I expect this will be somewhat controversial but I chose to go with no filter whatsoever – just coarse wire cloth with ½” square openings.

 

General thoughts

The Lancair filter/ram-air setup doesn’t truly provide ram air. When the ram air is activated, the snorkel directs air to both the throttle body and the inside of the air filter. Any excess pressure will actually flow backwards out the air filter. This is probably a good thing since the injectors on the normally aspirated IO550 don’t like it when manifold pressure exceeds ambient. The pressure difference causes fuel to come out through the injector vent screens causing blue stains on the cylinders. When people claim a 1-2” gain in MAP with ram air, it really means they’re bypassing the MAP loss caused by the air filter.

 

Using plenum pressure: My throttle body draws air from the upper plenum area. Assuming a reasonably well sealed cooling system, plenum pressure should be somewhat above ambient. As a bonus, the normally aspirated injectors will still work fine since manifold pressure can never exceed plenum pressure.

 

FOD and induction icing: The physical arrangement of the cowl and air intake should provide protection against ingesting large objects. Any object entering the cooling air inlets would have to make two 90-degree turns before entering the throttle body. This is not true for Lycoming engines since their intakes face directly into the air stream.

 

Measured results

I did a side-by-side comparison with another Legacy which has the standard Lancair induction system. At the same altitude and RPM we showed identical manifold pressures at full throttle.

 

Of course the big question is dirt entering the engine. Like most Legacys I only operate from paved runways, and only taxied on grass twice last year when parking at fly-ins. I have attached my oil analysis report from Blackstone labs. It clearly shows that my silica levels are well within allowable limits, which means that I’m not sucking huge amounts of dust into the oil.

 

Conclusion – I was able to save a significant amount of weight, complexity and cost by going with a stone-simple induction system. After nearly two years and over 200 hours of flight time I haven’t found any downside to this setup.

 

Adam Molny

N181AM

215hrs and counting