pan-Evac systems. Today, manipulating the pan vacuum with a dedicated pump — such as those offered by GZ Motorsports and Moroso — is more effective and gives the engine builder another tuning tool.

This Reher-Morrison big-block Chevy illustrates the simplicity of an external vacuum pump system. Crankcase pressure is evacuated via the valve cover and pumped to an external tank which vents the pressure and retains excess oil for periodic draining.

The concept was first introduced by Bill Million at Hedman Hedders and initially developed on the engine dynos at both Edelbrock and Traco Engineering. Traco-powered Penske Trans Am Camaros were among the first cars to run it in competition.

A Pan-Evac system connects the valve covers on both sides of the engine to the exhaust header collectors with one-way check valves and vent tubes that blend into the exhaust stream at approximately 45 degrees. Exiting exhaust gasses pull a vacuum on the tubes, thus drawing excess crankcase pressure from the sump and discharging it out the collector. This action relieves blow-by pressure under the rings, reducing windage and contamination. Although purely passive, the system proved remarkably efficient, and thousands of racers used them religiously until the emergence of modern external vacuum pumps.

All engines generate “blow-by” or leakage past the rings, creating positive pressure in the crankcase. Blow-by consists of compressed air-and-fuel mixture from the compression stroke and combustion leakage past the rings with associated by-products from the power stroke. It may include fuel, exhaust gasses, carbon and other contaminants. Crankcase pressure hurts power by upsetting ring seal, and it opposes piston motion by creating drag and windage. It also introduces harmful contaminants to the lubrication system and poisons the incoming mixture in the combustion chamber.

Vacuum Pump Advantages

Pulling vacuum with a dedicated pump has been the preferred method for well over two decades now and the technology has steadily advanced with important performance benefits that include:

  • Improved ring seal by relieving crankcase pressure below the rings
  • Reduced friction by allowing lighter tension rings
  • Reduced windage around the rotating assembly
  • Improved oil scavenging
  • Reduced parasitic pumping losses
  •  Minimized cavitation (in wet sump applications)
  • Reduced lubricant contamination
  • Improved bay to bay breathing in Y-block configurations
  • Improved power via reduced mixture contamination
  • Reduced gas port contamination

An external vacuum pump evacuates excessive crankcase pressure, resulting in less-than-atmospheric pressure in the sump. This pressure is readable with a vacuum gauge. The size and speed of the evacuation pump, along with the size of the vacuum line, control the amount of vacuum. Engine builders still debate the amount of vacuum required to neutralize crankcase pressure with optimal effect on windage and ring seal, while still minimizing combustion chamber contamination. While the rotating assembly is largely credited with creating windage in the crankcase, we must also remember that with pressure in the crankcase, rapidly descending pistons tend to pump wind toward the rotating assembly, exacerbating windage and parasitic drag.

Pressure evacuation reduces this effect. Conventional wisdom says that 12-to-14 inches of vacuum is required to generate beneficial power gains. Anything above 15 inches may require closer attention to wrist pin oiling (pin oilers) and cylinder wall lubrication. Within this narrow range, builders can employ lower tension rings to reduce friction without fear of losing effective ring seal. This move, however, is very much dependent on ring type, cylinder finish, ring land tolerances and even stroke length and engine speed, all of which affect the frequency and dynamics of piston reversal and its effect on ring performance. That being said; the benefits of crankcase pressure evacuation are particularly effective on larger displacement engines or those with power adders that generate higher cylinder pressure and more blow-by.

Wet-sump applications

Wet-sump systems utilize a belt-driven external vacuum pump plumbed to pull vacuum from one or both of the valve covers and discharge to an external oil separator and breather mounted nearby.

Moroso offers convenient pump mounting kits that accept a vacuum pump and alternator off the front of its electric water pump housing via a dedicated aluminum mounting plate.

“Fifteen inches of vacuum without pressure-fed pin oiling is generally a safe upper limit, particularly in wet sump drag racing application if longevity is an issue,” says Greg Zucco at GZ Motorsports, noting some engine builders prefer more or less vacuum depending on the particulars of their engine combination like the ring package, sump type, engine speed and so on. “We have stayed with 15-inches, even with folks wanting 20-inches, because it’s risky for longevity unless using a dry sump,” Zucco adds.

Moroso’s Scott Hall agrees that 12-15 inches of vacuum is the common number that most engine builders like to see, because it yields good power gains and it is relatively easy to achieve that amount of vacuum. He also attributes wrist-pin issues to late ‘90s ring configurations that often promoted air movement on the intake stroke and pulled oil off the wrist pins. Of course, many racers run pin oilers for the added security and also to help pull heat out of the piston crowns.

GZ Motorosports offers complete kits for numerous applications, including LSX builds (PN SPVPKLSX). It includes the VP 104 pump, pulleys, Gilmer belt, braided hose, breather tank and vacuum-control valve.

