[SkyKing]

Jerry,

I think what you're trying to describe is the sonic venturi, which actually serves as an air flow limiter, that comes off the engine's induction system - just before the pressure relief valve - and routes over to the air selector valve with two possible positions, (1) going to the heat exchanger if one desires cool air for the cabin; or, (2) going to the heater if one wants to fire-up the Janitrol. This venturi, which is a contoured, cone shaped aluminum tube, LIMITS the amount of pressure to the cabin. If that device wasn't there, the turbo system on the P's would be MORE than capable of OVERpressuring the vessel far beyond the design limits of 3.35 PSI at max differential.

As I recall, the sonic venturi device connects to the induction system via a short piece of rubber hose and a couple of band-clamps. IF there were a leak AHEAD of this device, then yes, you would definitely have a difficult time maintaining sufficient MP at altitude. But, if the leak was somewhere on the other end of the venturi, then it wouldn't matter, as it would be the same as pulling the dump valve and letting that 'limited' amount of pressurized air flow overboard.

With regard to Kevin's remarks, insofar as being able to maintain MP at critical altitudes, that has nothing to do with the cabin pressurization... they are as separate as apples and oranges and one of the reasons why I was having trouble with the purported diagnosis of Million $ Air, i.e., that they could seal off the cabin and wa-la, we now have sufficient MP from the turbo on the gauges at altitude. Something doesn't jive here, as like Kevin says, when you dump the cabin pressure with the dump valve, the already limited air pressure to the cabin coming from the induction system through the venturi is being dumped overboard... and at the same time there is a check valve in the plenum on the firewall side that seals shut... making it possible for the other engine's independent sonic flow limiter to sufficiently pressurize the cabin.

Either of the engines will sufficiently pressurize the cabin, that is, IF the engine is developing proper amounts of MP/RPM and you're not bootstrapping. UNLESS, of course, if the pressurization switch is in the OFF position... if it's OFF, electrical power is being applied to the safety/dump valve solenoid on the rear firewall and the valve is held in its OPEN position to prevent pressurization of the cabin. This is not the same thing as the dump valve controls on the lower pedestal.

If the turbo, wastegate, controller and pressure relief valve are all working like they're suppose to, then my bet is that there is either a leak in the upper deck reference lines -OR- a leak in the engine's exhaust system; or, as mentioned earlier, possibly a leak just ahead of the sonic venturi.

What Million $ Air should really do is perform the check as in the service manual to maximum critical altitude to help identify the problem.

SkyKing