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By Jay Storer

[Part 1] of this story may have convinced you that a proper crankcase evacuation system is right for your car - we could all use a tenth or two - but how do you choose the best setup for your application?
A complete vacuum system includes the pump, crank and pump pulleys, belt, 200-psi hose, push-on fittings and all hardware, plus a baffled breather tank and a vacuum control valve.

Tailoring pumps and vacuum capacity for various engine combinations involves three factors in the pump: pump displacement, number of vanes, and pump speed. The first is the biggest factor and mostly a result of pump-body length. The longer pumps have more displacement, and a three-vane pump outflows a two-vane, but a four-vane only flows a little more than the three-vane, so there is a diminishing return on number of vanes.

Within the three basic sizes of pumps GZMS offers, the Sportsman, Pro and Super-Pro, variations to suit engines size and other needs are achieved by varying the pump’s speed. Airflow from a vacuum pump increases with speed, i.e. increasing the pump speed by 10 percent equates to flowing 10 percent more airflow, although the increase in airflow flattens out at about 5000 pump rpm. Greg Zucco keeps his pumps below that threshold by underdriving them by 37 percent (63 percent of engine speed) with the use of a 5.5- inch-diameter pulley on the pump. All of the GZMS kits include a crank pulley and pulley mounting kit for the correct ratio.

A vacuum control valve like this high-flow GZMS billet model can be used with any pump and adjusts for a range of 5-25 inches of vacuum. Conventional wisdom suggests keeping the maximum vacuum to 12-15 inches.

If your needs fall somewhere in between kits, there are optional pulleys of 4.5-inch (25 percent underdrive) and four-inch (8 percent underdrive) to spin a pump faster. Pump selection must take in a number of factors, but to use a simplistic example, if you run an engine of less than 427 cubic inches with typical wet-sump oiling and normally-aspirated on gas, GZMS would recommend their VP101 Sportsman pump kit with 5.5-inch pulley. That same engine with low-tension rings and maybe a small nitrous dose (up to 150 hp) would use the same pump but spin it a little higher with a 4.5-inch pulley. That same engine but with a bigger dose of foo-gas or a supercharger would call for either the VP101 at 75 percent or the VP103 Pro pump spinning only 64 percent of crankshaft speed. Increasing speed on these pumps doesn’t increase the parasitic drag, as it would on centrifugal-type vacuum pumps.

An accurate vacuum gauge on your racecar will tell you if you’re over-evacuating the engine. There are some engine builders who don’t like to see too much vacuum on engines that don’t have pressure-feed-oiling of the wrist pins, theorizing that removing too much oil-mist from the system may rob some lubrication from the pins. If you install a vacuum gauge in your evacuation system, you should glance at the gauge when you’re at your top rpm going through the lights. GZ Motorsports recommends that you have no more than 15 inches of vacuum showing (remember we’re sampling the crankcase vacuum, not the usual intake vacuum most automotive vacuum gauges are intended to read).

In many cases, the efficient GZMS pumps can exceed this number, and so most kits are ordered with an adjustable vacuum control valve that allows tailoring the vacuum for your specific application. Eventually, as wear may reduce the total vacuum of a pump and ring wear may increase your engine’s blowby, the valve can be adjusted to maintain the proper 12-15 inches you need for maximum power gain. The valve just lets a little less air into the system to compensate for engine wear.


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