Turbo Engines, wategates & MP

[Ozgrade3]

A question posed to me today but no only partial explination in the AGK book.

Waste gate fully closed(above critical altitude)
Increase rpm gives you a higher MP - Decrease RPM MP falls

Wastegate operating normally (below critical altitude)

Increase or decrease RPM - MP stays the same as the wastegate opens and closes as required to maintain the selected uperdeck pressure.

Wastegate failed to fully open position
Increase RPM causes MP to fall and decrease RPM causes MP to rise - just like a non turbo engine.

Have i got it right or am i barking up the wrong tree?

[eocvictim]

Im 90% sure that in theory you're correct but going from my time with manual and auto waste gates on the same ac type RPM has a negligable effect on MP.

The theory being with a closed wastegate as you increase RPM you increase exhaust pressure increasing the turbine speed. Of course in doing so you increase the need for aspiration, so any increase in pressure from increased turbine speed is negated by the extra air required.

If the wastegate is auto any minor difference is controlled by the uperdeck pressure.

If the wastegate is open it is as you said acting as an N/A engine, but again MP changes are still negligable.

[leg man]

The Seneca II has a fixed waste gate. It's been a while but from memory we used to climb at 34" MP and 2450 RPM. At top of climb the throttle was left at the climb position and the RPM reduced to 2200RPM. With the RPM reduction the MP reduced from 34" to 31" which we used for cruise. So reducing RPM makes quite a difference to MP with a fixed waste gate.

[tmpffisch]

Also worth having a read of Pelican's Perch #15:<br>Manifold Pressure Sucks! (http://www.avweb.com/news/pelican/182081-1.html) to gain a good understanding behind manifold pressure.

Plus at Pelican's Perch - More Stories (http://www.avweb.com/news/pelican/list.html) there is a 6 part series on turbos and engine management, along with other great learning stuff on engines.

[Jabawocky]

tmpf...........keep studying that stuff.....will do you a lot of good one day young fella! :ok: