[69LS1]

Vac is dependant on several things.Atmospheric pressure is a main ingrediant.Imagine your at sea level with 14.7 PSI atmospheric pressure above your carb..... Your at an idle and the throttle is closed. An intake valve is open and a piston is decending allowing air to enter the cylinder.... But not much air can enter the cylinder due to the closed throttles.... The piston decending is trying to draw air in and atmospheric pressure is trying to force air in but the restriction of the closed throttles wont allow much to happen so the vacuum level is high in the intake manifold due much to the drawing in effect of the piston against this restriction.Now quickly open the throttle and the restriction goes away and atmospheric pressure of 14.7 PSI rushes in overcoming the cylinder trying to draw air in....intake manifold vacuum drops.

Engine load is a major factor in vacuum.High load generally means that the throttles are open more demanding more air and vacuum drops.Light load low throttle opening just easy cruzing doesnt require much air and the vacuum is generally higher.

Try driving with a vacuum guage in your passenger compartment...especially with a manual trasmission car.Start out in 1st gear and observe how your engine and the vacuum react...then try starting out in 3rd gear from a stop instead of 1st...This will load the engine much harder and you will see a dramatic difference is how the engine runs and how low your vacuum will drop as you are trying to start out.

When driving with a vacuum guage is cruzing along at say 25 MPH and say pulling 15 in of vacuum I have been able to lift the throttle just a hair... and vacuum may jump to 20 inches.... With no difference in MPH change...... IE vac is very sensitive to throttle opening and engine loads.

Under racing conditions if you can achive manifold vac down to zero while accelerating and not producing any bog then things are good.That is allowing atmospheric pressure to fill the cylinders and create HP.... If you are still pulling vacuum in that situation or a bog then that tells you something else is needed.... IE more gearing , a longer cam , more carb , ign timing ect ect ect.

This is unfortunatly a big subject and this just barely scratches the surface of the subject of vacuum..... Hope this help in some way.

 

[69LS1]

As far as cams go any cam is ...even real small ones open the intake BTDC and close the intake ABDC.
Reversion is a killer of vacuum.

As the piston is approaching TDC on the exhaust stroke ...X amount of deg BTDC the intake valve opens up.... If there is manifold vacuum present in the intake manifold it can draw some air out of the cylinder and into the intake ports of manifold... This is a disruptive pulsation in the intake manifold....

Also reversion can happen from the intake valve closeing.All cams close their intake valve X amount of degrees ABDC. As the piston dwells at BTC and then starts back up on the compression stroke the intake is still open some amount. The longer this stays open the more the piston can literly push A/F right back out the cylinnder and into the intake port and manifold. This can be a very strong pulsation and is very disruptive to the creation of vacuume.

Longer duration cams tend to need more compression due in part to these reversion pulsations.....Shorter duration cams in general produce far less reversion pulsations than do the long duration cams.

As typical short cam may have timing of 20-56 and a long duration cam of say 62-98..... The long duration cam opens much sooner and closes much later than the short cam and will produce much less vacuum and more pulsations than will the short duration cam.

The intake manifold sees these pulsations and the carb has to deal with them also....Especially with single plane and most especially with IR manifolds.