Boost on vintage aircraft
 

On warbird era aircraft the index for power was boost, i.e. +14
lbs per sq. in . This boost could be positive or negative.

How does boost compare to a manifold pressure gauge?
What would the boost indicator show when the engine is not running?

Thanks .

Boost is the inches MP above or below ambient in the induction manifold. The more familiar MP gauge measures actual pressure in the manifold. They're both measuring the same thing - it's just the reference point that changes.


With the engine not running the boost would be zero. In a non-turbo engine it would always be at least slightly negative (think about a 'normal' MAP gauge. When does it ever go above ambient?). In a forced induction engine the boost would go above zero when the turbo- or super-charger is compressing the induction charge.

Tinstaafl thanks, that clears it up.

Tinstaafl,

Not quite correct in one aspect. As you say, a boost gauge is just a MAP gauge using a different reference. 0 psi boost equates to 30" Hg (1 atmosphere). Therefore, if you have a low pressure day, "static" boost (i.e. with the engine not running) will show slightly less than 0 psi in the same way as a MAP gauge will show less than 30", and vice versa.

IHL,

It is worth remembering that 1 atmosphere is 30" Hg and 14.7 psi. Therefore, as a rough rule of thumb 1 psi boost equates to 2" Hg. Therefore, +14 psi boost is approximately 59" MAP.

Rgds

L