Fantastic..care to share how your Holleys respond to vacuum created by a roots blower vs a naturally aspiratred set up?Yep..we are all in tune to the pv boost referance
.lets go further.Is the supercharger drying up the idle/transition ckt at idle and leading to Detroit roll?Iwould like your thoughts on a open power valve at idle,not a failed pv.I have not come across anyone that can explain this in full.I always get the Yes..the engine will idle rich with an open pv.Why?.How do the ifr,s that drink from the main well flow more fuel with an open pv?is it fuel weight atop the ifr?
You've just about hit the nail on the head. Because the ifr is connected to the main well, fuel flow through it will be affected by the fuel level in the well. Think of it as the idle and main systems playing tug of war. At idle, the idle system draws fuel from the main well. Since the main jet offers some restriction, the fuel level in the main well drops. This results in a lower head of fuel (height of the fuel above the ifr). If the pv is opened, itís just like opening up the main jet. This lower restriction raises main well fuel level and the idle becomes richer.
It is by design that the idle circuit feeds from the main well. With the idle system pulling the main well level down, a higher airflow is required to get the main system started. This helps prevent the main nozzle (boost venturi) from dripping or starting prematurely. As the throttle is opened, the venturi signal increases, and the idle signal eventually decreases, so there is a smooth transition to the main system. When the throttle is opened suddenly, the idle system drops out because it is sensitive to vacuum, not airflow. This raises the fuel level in the main well and the main system will now start faster to compensate for the weaker idle signal.
You can observe this effect - with the vehicle in neutral, open the throttle with the idle speed screw slowly until you see fuel flow just beginning to discharge from the booster. Holding the throttle at this position, open the mixture screw and the nozzle should stop.
I've not heard of a Detroit roll. Is that like getting mugged downtown at night? lol
I haven't experience with Roots blower applications, but as long as the vacuum under the carburetor at idle is about 16 in. Hg or higher, the air flow and fuel flow will remain constant. It doesn't matter if there's a blower or not, the carburetor will not know the difference. This is because the carb has reached what is called critical flow - you could increase the vacuum to 24 in. Hg and the carb will meter exactly the same as at 16 in. Hg. Let's look at this possibility - assume the carb idle is adjusted too rich initially and the engine develops less than 16 in. at idle. The engine is started and the vacuum under the carb is less than 16. So, the engine speed drops due to the rich mixture, the vacuum drops, the fuel flow drops, and the mixture now becomes correct. The engine speed and vacuum increase, and the cycle repeats itself as the engine rolling condition. This could also possibly happen with a too-lean idle mixture and the power valve opens when the vacuum drops. Another cause for the roll could be the vacuum advance or the centrifugal advance changing timing under these conditions.
I hope this is helpful and not too long winded.