Using the vacuum from the intake manifold is something that used to be installed as an aftermarket *EMERGENCY*?suction source for some light aircraft. However, it should be noted that it was for emergency use only, with good reason. Your vacuum instruments are labeled as requiring 4-1/2 - 5 inches of suction to work properly. Generally speaking, they will work OK on 3-1/2" of suction (2-1/2" suction for T&B only).
Now think about the throttle setting and manifold pressure you use when flying your aircraft. At full throttle, your manifold pressure is the same as ambient pressure, so there is no vacuum to be sourced at the manifold. So your vacuum instruments will spin up while you are throttled back during your run up, but will be spinning down while at full throttle during take off and climb. Now, let's think about cruising at 7500'. You would nominally have ~23" manifold pressure at full throttle at 7500'. In order to generate 3-1/2" of suction for the gyros, you would have to throttle back substantially to 19-1/2" manifold pressure. It is a very rare day that I actually throttle my KR back far enough to have 3 - 4 " of manifold pressure available to be used as a source to operate gyros. Additionally, the air that moves through the gyros is going into the intake manifold. That is a huge air leak into the intake system. It not only substantially leans the mixture, possibly more than you can compensate with the mixture control, but unless that vacuum leak is dead center in the manifold, it is likely to cause one cylinder to run extremely lean, likely causing valve damage by extended operations under those conditions. One solution I've seen on a certificated plane was to mount venturies on the bottom of the plane just aft of the engine in the slipstream of engine cooling air. The venturies are then running in already disturbed air, and are not prone to freezing in icing conditions, although I can't imagine why one would fly their KR in icing conditions anyway. Venturies will still generate drag, but Not as much as out in the undisturbed slip stream, and would be a much safer system than creating a large vacuum leak in the intake manifold, then planning to operate at partial throttle to keep the gyros spinning. Gyros are heavy and require a vacuum source with a lot of associated heavy plumbing. Mike Styrewalt has his Dynon D2 for sale for a very reasonable price. That weighs next to nothing and requires almost nothing in the way of power while replacing all three gyros.? There's a reason why mechanics are giving away perfectly functional gyro instruments. -Jeff Scott Los Alamos, NM ? -------------- Hi, Is there anybody on the list having experience with a vacuum for gyro/turn-bank supply generated by the carburetor throttle? If yes, could you please share it. FYI, I found this interesting reference: http://cybra.p.lodz.pl/Content/6351/AER_56_6.pdf Stefan s_sbal "at" hotmail.com