“Wet sump engines can now run as much as 16 to 18 inches with modern piston-and-ring technology,” he adds. “We have run our own 632ci above 16 inches for many runs with no issues on the wrist pins or exhaust valve guides.”

Hall also cautions that pulling too much vacuum in a wet sump system can affect overall oil pressure. Vacuum in the system increases the oil volume that the engine flows based on the reduction in resistance the oil pump sees on the pressure side. Vacuum may draw oil out from between the bearings and journals like a straw; thus, flow increases and the system pressure drops. Running a high-volume pump is often necessary.

Determining proper pump size

“If we could conveniently measure oil volume it would tell us more about the system than oil pressure,” explains Hall. “Moroso builds both vacuum systems and oiling systems so we have been intensively studying and resolving these issues for a long time. It is common to see an 8-to-12 psi oil-pressure drop when running 12-16 inches of vacuum because of this.”

Fifteen inches of vacuum without pressure-fed pin oiling is generally a safe upper limit.–Greg Zucco, GZ Motorsports

External pumps are rated by airflow capacity in cubic feet per minute (cfm) while crankcase vacuum is measured in “inches of mercury.” A larger pump moves more air and creates higher vacuum if operated at the proper speed and supported by non-restrictive plumbing. Most applications use a single -10AN or -12AN input line and in some cases a pair of -10A input lines on larger engines. In every case the pump size must adequately support the desired airflow level throughout the engine’s effective power band. Pump suppliers such as Moroso and GZ Motorsports provide a selection of pump sizes and accessories to support popular applications.

Pump speed is determined by the drive ratio between the crank pulley and the pump pulley. Zucco suggests limiting pump speed to no more than 6,000 rpm for optimum durability, and GZ recommends spinning the pump at 54 to 75 percent of crankshaft speed, depending on the application. As a rule, it’s better running a larger pump at slower speed to achieve the level of vacuum required while preserving pump longevity.

Dry Sump Applications

Racing dry-sump applications are a little different. A scavenge stage not pulling oil is pulling some amount of vacuum. Depending on engine size and the number of scavenge stages, a dry-sump engine will likely not require a separate external vacuum pump. The amount of vacuum produced with a dry-sump pump depends on pump speed, the number of stages and the amount of time each stage spends scavenging oil or creating vacuum.

“Dry-sump scavenge stages often do not pull adequate vacuum and the gerotor design tends to have a lot of parasitic drag,” says Zucco. “So many of our customers add our Super Pro vacuum pump to their dry-sump system to develop the 20-plus inches of vacuum they desire.”

High-end systems running as many as five or more stages often dedicate all but one, sometimes two, stages specifically to vacuum. In effect, a dry-sump pump is also an external vacuum pump. In some cases one stage may be connected to a valve cover or even the lifter valley area. In both dry- and wet-sump applications it is advisable to plumb an independent pressure balance line connecting the crankcase or oil pan to a valve cover or the lifter valley. It prevents excess pressure exiting the crankcase through the oil drain-back holes in the cylinder heads, which could prevent oil return flow, affect the oil level and create unwanted aeration in the oil supply. An external line is often conveniently connected via the unused mechanical fuel pump port on small- and big-block Chevys. Most dry-sump storage tanks also incorporate an integral breather tank, so a remote unit is often unnecessary.

Engines running on alcohol have always produced lower vacuum readings. — Scott Hall, Moroso

Best performance occurs when optimum vacuum is achieved early in the power band and maintained across the effective power band of the engine. The correct pump is sized and operated at the desired speed to accomplish that goal. As a rule that means using a pump with enough air flow capacity and a vacuum control valve to maintain the desired amount of vacuum. If the pump begins to exceed the desired vacuum level, the control valve provides an adjustable air leak to prevent it. Adjustability is an important factor as it follows engine speed and vacuum. It is further affected by normal vacuum leak sources present, such as faulty gaskets or seals and even the dipstick.

A vacuum pump must be sized to compensate for whatever level of leakage might come from these sources. In the early days, extreme measures were taken to eliminate all leakage. These steps included special reverse-lip crank seals and silicone sealing the valve covers, intake manifold and so on. These steps are now unnecessary with current pump technology that applies proper pump sizing and speed along with vacuum control valves to produce predictable results. Properly sized modern pumps easily overcome these smaller vacuum leaks while using the control valve to maintain the desired vacuum.

Moroso regulator (PN 22633), top left) screws directly into the vacuum pump instead of a valve cover to prevent pulling excess oil out of the crankcase. Moroso’s regulator (PN 22629), bottom left, offers 360-degree adjustability. Made from 6061 billet, it can be easily disassembled for cleaning and works with any vacuum level. The Moroso oil breather tank (PN 85467) provides air/oil separation through interior baffling made from mesh media. Includes a billet clamp for remote mounting and a -8AN drain plug.

