Manifold pressure gauge

[Mason]

Aircraft C401 TSIO-520

At idle, one engine indicates about 15in MP (normal) and the other indicates around 20in MP with RPM equal on both engines. At t/o and cruise, all indications are normal. I'm thinking that it is some sort of leak in the MP gauge or line. Anyone seen this before? What could be some other causes.

 

[Torqued]

There's a couple of things that could affect your idle MP. One is the idle mixture setting. A too rich mixture can give a high MP. Try leaning the high MP engine and see if that reduces the MP.
Next, it could be the prop low pitch setting. During runup, does the split in MP continue as you increase RPM until the prop governors start to limit RPM? If so, the side with the higher MP may have its low pitch set a little coarser than the other. I'm not familiar with the C401 so I can't say whether it could be adjusted on the aircraft or not.

 

[erj-145mech]

I wouldn't spend a lot of time or effort to marry up the needles at idle, since you don't have any specs at that power setting. But it does sound like an idle mixture issue.

Does the engine sounds like its "loping"?

 

[bocefus]

Never saw a TCM fuel injected engine that didn't "lope" at idle

 

[RedGuy]

I fly 310's and 402's both with IO/TSIO 520's and I can tell you that the MP very rarely matches up at idle. It's not uncomon to see a 5 in split at idle, but usually it's more like 2-3 inches. I suspect it's a mixture issue since once you start bringing the power up the needles keep getting closer and closer as the power is increased.

 

[Bus Driver]

Having absolutely no experience with the TSIO- 520, I am curious as to why you gentlemen are leaning toward an idle mixture problem vs. an induction leak or possibly a wastegate problem.

 

[avbug]

There's a couple of things that could affect your idle MP. One is the idle mixture setting. A too rich mixture can give a high MP. Try leaning the high MP engine and see if that reduces the MP.

What??? How do you suppose mixture has any effect on manifold pressure? It doesn't. Mixture has an effect on power, and temperature, but not on manifold pressure, unless it changes RPM. For a given RPM, mixture has no effect on manifold pressure.

Manifold pressure is a function of throttle position and engine RPM, period. For a given engine RPM, increasing throttle will increase manifold pressure by reducing the obstruction to engine suction through the induction. For a given throttle plate position setting, increasing RPM decrease manifold pressure.

In normal operation, increasing throttle plate position increases manifold pressure in much the same way that removing an obstruction from a vacum cleaner hose increases the pressure in the hose...by removing the obstrution to the hose. Your engine is an air pump, and consequently a suction machine. The induction represents the vacum cleaner hose, or suction end, and the exhaust represents, well...the exhaust. Put your hand over a vacum cleaner hose, pressure drops in the hose, just lke closing the throttle on an engine.

For two otherwise identical engines to have differing manifold pressure for a given throttle setting with identical RPM, you either have an induction leak or an indication error. This assumes the same engine RPM.

Take the theoretical shopvac, turn it on, put your hand over the hose. Measure the pressure in the hose. There's your manifold pressure; the hose is the induction manifold. Cut a hole in the hose. Hose pressure will rise, and there's your induction leak.

If it's an indication problem, you may see a higher manifold pressure, but you're also going to see temperature differences if you have a means of monitoring them. The same is true of an induction leak.

How matched are the engines at idle?

 

[Mason]

What??? How do you suppose mixture has any effect on manifold pressure? It doesn't. Mixture has an effect on power, and temperature, but not on manifold pressure, unless it changes RPM. For a given RPM, mixture has no effect on manifold pressure.

Manifold pressure is a function of throttle position and engine RPM, period. For a given engine RPM, increasing throttle will increase manifold pressure by reducing the obstruction to engine suction through the induction. For a given throttle plate position setting, increasing RPM decrease manifold pressure.

In normal operation, increasing throttle plate position increases manifold pressure in much the same way that removing an obstruction from a vacum cleaner hose increases the pressure in the hose...by removing the obstrution to the hose. Your engine is an air pump, and consequently a suction machine. The induction represents the vacum cleaner hose, or suction end, and the exhaust represents, well...the exhaust. Put your hand over a vacum cleaner hose, pressure drops in the hose, just lke closing the throttle on an engine.

For two otherwise identical engines to have differing manifold pressure for a given throttle setting with identical RPM, you either have an induction leak or an indication error. This assumes the same engine RPM.

Take the theoretical shopvac, turn it on, put your hand over the hose. Measure the pressure in the hose. There's your manifold pressure; the hose is the induction manifold. Cut a hole in the hose. Hose pressure will rise, and there's your induction leak.

If it's an indication problem, you may see a higher manifold pressure, but you're also going to see temperature differences if you have a means of monitoring them. The same is true of an induction leak.

How matched are the engines at idle?

This issue occurs at idle RPM (1000), approx 5+ inch MP split. As the throttles are advanced the split decreases until the turbos really start to come online around 29-30in MP then there is no split up to full power.

 

[Torqued]

What??? How do you suppose mixture has any effect on manifold pressure? It doesn't. Mixture has an effect on power, and temperature, but not on manifold pressure, unless it changes RPM. For a given RPM, mixture has no effect on manifold pressure.

I agree with the theory of what you said but I have seen this many, many, many times. I can only surmise that the rich mixture must have something to do with the combustion efficiency and that affects the airflow through the engine. I must repeat also that I'm not familiar with his particular aircraft so other things may apply.
I'm not posting here to start an argument. I only post to share some experience and maybe help someone. If the shoe fits...

 

[avbug]

If you see a change in manifold pessure when adjusting the mixture, you've chaned the engine RPM to do so. If the RPM stays constant the the manifold pressure stays constant.

What type of temperature monitoring do you have onthe engine and what temperatures are you seeing?

At 1000 RPM, you are seeing a 5" split in manifold pressures? Which way, and what values? The correct value should be approximately 12" of manifold pressure. What is being displayed?