Manifold Pressure......revisited.

[Flymach2]

Hi...

I noticed something the other day that I had never seen before and perhaps someone could enlighten me.

On two different normally-aspirated, fuel injected engines that I was looking at, the manifold pressure line had a machined "pinhole" approx. 1/32nd in diameter near the attachment point into the manifold. I have never seen a reference to this anywhere. The A&P told me it was there for a reason but couldn't actually tell me why. Thanks in advance for any help.

Regards

 

[maverick_fp00]

This has nothing to do with your post, but my question has to do with manifold pressure, so I figured I'd put it in here.

When leveling out for cruise in the Navajo, I'll bring the manifold pressure back to 32" and props back to 22RPM. When I bring the props back, the right mp increases about 3" and the left one will decrease about 3". What does that indicate?

Thanks,
Nick

 

[Flymach2]

Hi...

Not really sure why the MP in the left engine would decrease when RPM's are reduced. The only thing I can think of is that the left engine turbocharger wastegate is sticking. Just a guess. Hopefully someone can answer your question with a bit more authority as well. Good luck.

Regards

 

[jetdriven]

the navajo's have really cheapo wastegates. thats why you chase the MP all over the place for a minute after you change power settings. Plus it drifts from the cruise setting over the first minute after you get it set to 30".
Here is what might be happening. At high altitudes the turbocharger wastegate closes as you climb. it continues to close (to maintain selected manifold pressure via turbine speed) until the wastegate closes fully. At this point the turbocharger is maxed out and depends on every bit of exhaust gas to spin the turbine. When you decrease RPM the right engine isnt at its critical altitude yet and the turbine speed remains constant, IE the wastegate closes slightly to compensate for the reduced exhaust flow. Turbine remains on speed. The left engine wastegate is already fully closed, so the turbine speed decreases on less exhaust volume, and MP decreases. Then the turbine settles out on its new speed and now you have less MP.

Suspect a worn engine or loose clamps on the exhaust?? If it happens below 5 or 10,000 altitude who knows?

This has nothing to do with your post, but my question has to do with manifold pressure, so I figured I'd put it in here.

When leveling out for cruise in the Navajo, I'll bring the manifold pressure back to 32" and props back to 22RPM. When I bring the props back, the right mp increases about 3" and the left one will decrease about 3". What does that indicate?

Thanks,
Nick

 

[tarp]

Pinhole -

This is just a guess but it is something that drives the scientific types crazy when working with pressure/vacuum lines.

Collapse, ballooning and fatigue.

Huh? Lines which carry a vacuum (hoses and glass, metal or plastic) will collapse on themselves if the vacuum becomes perfect (i.e. no leaks). If they don't collapse outright, the constant attempt to collapse and then return to normal rigidity causes fatigue in the pipe. (Under pressure, just the opposite - lines balloon and then fatigue.)

The problem in scientific experiments is that you now have to allow for a "measured" contamination of your experiment, since you have to "bleed" a leak through your system. This is really nasty when the thing you are measuring is not just "air". What if you need a vacuum pulling pure oxygen or nitrogen - the plumbing and containment are a nightmare.

As simple as the intake manifold venturi is in an aircraft engine, the airstream would create an instant and powerful vacuum as soon as the engine is started - my guess - pinhole helps relieve pressure on the line.

 

[Flymach2]

Hi...

Thanks for the reply, Tarp. I also thought of the possibility of some sort of "relief" but I can't seem to logically make it work it my head. I guess where I'm having the problem is that the MP is measuring absolute pressure in the manifold. I think the relief would make more sense to me if the measurement was differential pressure. Hmmm.....another obscure detail to figure out. Thanks again for your thoughts.

Regards

 

[tarp]

Flymach,

It still would measure "absolute" not differential. The pinhole is on the same side of the meter/guage as the venturi. The "leak" is not a static leak on the "other" side of the guage.

Let's just say that the theorectical maximum pull of the intake manifold at the venturi is 10.0". We drill a pinhole at 1/32" allowing an ambient pressure leak to relieve the tube. With the "leak", the vacuum line loses 0.2" of pressure.

Ergo, at the meter/guage the vacuum is 10.2" instead of 10.0" and since the orifice is a constant 1/32", we can now adjust the guage so that it is 0.2" off or corrected/adjusted for the leak.

The leak is only significant when the pressure would be within 0.2" of ambient pressure - however, ambient pressure, say 29.00" is really our least significant value on the meter and at greatest is only 0.2" off.

So on a standard day at sea level - the outside air pressure is 29.92" and we just so happen to have a turbo charged engine and we run it at about 3/4 throttle up to a value of 29.92"MP - OK, it could 0.2" off - does it matter? Ah, not really. That's not a significant number.

So the pinhole leak on the same side of vacuum does represent "absolute" pressure (adjusted for the leak). The leak has only one job - keep the tube from collapsing. And I know, it's just crushing to think that the guage in your airplane is not absolutely correct all the time but sacrifices must be made.

By the way, your vaccum pump fails in flight but you have a handy "standby vacuum system" plumbed into your intake manifold. How do you create the 5.0" of vacuum needed to run your gyros? Answer - assume standard day - take your altitude times a thousand and subtract from thirty (OK 2992) and then set your MP (low enough or engine speed slow enough) to create a "differential" of 5.0 inches. For example, IFR at 4000. 4 inches from thirty gives 26" of outside ambient air pressure - you need to run your engine at 21" (or 2100RPM if you don't have a pressure engine) to keep 5.0" of differential on the gyros. This is an example of vacuum "differential" being used.

 

[KeroseneDream]

Could the opposite still work. If you were at 5,000' the ambient pressure would be 24.92 in. So could you run your turbocharged engine at 30" and be good?

 

[tarp]

Kero,

Pretty funny.

 

[Flymach2]

Hi...

Tarp, thanks for your input...greatly appreciated.

After finally reaching the manufacturer, I have been given the definitive answer. The "pinhole" is there for drainage. Condesation in the line or perhaps excess fuel. And, as mentioned, the drain hole will not have an effect on the instrument. Interesting little detail....

Regards