If you pull back the throttle far enough, you will quickly see the disadvantage of relying on the turbo for a cabin air source.

That fixed venturi bleeds a fixed percentage of the compressor airflow, a high enough percentage to be adequate even in the worst case (max altitude, min power, single engine, whatever). All the rest of the time, it's bleeding more air than necessary. The result is reduced power from higher exhaust back pressure, a lower turbo critical altitude, and reduced MP and HP above that. Plus, that compressor bleed air needs to be cooled, so drag from an intercooler and/or A/C. An electric blower, on the other hand, would at all times take only as much power as necessary to maintain cabin pressure, and would require less cooling. At the worst-case design point, the tradeoffs probably favor turbo bleed air, but everywhere else in the flight envelope, electric wins.

For the same cabin pressure and outside pressure, how is the blower pressurization going to require less cooling? The ideal gas law applies to both methods. Even though the upper deck pressure may be higher than the cabin, if the air expands into the cabin, it will cool naturally the same way as if the upper deck was the same as the cabin.

Why electric...

Wires are easier to route.

Electric works when you pull the power back on descent.

Zero chance of having a bad turbo pollute the cabin air...

If you have a turbo let go and blow hot oil into the complex cabin pressurization ducting...
you will never get all the smell out.

No need for a pressurization disconnect valve.

No need for a cabin air intercooler.

Alternators seem to be less costly and more reliable than Turbos.
Modern brushless motors should run 5000+ hours, there are ZERO wear items beyond the main bearings.


Less load on the A/C system.

P.S. not all piston pressurization systems have a sonic choke.
none on my Aerostar. The Factory 700's have chokes, the 601P does not.

Paul, I'm with you on most of that - but you probably still need an intercooler (actually an aftercooler if we want to be pedantic)

Seems like the pressurization on the Malibu/Mirage/M350 is pretty close to free. I owned an unpressurized Matrix and a pressurized Mirage, there is no difference in the performance tables, fuel flow, critical altitude or engine temps. Adding a 200 amp compressor 2 giant alternators and all the cabling would not be free.

There is an easy way of achieving the same tables for two airframes with slight difference in weight and available power -> you build table for the less performing variant and use it for both. Additional marketing gain -> many pilots will now claim that the better variant easily makes book numbers!

There is no free lunch.

It will cool some, but not enough. Over compression, followed by expansion, will leave it hotter than if it were only compressed enough in the first place. If it were not so, then the bleed air from turbines (~500F) would also not require cooling before going into the cabin. The reality is that these are not ideal gases, inefficiency in compression becomes more heat than from just PV=NRT, and the more the compression, the more heat. Pressure at 24,000 ft. is about 11", compressing that 4:1 up to 44" (approx. upper deck pressure for a TIO-540), then expanding back down to 22" (~8,000' cabin altitude) is going to leave it a lot hotter than just compressing it 2:1 in the first place.

Related topic, handled in more depth:
viewtopic.php?f=4&t=157750

I flew them both. No difference in performance. They both did book numbers. Doesn't look like a marketing gimmick. My Matrix did not have radar, so could conceive maybe having a knot or three advantage, but really didn't notice any difference. Just a little less useful load in the Mirage with the extra systems.

How much impact on critical altitude does bleed air pressurization have on a TN or TC piston engine?

How much impact on critical altitude does bleed air pressurization have on a TN or TC piston engine?

In the case of the pressurized Mirage, and the non-pressurized Matrix they both have a critical altitude of 20,400 feet for 42 inches MP. Above that you get a linear drop off of ?0.8 inches per thousand feet iirc. Could make 35+ inches up to 25,000 feet. 2 big turbos though, designed for the pressurized airframe.

That's not how it works. They are a sonic nozzles, so more or less once the pressure differential across the nozzle gets high enough a shockwave forms and the mass flow becomes constant. In the Malibu that's around 25" MAP. Above that MAP no additional bleed is taken from the motor. Below 25" the outflow valve does have to move with any power changes but it will keep it fully pressurized down to about 20".

Like Chuck said no real performance lost due to the bleed air. My newly OH'd TSIO550C's critical altitude is 21,900, I doubt a TTx driver does any better with the same motor.

>Paul, I'm with you on most of that - but you probably still need an intercooler (actually an aftercooler if we want to be pedantic)

At altitudes where you need pressurization its usually cooler than you want the cabin.
Unless I'm in the summer Southwest I usually want heat at altitude.
My guess is that with electric pressurization that need is greatly reduced, is it zero?
Don't know. Is it less than an over pumping compressor 2 inches away from a 1500F turbo... yes, yes it is.

If heating is desired you might be able to use the heat from the electric motor by running the air across it before going into the blower. Add a valve to switch to unheated external air for when you don't need the warmth.