On high-rpm race engines, enough positive pressure can build up in the crankcase during the piston's downward travel to adversely affect cylinder wall ring sealing. Overcoming positive crankcase pressure also wastes horsepower and blows oil out of every crack and crevice of the engine.
Solving that problem is what crankcase evacuation systems do by creating a negative pressure in the crankcase when the engine is running. Besides improving ring seal while reducing parasitic power losses on conventional engines, a well-engineered evacuation system also permits the use of low-tension oil rings and low-drag compression rings for even greater frictional loss reductions. Negative pressure is also said to better control windage and raise crankshaft oil seal effectiveness.
The original evacuation systems used hoses that connected the valve covers to the header collectors. At high rpm, the high-speed exhaust gases rushing through the collector induce vacuum (suction) in the hose, in turn reducing crankcase pressures.
On the theory that "if a little is good, more must be better," some racers began using mechanically driven AIR (smog) pumps hooked up "backward" to mechanically suck air out of the crankcase. However, some racers reported durability problems because these pumps aren't designed for high-rpm usage. Enter Moroso, introducing a heavy-duty vacuum pump (PN 22641) specifically engineered for crankcase evacuation duty.
Moroso Vacuum Pump
Recently, we had a chance to try out the Moroso vacuum pump on a 14.5:1 509ci GM Rat motor. It was a serious unit with equipment such as '88 vintage Pontiac Pro Stock large-port heads, a JET-built 1,150-cfm Holley carb atop a Hogan's sheetmetal intake, an MSD-6AL ignition system, a big Comp Cams mechanical roller cam, a Hamburger oil pan, a Crower crank, BRC aluminum rods, and gas-ported Ross pistons with low-tension oil rings. We may have revealed that the '88 vintage assembly was not specifically optimized for an evacuation system.
We tested the engine on Westech Performance's SuperFlow dyno using 116-octane VP C-16 race gas and 42 degrees total timing advance. Westech ran the engine with no evacuation system, with a traditional exhaust collector evacuation system, and then with a Moroso vacuum pump. For the vacuum pump test, Moroso's recommended default pulleys-a 2.5-inch-od V-belt crank pulley (PN 23520) and a 5-inch-od AIR pulley (PN 64895)-were installed to drive the pump at 50 percent of engine speed.
Peak power numbers for the Moroso vacuum pump evacuation system were 673 lb-ft of torque at 6,000 rpm and 897.2 hp at 7,900 rpm, compared to 658.6 lb-ft at 6,000 and 887.8 hp at 8,000 rpm with no exac system. Peak to peak, the pump in its as-run configuration on this engine was worth about 14 lb-ft of torque and 10 hp over the no-system configuration. It was also up about 17 lb-ft and 6 hp compared to the exhaust evacuation setup.
However, overall power gains proved inconclusive. When you look at each rpm test increment, the numbers bounce around-sometimes they're up, sometimes they're down. Between 6,100 and 8,100 rpm (where we have good data for all three test regimes), there's little difference in overall average numbers.
There are several explanations for this inconsistent performance: The engine had been in storage for several years; some gaskets may have dried out, leading to air inclusion under suction pressure (special attention should be paid to ensure all sealing surfaces are as flat as possible, and dipsticks should not be used). Plus, it did not have the new lip seals specifically designed for evacuation system use that better resist positive outside air pressure. We also may have needed to turn the pump faster. Pro engines using evacuation systems typically tach over 8,200 rpm; according to Moroso's catalog, lower-speed engines like our test example should run the pump at 66 percent of engine speed, which requires a different pulley that unfortunately did not arrive in time for the test. Although the pump pulled more than 6 inches Hg of vacuum through 8,200 rpm once all the leaks were plugged, ideally you need 10-17 inches of vacuum for the system to really work.
Based on these preliminary results, we'd say you need a fresh engine with a superior gasket seal combined with low-friction components to make this system work at its full potential. In such conditions, we've heard from NMCA EZ Street racers that the Moroso pump can be worth as much as 40 hp at peak on small-block nitrous engines. In theory, the vacuum pump should have an even greater affect on large-bore big-blocks since they have more area on the underside of the pistons. That makes our particular test even more of a curiosity compared to the talk in the pits. Another benefit of the evac system is eliminating smoking under heavy nitrous loads. Despite our dyno test, it's an interesting technology that demands a closer look.
Plumb the inlet side of the pump to the front of a valve cover using an AN-12 hose, hose-e
...a Teflon sealing washer. This position creates max vacuum while allowing some oil mist
Westech Performance Group
Moroso Performance Products
80 Carter Drive