Vacuum

[yoshi]

On an SBC 350 what creates the vacuum at the carburettor? Thanks

[MikeB]

Most carbs have 3 sources of vacuum -- a large port for the power brake booster, and two smaller ports.

The large port and the small port (closest to the carb base) are manifold vacuum.

The small port that's a little higher up on the carb is known as "ported" vacuum. There is little to no vacuum at this port until the throttle blades begin to open.

The small ports are used for distributor vacuum advance, transmission modulator, and emission controls, depending on the vehicle.

There is some debate on which small port to use for vacuum advance. Personally I've had success using both, depending on engine idle vacuum (mostly a function of camshaft duration and overlap).

[yoshi]

But where or how is the vacuum created?

[68post]

Pistons moving down in their bore with the intake valve open. This lets air rush down thru the carb from above and thru the intake and into the cyl. head past the valve and into the cylinder.

14.7 psi of atmospheric pressure above the carb and zero psi in the cylinder because the piston is moving down and creates space for the air to rush in. Air flows thru to fill that space !

[yoshi]

Ah right thanks, low vacuum would be because if a leak or worn rings then

[86c20]

just remember if the blads on the carb are closed you have high vacuum open no vaccum.
just like was said the piston needs to make a low pressure area and then the carb blads need to make a restriction, that gives you vacuum. this is also why diesel's dont have vacuum as they have no restriction on the intake system