I've taken two longer cross country flights >3 hrs) lately and have experienced a change in turbo air temp suddenly after ~2 hours flight time.  It has happend four times now, so there seems to be a pattern.

Decribing the set up: standard "mixing box" on upper center firewall with dual inputs: hot air siphoned off the turbos (dual merged via scat before the box), and a single scat cooler air coming after the third intercooler, using three sonic nozzles total , all going into this box.  A pull cable on the panel runs through the firewall to the lever on the front of the box: pushed in cable moves the lever towards pilot side for cooler air, pulled cable pulls the lever towards copilot side for hot air.  During all flights the cable was stationary and pushed in...solidly.

If one pulls cable during flight, the air temperature coming into the cable changes (as expected), pressurization does not budge, and the noise level in the cabin does not change (this will be notably important).

About two hours into the flight, with cabin pressurized (4-5 psi differential), cool air selected, cable fully in, I hear this "thwap" sound suddenly coming from the front (after the first time, the initial shock of such a noise in the mid-teens has worn off).  The inlet temperature entering the cabin via the mixing box's inside opening is immediately higher (and the cabin temp begins to rise).  The ambient noise in the cockpit increases noticeably, making communications a bit more difficult for ATC to hear, plus bringing the Bose mic closer to the squelch threshold.  Pressurization does not change, as observed on the cabin altimeter (Duke's System).  To give you a sense of incoming temp, the internal avionics temp sensor in the Chelton (GRT) EAU shows the component increases from ~100* to ~132*F in 20-30 minutes.   There is no change in smell or odor.

If I pull and push the mixing box temperature select cable (whether this is done before the "thwap" or afterwards), it pulls normally and returns full-in normally.  Pressurization does not change, temperature increases with pull and returns to the recently observed higher temp when fully in.  Cabin ambient noise level does not change with this pull/push.

Door seal remains same.

Is there some sort of seal or gasket or flapper inside this brass colored anodized "mixing box" that might be yielding after the engine compartment heats the box up, allowing hotter compressed air to sneak around and join the cooler air?  I realize that the innards of this device are unknown to me (which I never like to admit about anything on the plane).

Also note that when I land, pull the cowling, inspect hoses and cables and position of the levers (both the hot/cool lever and the cabin dump bypass lever), all appear in normal position.  Having not changed anything, if I resume the flight, the temperature is again at its normal cooler level.  It's as if once the turbos stop pushing on whatever "thwap"d, the coller air is no longer getting mixed with hotter air.

I had first concluded the hot/cool lever on the box was somehow slipping just a wee-bit from its fully pushed over "cool" position due to some flex in the cable, so yesterday (after the outbound flight) I tie wrapped the lever tightly in the "cool" position to secure it for the flight home.  When the "thwap" occurred again, I thought the tie wrap must have quit on me, but inspection after the flight confirmed the tie wrap was in a well secure and stable position.

I have tried to meticulously describe the conditions and experience.  Being there and hearing the "thwap" and noting ambient noise and temperature increase can not be recreated even with careful recollection and descriptives, but I hope this has explained the events.

Anyone have any ideas?

Jeff Liegner (always debugging something, 51 hours)

LIVP (N334P)