quote:Originally posted by Detonator:
I have a hard time grasping the concept of needing to add a vacuum pump, is the engine not a giant vacuum/air pump, I would think a check valve and manifold pressure would be all you need? what am I not understanding?
I am being serious what did I miss?
In order for the engine to be the vacuum pump itself that will operate vacuum accessories like the brake assist booster, it must provide 1) pressure to the intake manifold (source of vacuum). That pressure is measured in inches of mercury rather that in pounds per square inch.
The reason is it is a negative number if stated in psi and that confuses too many people 2) the engine needs to produce the pressure across a large enough hole in the intake manifold. In the case of the stock Pantera that hole is 5/8" and the minimum pressure is 15in-hg.
Camshaft timing, most specifically the overlap of the camshaft, i.e., the amount of time both the intake valve and the exhaust valve are open, effects how high the manifold pressure will be.
These engines when new and "stock" produced about 16 or 17in-hg.
When you change to a longer duration camshaft with more valve overlap, the pressure produced in the intake manifold drops. Almost always under 15. Sometimes as low as 7 or 8in-hg at idle.
That isn't enough pressure for the accessories to operate on.
In my case, the design of the intake manifold does not have a common plenum. Each runner to an intake manifold has it's own. It's own carburetor as well.
In order to get a reading of the combined vacuums being produced by each cylinder, the intake runners need to be connected together to each other in some way. Sometimes that connection of plumbing to which they all connect is called a vacuum manifold.
In the case of independent runner manifolds using carburetion, not fuel injection, you have to minimize the amount of pressure lost into that vacuum manifold to insure that you can still tune the strength of the fuel mixture, or more correctly stated, minimize the amount of leaning out of the mixture due to the interference of the leak of cylinder pressures into the vacuum manifold.
The vacuum manifold on mine, as with many with Webers is made from 1/4" od metal tubing, and is strung together in series like a Christmas Tree light bulb set.
So the pressure can be read accurately now with a vacuum gauge because of the vacuum manifold, but the quantity of vacuum (negative pressure) does not add up to the amount you get from the 5/8" inside diameter hose that would be connected to an intake manifold with one common plenum under one carburetor that would supply the entire engine with atomized fuel.
I've also introduced a PCV valve into the equation by connecting it to the vacuum manifold which equates to a 2in-hg loss, AND I had to restrict that to keep it at only a 2in-hg loss at idle. So now I have 12in-hg at idle. This is not enough pressure, and not enough volume of pressure to operate the brake booster.
So one possible solution is to use a vacuum pump to supply the accessories, in this case just the power brakes because it is not dependent upon the engine itself to produce the vacuum. It is a self propelled vacuum pump producing up to 22-23in-hg on demand.
Get it now at all?