Many engine builders monitor crankcase vacuum throughout the engine’s power range. From a simple vacuum gauge to a dedicated channel on a data logger, crankcase vacuum pressure is often monitored and plotted along with all the other relevant engine data to provide a more detailed picture of how crankcase pressure affects performance. If an engine loses power at a certain point in the rpm range for example, the data logger may show a sudden increase in blow-by pressure, indicating the likelihood of lost ring seal due to high-speed ring flutter or detonation, among other conditions. Whatever the case, it alerts the engine tuner who can then take steps to rectify it.

“Engines running on alcohol have always produced lower vacuum readings, regardless of whether they are carbureted injected or using an EFI system,” explains Hall. “Because the amount of alcohol is nearly double that of gasoline, the ring tends to hydroplane off the cylinder wall on its way upward — causing both oil contamination and less ring seal during combustion.

External Pump Tips and Tricks

“Racers often see the worst vacuum readings earlier in the racing season when the air is colder and the engines run richer due to greater air density,” adds Hall. “As the air gets hotter later in the season and racers lean the engines out, they see vacuum readings go up, especially at idle and on the two step.”

External vacuum pumps are the ideal choice for hot street engines, bracket racing engines and other performance applications such as off-road racing or drag boats. The key to getting the most out of an external vacuum pump lays in choosing the setup that best suits your engine.

Shown are two vacuum relief valves from GZ Motorsports (PN 101A, 101B), top left. Both are adjustable adjustable from 5 to 20 inches and are suited for high flow-rate pumps. The only difference is the mounting threads — one is 1/2-inch NPT and the other is 7/8-14 NF. Pump speed is adjusted by sizing the pulleys. Shown top right is the GZ Motorsports 1-inch crank mandrel and three different Gilmer pulleys. GZ offers a a number of breather tanks, bottom, including this model designed for C5 and C6 Corvettes (PN BT2) but can be adapted to other applications. The double-baffled tank is 9-inches tall and includes the necessary fittings.

“While I would not suggest their use on stock vehicles, the LSX platform appears to work well regardless of modifications, and the pump eliminates the intake oil contamination that platform appears to have as a result of the PCV system,” says Zucco.

Pump choice is primarily based on engine size and horsepower level with appropriate attention to pump speed and vacuum line size. Manufacturers offer a variety of pump sizes to accommodate large and small engines and various horsepower ratings.  Smaller engines use smaller pumps and lines while large engines are just the opposite. Power adders such as turbochargers, superchargers or nitrous oxide increase cylinder pressure and blow by and thus require larger pumps and bigger lines to accommodate increased airflow.

Easy installation

Installation procedures for most engines are surprisingly easy, requiring only basic hand tools. Most pump kits include mounting hardware that provides the ideal pump location and proper drive pulley alignment. GZ Motorsports offers an extensive range of all inclusive kits that are sized and equipped for specific applications. Moroso provides a full range of pumps and individual hardware so builders can select the supporting components they want for their installation.

Illustration shows a typical Pan-Evac system that uses exhaust pulses to draw vacuum from the valve covers.

It is very important to carefully check drive-belt alignment and adjust when necessary to achieve perfect alignment between the pulleys. Some users will require modifications and/or minor fabrication if they plan to mount the pump in a non-standard location. Vacuum pumps require a dedicated belt that does not drive any other components. If you remote-mount the breather tank, you may have to fabricate a longer discharge hose, although most kits accommodate inner fender mounting arrangements. Pressure and oil mist evacuated from the crankcase is discharged to the external breather tank that vents the pressure through a filter and contains the excess oil for periodic draining.

“Make sure that the exhaust fitting is positioned between the 5 o’clock and 8 o’clock position if you can,” suggests Hall. “This allows oil to easily make its way out of the exhaust fitting into the breather tank. In any other position, excess oil will build up at the bottom of the pump and may cause the vanes to stick in the rotor slots.”

Clean arrangement with the pump pulling vacuum from both valve covers.

Variations in pump design may also affect your choice of pumps to suit your particular requirements. Pump lubrication is accomplished by oil mist in the air flowing through the pump. Early pump designs experienced contamination problems and required constant cleaning and maintenance to prevent the pump vanes from sticking. Pump manufacturers like GZ Motorsports and Moroso have each addressed this in their own way so the end consumer can make a thoughtful choice based on his needs and interpretation of their individual pump features. Direct consultation with the manufacturer will ensure the best possible choice.

External vacuum pumps enjoy a well documented performance record. Power gains ranging from 10 to 35 horsepower are common. These gains are on par with and sometimes exceed those provided by a cam or intake manifold changes and pump systems are much easier to install. That makes vacuum pumps one of the best bang-for-the-buck products available for both high performance and racing applications.

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