V twin vacuum engine

[Aerolight]

 

Bad weather so indoor filming only - no plane

This engine has floating flap valves which allow for the best seal (as far as Lego is concerned). The blocks the flaps pivots in are free to move forward and back, this allows the flap to sit flush against the  cylinder when closed. The inlet valve sits in side of each cylinder while the exhaust is external (the one your see oscillating in the video) so the pressure keeps the valves closed. 

No modified parts and all legal connections (I think at least)

I don't have enough rubber bands (close valves) so had to steal some from other MOCs.

[Gray Gear]

Very nice! Seems to be running very smooth with little friction, the low idle speed is really impressive.

[Davidz90]

Amazing sound!