THE SHOP

April '20

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36 THE SHOP APRIL 2020 n engine's ability to breathe is critical for a number of reasons, including relieving crankcase pressure. Excess pressure can result in external oil leaks, oil pressure blow-by and increased windage concerns, and is detrimental to piston ring sealing performance. While installing breathers on the valve covers can be adequate at times, depending on other build factors it may not be enough to control excess crankcase pressure. The remedy is installing an external, belt-driven vacuum pump, which creates a vacuum pull that also adds horsepower due to a reduction of parasitic oil cling. Running a vacuum pump also allows you to run lower-tension piston rings, since the negative pressure created by the pump increases the rings' ability to seal. THE BENEFITS OF VACUUM Generally speaking, operating at about 15 inches of vacuum at high engine speeds is the goal. The engine must be sealed, meaning eliminating valve cover-mounted breathers. The vacuum pump features a belt drive (usually radius tooth), with the pump driven by the crank. A setup using a tooth count combination of both the pump's driven gear and the crankshaft drive gear at about 50-51% is generally ideal to regu- late the speed of the pump. If the pump is driven too fast, it loses efficiency. Plumbing the pump is straightforward: a pair of -12 AN hoses is run from the pump to the front face of each valve cover. This allows the pump to pull enough of an oil mist to stay lubricated. If the hoses are connected to the roof of the valve covers, you may not pull an ade- quate amount of oil. Installing a vacuum relief valve (which can be mounted at the valve cover) allows a pre-set between 12 and 20 inches. The use of an external vacuum pump typi- cally results in horsepower gains of 10-35 hp. For purposes of illustration, in this article we installed a vacuum pump system on both a Chevy 422-ci racing small-block that fea- tures 14:1 compression, and a 632-ci race big-block at 15:1 compression. VALVE COVERS The valve covers were a straightforward selec- tion—Moroso welded aluminum models. Since we needed to eliminate breathers but retain an oil fill port, a Jones Racing weld-in female bung was installed to the roof of the left-side valve cover on the 632 big-block build. This is sealed with a screw- in aluminum fill plug that features an OD hex or finger grooves and a sealing O-ring. Installing the oil fill bung required cutting an 1-5/8-inch hole in the cover roof, in- line between the No. 1 and No. 3 cylinder rocker pairs where you have a clear shot for oil fill between rocker arm locations. The bung is then TIG welded to the aluminum valve cover, preferably at the underside of the roof to maintain a tidy appearance. On the 422 small-block build, I used an Earl's bung (requiring a 1.800- Breathe The benefits of installing a belt-driven vacuum pump. By Mike Mavrigian a Little Easier Adding a vacuum pump allows you to seal the crankcase and create negative pressure, reducing windage concerns, improving ring seal, reducing parasitic power loss and eliminating pressure leaks. The use of a vacuum pump may add 10-35 hp. This 422-ci small-block pulled 730 hp. Vacuum pumps are belt-driven, typically with a radius tooth belt. Various pulley diameters are available to achieve proper pump speed ratio for given applications, and various belt lengths are available. PRECISION ENGINE

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