Older Chevy transmissions, particularly those with THM (Turbo Hydra-Matic) transmissions, typically use a
ported vacuum connection to the modulator. This means the vacuum source is *after* the engine's intake manifold, usually tapped into a ported vacuum source on the intake manifold itself. This allows the vacuum signal to vary with throttle position, providing more precise shift control.
A straight vacuum connection (directly to manifold vacuum) was less common on these transmissions, resulting in less refined shifting. While some variations might exist depending on the exact year and transmission model, ported vacuum is the standard for most older Chevy THM transmissions. Always check your specific vehicle's service manual for definitive confirmation.
