Power Valve Selection: The Definitive Answer
The hard situation to be know whether the idle circuit is contributing or fuel or air to the delivery is part throttle higher speed.
Lets say a highway geared vehicle running steady 3000 rpm, 60 mph has 15"hg manifold vacuum.
15"Hg vacuum is 7.3 psi below atmospheric (7.4 psi absolute).
So does the booster create a strong enough drop in pressure to pull fuel out of the idle circuit?
My guess is normally not. There's less restriction pulling fuel at atmospheric pressure (14.7 psia) through the main jet.
But this is an example why its difficult to make blanket statements about carb behavior. And then when it comes to restrictions and flow, there are choking flows and changes in flow behaviour when it goes from laminar to turbulent, etc, etc.
We can take specific situations and try to understand them even when they don't behave as expected. Maybe that should be especially when they don't behave as expected.
Here's another see-thru version of a idle circuit from the bowl side.
Blue represents fuel just sitting at same level as in the fuel bowl.
H, P, L are the three possible location for idle feed restrictions.
As you can see, how the idle/transfer flows depends on the pressure at the slots, the air bleed, and the booster. But how much air or fuel moves through them depends on the restrictions. A restricted carb, such as a 2 bbl on a v-8 running WOT high rpm will pull fuel through through the idle/transfer ports. I think someone posted they might be running 6"Hg at WOT.
The hard situation to be know whether the idle circuit is contributing or fuel or air to the delivery is part throttle higher speed.
Lets say a highway geared vehicle running steady 3000 rpm, 60 mph has 15"hg manifold vacuum.
15"Hg vacuum is 7.3 psi below atmospheric (7.4 psi absolute).
So does the booster create a strong enough drop in pressure to pull fuel out of the idle circuit?
My guess is normally not. There's less restriction pulling fuel at atmospheric pressure (14.7 psia) through the main jet.
But this is an example why its difficult to make blanket statements about carb behavior. And then when it comes to restrictions and flow, there are choking flows and changes in flow behaviour when it goes from laminar to turbulent, etc, etc.
We can take specific situations and try to understand them even when they don't behave as expected. Maybe that should be especially when they don't behave as expected.
Here's another see-thru version of a idle circuit from the bowl side.
Blue represents fuel just sitting at same level as in the fuel bowl.
H, P, L are the three possible location for idle feed restrictions.
