Pete:
I have a 1980 Sundowner with the O-360-A4K and have a Precise Flight Stand-by vac system. My friend wants to have this system installed in his E33 Bonanza, but was told that the SVS does not work on a pressure system. Does this make sense?
That is correct. He cannot use an SVS backup in a pressure air system.
In the pressure system, the vacuum pump is used sort of like a high-volume, low-pressure air compressor. It sucks ambient air in through a filter, then blows it through the instruments, usually through a secondary filter. The secondary filter is there to protect the instruments in the event of a vacuum pump failure.
In a standard vacuum system, the pump sucks the air in through a filter, but the air passes through the instruments on its way to the pump. The pump exhaust is then discharged overboard.
The SVS backup vacuum system uses a shuttle valve and pilot-controlled valve to enable the engine to serve as the air pump, as long as you keep manifold pressure at least four inches below full throttle. You go to full rpm and full throttle, then retard the throttle until the manifold pressure drops at least four inches. That four-inch differential enables the engine to serve as the air pump, on a vacuum system. The only way the engine could provide the air for a pressure system would be to pump the engine exhaust through the instruments.
While I like it, and I have one on my plane, there are drawbacks to the SVS system. When activated, it will limit your altitude and airspeed, so it may not be usable in the Western mountains. For example, in my Sierra I can maintain 10,000 feet and 90 knots IAS with the SVS activated. And during an emergency approach while on backup, you’ll have plenty of vacuum and reliable instruments. But if you suddenly and unexpectedly have to do a missed approach, such as something obstructing the runway when you drop out of the clag, you’d better be very careful how you react. If you just automatically go to full throttle and head back upstairs, you’ll be entering the clouds with no vacuum and gyros that are winding down and slowly rolling over… and you won’t know it unless you pay attention to that red light. Make sure it is somewhere you will see it! You have to plan to make your missed approach climb with reduced power. That’s not a huge problem when it is done under controlled circumstances, such as if you don’t break out at MDS or DH. But if you see an obstruction at the last minute and have to hit full power, make sure you get leveled off and power reset to provide vacuum, before you climb back into the clouds. And while you are doing this (very quickly), you have to make sure you don’t exit the airport environs and hit something else that is sticking up from the ground!
Definitely no free lunch, is there. The SVS is very practical and relatively affordable, for non-turbocharged vacuum systems (they can be used in turbo applications, but require even greater care to use right). But they do not have the capabilities of the heavier and much more costly electrically-driven backup pumps. Your friend will need to explore the electric units, for his pressure system.