Boost pressure is primarily indicated on which gauge?

Boost pressure is primarily indicated on the manifold pressure gauge. This gauge measures the pressure within the intake manifold of an engine, which reflects how much forced induction (like that produced by a supercharger or turbocharger) is being supplied to the engine. When a boost is applied, the manifold pressure increases above atmospheric pressure, and this increased pressure is what the manifold pressure gauge is specifically designed to measure.

The cylinder head temperature gauge monitors the temperature of the engine cylinder head, which is critical for maintaining proper operating temperatures but does not reflect pressure levels. The fuel pressure gauge tracks the pressure of the fuel being supplied to the injectors, which is important for engine performance but unrelated to boost pressure. The R.P.M. gauge records the engine's rotational speed, also not related to measuring boost pressure.

Thus, the manifold pressure gauge is the correct instrument for measuring boost pressure, as it directly indicates the level of pressure within the engine's intake system where the boost effect occurs.