[Grandy]

In doing my sniffle valve education I found on the Van's site that a sniffle valve is not used in vertical induction sumps. Apparently they are only used in the horizontal sumps. Does anyone know the reason why?

 

[frankh]

I don't have one on the horizontal induction sump either. Mattituck told me I didn't need one..presumably less drainback on a fi'd motor maybe?

 

[scsmith]

no drainback out of carburetors

I believe its the fuel injection that is the issue. After shutdown, fuel in the injector lines percolates into the intake chambers and drains down into the intake manifold and pools there. A sufficient quantity can cause a little excitement.

others please correct if I'm wrong.


My Lycoming manual shows that sniffle valve was not originally fitted on the IO-360-A1A, but was fitted on the -A1B6. I put one on my -A1A and it does dribble a bit of fuel after a flight.

 

[JoeBlank]

I believe its the fuel injection that is the issue. After shutdown, fuel in the injector lines percolates into the intake chambers and drains down into the intake manifold and pools there. A sufficient quantity can cause a little excitement.

Steve,

That's exactly correct. The bottom end of the sump on the horizontal induction engine acts as a 'fluid trap'. Generally, fuel will percolate down into that area after shutdown and pool there. Water can also accumulate there if the aircraft is parked outside and collects rain water. The sniffle valve allows those fluids to drain off through the drain line and overboard.

On the vertical induction configuration, the fuel will simply drain through the carb or servo into the filtered air box. It's possible for fuel and water to accumulate there also, so one needs to drill a drain hole in the aft bottom of the FAB to facilitate drainage.

 

[chaskuss]

Randy
I'd like to add to what others have said, The horizontal sumps [both carburetor and fuel injection] will not allow any pooled fuel to drain out the servo or carburetor. This issue can happen with any landing gear arrangement. However, more fuel tends to pool into the rear of these sumps on conventional gear aircraft.
Without the sniffle valve, you may get a fuel back fire on the ensuing start up. The back fire can damage a number of components on the engine.
Charlie Kuss

 

[Grandy]

Thanks guys

The sum of knowledge offered makes sense to me. The vertical sumps must be able to drain on their own so no problem. Thanks for clearing that up. I spent a half day finding out what a sniffle valve was and then that I didn't need one. Funny how the education part of this takes time.