>> I think the cheapest, lightest, and most reliable way to
      > create vacuum
      >> for your steam gauges is with an exhaust bung.
      >>
      >> The exhaust bung would enter the exhaust pipe at a small
      > angle, say 20
      >> degrees, pointed towards the exit, and extend into the exhaust gas
      >> stream about 1/3 of pipe diameter. This creates a venturi,
      > and LOTS
      >> of vacuum.
      > <snip>
      >> IMPORTANT NOTE: you will have to regulate the vacuum (with a
      >> regulator), but it will work very well. (Almost TOO well.)
      >>
      >> So:
      >> lightweight - a few ounces of bung material and tubing
      >> reliable - as long as the engine is running (same as vacuum
      > pumps)
      >> durable - no moving parts
      >> cheap - tens of dollars with no annual maintenance
      >>
      >> How's THAT for Experimental Aviation?
      >>
      >> - Vince Orton
      >
      >>
      >> Wow that might be a great idea pulling on the pistons. Super
      > cheap
      >> turbo compound. How do they keep the oil vapor in there? For
      >> drag racing I guess they don't care.
      > <snip>
      >> For use as a vacuum pump to drive the instruments it sounds
      > great.
      >> These are also called "air ejectors" if you want to look
      > them up on
      >> Google. A simple venturi might generate the -5 psi required
      > for vac
      >> instruments without all the back pressure.
      >>
      >> Paul Lamar ...No rotor no motor.
      > <snip>
      >> Paul,
      >> Modern street motorcycles have used this technique for some
      > years now.
      >> They run at very high RPM's of course. Kawasaki was the first
      > to use this
      >> system to my knowledge. (In the late 80's)They have had a
      > exhaust reed
      >> valve system with a small orifice to the crankcase to pull vacuum.
      > <snip>
      >> If your instrument will tolerate oil you're OK. Otherwise be sure
      >> the crankcase is NOT connected to the vacuum generator.
      >> Bill Jepson
      >>
      >>
      >> I can see how it would reduce the windage HP loss of pushing
      > air out
      >> of the cylinder with the piston going down and up into the
      > cylinder
      >> with the piston going up. The rotary does not have that problem
      >> fortunately so I don't see a need for vacuum in the crankcase.
      >>
      >> Paul Lamar ...No rotor no motor.
      >
      >
     *************************************************************************
      >
      >
      > ACRE -
      >
      > Wow. This thread has gone far afield.
      >
      > Forget crankcases. Forget oil vapor. Forget H.P. increases.
      >
      > Use the exhaust bung to SIMPLY create vacuum, regulate it, and
      > use it to
      > drive your steam gages. That's it.
      >
      > IMPORTANT NOTE: This will create TOO MUCH VACUUM. Something on the
      > order of 15 psi, if memory serves. You WILL need to regulate it.
      >
      > The point was that with this solution, you will not need to
      > worry about
      > mechanical problems, shearing shafts, pump maintenance, pump
      > weight, and
      > all the other things that go along with a mechanical solution.
      >
      > Yes, smog pumps are cheap, but an exhaust bung in cheaper, more
      > reliable, more durable, less maintenance, and ... LIGHTER.
      >
      > - Vince Orton
      >
      > Vince what do you mean by an "exhaust bung"? Do you have a dwg?
      >
      > Paul Lamar ...No rotor no motor.
      >
      >
      > How much vacuum is generated during engine idle? Consider a long
      > 'slam dunk' power off descent through the clouds from 12k to sea
     level.
      >
      >
      > Matt-
      >
      >
      > Answering my own query, I suppose one could arrange for an exhaust
      > throttle for such situations. The throttle would route exhaust
     through
      > a venturi (restriction). Diesels use them to generate braking
     force..
      >
      >
      > Matt-


     The successful use of any venturi system assumes that the inlet is under
     greater pressure than the outlet. Thus it has a small taper to
     create the
     restriction followed by a more gradual longer taper to recover the
     gas flow
     speed. The lowest pressure in the venturi is just a wee bit past the
     narrowest point. OK we all know this and that these devices are
     commonly
     stuck to the outside of all vintage aircraft cowlings. I am
     unclear how
     just sticking a tube in a gas stream will pull enough vaccumn to
     power the
     instruments. Perhaps the effect is more like blowing across the top
     of a
     open bottle?

     Doug in Japan



     Yes, like blowing across the top of an open bottle. A low efficiency
     venturi..

     It seems likely that the efficiency of a venturi is defined by a number
     of factors. One of them is certainly the change in diameter of the flow
     path. That determines the pressure change - the relationship between
     increased flow velocity and decreased pressure causes gas to be ingested
     into the stream if a port is placed in a region of reduced pressure..

     Certainly another factor in the efficiency of a venturi is related to
     the viscosity of the fluid system. Reynolds number is about viscosity,
     and viscosity is about shear resistance of the fluid. That says that
     fluid flowing past an orifice in a surface will tend to entrain fluid
     from the orifice - drawing a vacuum - however weak.

     Anyway, while you will get some vacuum with no narrowing of the exhaust
     stack, it seems like a little narrowing might be good if it meant that
     the efficiency could be improved significantly. I imagine that without
     a restriction, the pressure in the vacuum system might not be very
     negative as the leading edge of each exhaust pulse traversed the port.

     Who reads Hoerner?? :)


     Matt-


What is what now. The first poster says inserting a bung into the
exhaust will create a vacuum so powerfull that you will have to regulate
it for instrument use.

The last poster says that the vacuum will be so weak without a
restriction in the exhaust as to be almost useless (my own wording).

Which is it now: a strong vacuum or a worthless vacuum? Has anybody
tried it?

--
Erik Snyman
Messina
South Africa.

It will definitely work Erik. The problem is nobody has posted a dwg yet.

Paul Lamar