Got this from Geo a while back.
http://users.bigpond.net.au/ergoff/vac1.htm
On Chevy/GM smallblocks there's access to the manifold vacuum by way of a pipe plug/fitting between the carb and distributor. You can connect temporarily for diagnosis or run a nylon tube to the dash like I have. Not only are you directly connected to engine performance but by reading the vacuum/throttle opening and rpm, it's easy to run at the most economical throttle setting. This means you want to run with the throttle at the highest vacuum reading.
http://users.bigpond.net.au/ergoff/vac1.htm
On Chevy/GM smallblocks there's access to the manifold vacuum by way of a pipe plug/fitting between the carb and distributor. You can connect temporarily for diagnosis or run a nylon tube to the dash like I have. Not only are you directly connected to engine performance but by reading the vacuum/throttle opening and rpm, it's easy to run at the most economical throttle setting. This means you want to run with the throttle at the highest vacuum reading.