Question for the Motor Heads

[ATAG_Bliss]

Originally Posted by MadBlaster

what would make the manifold pressure increase if the rpms of the engine and supercharger are both proportionally decreasing from the increased prop load and fuel being metered is unchanged?

Manifold pressure is going to increase when you compensate for the loss in RPM's through engine load and throttle increase. But that same metered fuel that required 50% throttle to maintain the RPMs you wanted to achieve, is now doing it at less RPM's. This creates positive pressure - boost (but only until the engine is normalized) then the boost guage (depending on the engine / charger combo) will remain fairly steady, but again, this is under a load. Superchargers do take HP to turn and cause a load on themselves, but they are generally designed for the application it's being used on.

A boat is the easiest way to see a supercharger in action with regards to boost, because the prop is always submerged in water at any RPM.

As far as back pressure is concerned, I think you meant to say exhaust manifold instead of intake, but your points are exactly right. That all factors into the equation of a boosted combustion engine.