Question for the Motor Heads

[ATAG_Bliss]

Originally Posted by Dutch_851

Why is it that when you coarsen the prop pitch and RPM's drop, the boost gauge goes up? Conversely, when you reselect fine pitch, boost goes down?

If driven mechanically, surely supercharger boost should increase with RPM.

Or does a waste gate in the manifold close in relation to prop pitch setting?

Also is the RPM gauge telling you prop RPM or engine RPM?

Thanks in advance!

Boost is relative to both engine RPM and engine load. Typically speaking, say in a blown (supercharged) car, holding a steady 2000 RPM's (with the transmission in neutral) will hardly create any boost. But take that same car and lug it (put a load on it) at 2000 RPM's and you'll see an exponential amount of boost displayed or a huge difference between the amount of boost seen when the engine is at the same RPM.

Changing the prop pitch can induce more of a load (lowering RPM and increase boost) depending on what you were running before hand. Just think of it this way. Say you are in a huge diesel truck with a manual transmission. You take off from a stop sign/light and are at high RPM's in 1st gear just holding steady (like you are on the governor/ready to shift). There's not much load on the engine and it's easy to maintain that speed given the low gear ratio and high RPMs. Now take off from that same stop sign in 3rd gear. It'll take much more fuel needed to burn to achieve the same result, even though you'll get to that same speed you were in 1st gear with much less RPM's. But because you are using twice the fuel, (lets say you are at a stoichiometric fuel rate of around 14.7:1), you are also going to be using much more air. Now if think about it, you are burning more fuel and sucking in more air in the last scenario using less RPM's. So the load of the engine (easily calculated by fuel consumption or air flow at any RPM) is a huge driving factor of your base engine with regards to how much manifold absolute pressure (boost) you have.

I'm not that familiar with aircraft engines, but all combustion engines are based off the same principles. I would assume that the RPM gauge in-game only measures engine RPM, but I not 100% sure.