I'd love it if somebody could point me to a pic of the breather outlet on a 4-pipe system. Also, any pics of the breather routing on a -4 would be terrific. Thanks.
Steve you are talking about crankcase breather right? It's not different than any other exhaust. Just route the breather close over one of the (4) pipes with an angle cut facing fwd. With 4 separate pipes, you might aim the breather between two if they are close together. Here's a pic I found on the web. (Note: cutting the tube square can cause excess suction, and excess oil loss.):
I have done a bit of work in this area and it is quite possible that this location may actually add pressure to your crankcase. I was having problems with oil on the belly, excess oil consumption and a bunch of nuisance oil leaks all around the engine. Tests with a manometer connected to various locations in the cowling showed that I as actually adding pressure to the crankcase venting in this location. After I routed the tube outside and aft of the cowling my oil consumption went down and all the oil leaks around the engine compartment went away. At least one other builder has followed my lead with the same results. I guess there is a reason that almost all certified aircraft have their crankcase vents end outside the cowling.
I have done a bit of work in this area and it is quite possible that this location may actually add pressure to your crankcase. I guess there is a reason that almost all certified aircraft have their crankcase vents end outside the cowling.
I hear you Tom there may be local negative pressure under the cowl. I agree, but It should not matter if its outside or inside the cowl if you follow Vans plans / recommendation, which is like I drew above. The angle cut should be facing forward. I think its shown in the plans. The tube should also be relatively vertical. That sould nutralize any local positive pressure. Also the breather does not have to be real close to the pipe.
The problem is having too much SUCK or negative pressure at the breather end, not too much positive pressure into the crank case (I think). Try blowing across straw sitting in a glass of water. Blowing across it causes the fluid rise. Blow down into the straw the water goes down in the staw.
The best way to check crank case pressure is hook up an airspeed gauge to the oil filler cap/dip stick. You have to make some kind of cap adapter to connect the pitot line from the oil filler tube to airspeed indicator.
Normal Lyc crankcase positive pressure (from ambient) for a Lyc = 0.98 to 1.46 inches-H2O. Using an airspeed indicator to measure differential pressure is clever. 45-55 mph is 0.98-1.46" H2O, pitot port goes to oil filler cap is what you are lookinfgfor. Static to ambient static port.
