Manifold increase on magneto check?

[MrEdd]

Hi lads.

I have a question that is not important but i has been running around in my head and the buggar don´t want to stop.:ugh:
So if any of you have a good idea aboute i would be happy.

So here goes, i were doing a magneto check on a Schweizer 300 CBI and got a increase in manifold pressure while doing so. Engine work like a charme, spinnig like a cat.
But how come that the manifold pressure acually increased, cant figure oute how it could?
Is it just so that i withoute feeling it accidentiley raised the collective:confused: , i really don´t think, but that would be an answere.

[Flingwing207]

With a fixed throttle position, MP goes up when RPM goes down (RPM dropping in this case due to one set of plugs being shut off). MP would be at maximum for any throttle setting when the engine was at zero RPM. (thanks for the catch, WHK4)

[WHK4]

Flingwing, don't you mean MP goes up when RPM goes down, for a fixed throttle position?
MrEdd, what you describe makes sense to me.
As FW said, the engine is slowing down due to one mag being turned off.
The slower engine is now sucking in air at a slower rate.
This allows pressure in the manifold to become closer to the ambient pressure.
As FW suggests in his example:in the extreme case that the engine were to stop altogether at that throttle setting, the MAP guage would read ambient pressure.
Interesting question, and I'm going to look for this effect at my very next Mag Check.
Cheers
WHK4

[bellfest]

Interesting question, and I'm going to look for this effect at my very next Mag Check

:confused: :confused: :confused: :confused:
Better late then never I guess:ok:

[RobboRider]

Hang on, Guys. Before developing theories about what makes it OK to go up can we go back to the basics.

Mr. Ed is right to wonder because it is not supposed to go up it is supposed to go DOWN during the mag check! (At least in the R22, R44 and the FWs I fly. No question about that.

So back to the original question - why is it going up?

Could there be a problem with the switch/wiring and it is not firing on both mags when you think it is and firing on both when you think its only firing on one?

[WHK4]

RobboRider, he's talking about MAP increasing, not RPM.

[ShyTorque]

An engine acts like an pump (in fact that's just what it is), powered by AVGAS.

With the engine running, the atmosphere is constantly trying to fill the inlet manifold with air which would tend to increase the MAP to ambient.

The engine is pumping out the inlet manifold, which tends to decrease the MAP. At idle or near idle, the throttle plate is almost fully closed, so the engine tends to win, giving a low MAP reading.

If the engine becomes less efficient or less powerful, the atmosphere gains and increases the MAP reading.

With a fixed throttle plate setting, (governor not working), switching off a set of plugs during the magneto check makes the engine less efficient due to reduced combustion. The RPM should drop and the MAP should go up.

Watch the MAP gauge when you switch off the engine completely. As the RPM falls to zero, the MAP increases to read full atmospheric pressure, where it will stay until next time you start the engine.

[MrEdd]

Thanks Shytorque, that explanation was good and seems to make sense.
Thanks a lot.

[RobboRider]

"RobboRider, he's talking about MAP increasing, not RPM."


My mistake, guys. I will try and blame my 'flu' adled brain but I suspect it's more my lack of recent flying. Now the wet season has passed I should be turning Avgas into noise a bit more regularly:O

[WHK4]

No worries, RobboRider.

Anyway I looked for this MAP rise during a mag check yesterday in an R22 and it did happen.

When the RPM dropped a few percent, the MAP rose about 1/2 inch.

Cheers