Carb shop question

[Blown 472]

When you boost ref a carb, what psi do you pick to set it up or is it just when it sees boost?

[Hotcrusader76]

]what psi do you pick to set it up?
Do you mean 'in/hg'?
Selection of the correct PV when boost referenced depends on many different factors, same as when choosing one for a stock application.
~Ty

[Blown 472]

nope, boost ref, taking a line from the intake to the pv's to blow them open due to the large vacuum from the blower.

[Hotcrusader76]

Blown472,
Well now that I understand your question, here is some simple insight to your inquiry.
Boost referencing the carburetor refers to utilizing a source of vacuum from the engine that is below the supercharger.
The modification is performed so that the source of vacuum is routed around the supercharger and into the PV cavity. With modification, this can be routed through the ported vacuum source or taken through the baseplate just below the original PV cavity vacuum source.
Now that you can read the proper vacuum signal the PV is then selected as per application specs. Selection of the proper PV in ‘in/hg’ has many factors taken into consideration in order for it to open during the right time. Some items to consider when selecting are cam specs, amount of boost, and cylinder head modifications for starters.
I hope this helped.

[schiada96]

The boost ref carbs I have used and messed with have 2.5's in them

[Blown 472]

What about when the intake goes from vac to boost?

[Infomaniac]

The main reason to boost ref is because the power valve is seeing the vacuum under the carb before the blower and not the actual manifold vacuum. The original design of the power valve is to operate according to manifold vacuum. So you have to reference that manifold pressure to the power valve.
Some boat engines at cruise are under a small amount of boost while the throttles are not all the way open. The power valve is seeing the vacuum of the blower drawing on the carb and will remain closed. The power valve should be open and adding more fuel because the engine is under boost. This can cause an excessive lean condition and you can guess what can happen.
Vacuum holds the power valve closed. There is vacuum most of the time under carbs on top of blowers even at wide open because it is hard to have carbs that flow the same amount of air a blower draws.

[Blown 472]

Ok, so how do you get the valve to open?? you use boost to over ride the spring to open the valve, at what psi do you do that or can you stagger it via a delay valve or something?

[Infomaniac]

Vacuum holds it closed. If you are under boost there is no vacuum and it is open.
It is a normally open valve.
As soon as vacuum drops below the value of the valve, it opens.
A 6.5 power valve requires 6.5 in/hg to hold it closed, as soon as vacuum falls below that value it opens.
[ April 07, 2003, 12:47 PM: Message edited by: Infomaniac ]

[Blown 472]

Infomaniac:
Vacuum holds it closed.
It is a normally open valve.
As soon as vacuum drops below the value of the valve, it opens.
A 6.5 power valve requires 6.5 in/hg to hold it closed, as soon as vacuum falls below that value it opens. Ok, so why then plump boost into it? or should I put tha crack pipe down??

[Infomaniac]

Because if you don't. the power valve acts on the pressure under the carb and not the pressure in the manifold.
The blower is between the power valve and manifold.
The blower sucks really hard on the carb. and may suck the power valve shut when the engine is under boost.
Dude: The power valve needs to see manifold pressure. The blower is in the way. You have to run a hose from the manifold to the power valve.
[ April 07, 2003, 12:56 PM: Message edited by: Infomaniac ]

[Blown 472]

Ok, so are you plugging the hole that leads from the valve to the vac source?? and if you plump from the intake to the valve under boost have you not created a positive pressure on the diapham side of the valve rather than a vac??

[Infomaniac]

There you go.
But the manifold pressure is not always boost. It will be vacuum sometimes.
To answer your original question: It is called boost ref. but it is not always boost.
It is just giving the power valve the pressure reference is should have. Manifold pressure. Instead of under carb (blower inlet) pressure.
I think it should be called "manifold referenced" power valves instead.
[ April 07, 2003, 01:09 PM: Message edited by: Infomaniac ]

[Blown 472]

I get cha, so by doing this you are providing a vac source to the valve that is not huge like the vac at the base of the carb, like my car for example, I pull ten inches of vac while crusing, so if I had the right valve it would be closed, but when I stab it, it starts making boost and then the vac drops, valve opens and the boost helps keep the valve open per say. Thanks for the info, info. :D