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Power Valve Dissertation
Saw this on Parker Carbs' website. Looked so good I just had to throw it on here...
The term power valve is a misnomer likely born of advertising- a more descriptive term might be ‘vacuum actuated enrichment device’.
The whole idea behind a main venturi and auxiliary (booster) venturi is that the correct amount of fuel will be mixed with the air flowing through the venturi at any throttle opening and/or RPM. More specifically, the idea is that as airflow increases or decreases the amount of fuel dispensed into the airstream will also increase or decrease and that the air/fuel ratio will remain constant.
When carburetors with venturis appeared on the scene it became obvious that a simple, unassisted venturi left a little to be desired when it came to maintaining a constant air/fuel ratio under all conditions. The various problems encountered led to the development of a transfer idle system, an accelerator system and a main system enrichment device- the power valve.
The power valve was created to solve a main jet sizing problem. Fairly early in carburetor development it was discovered that if main jets were small enough to provide good performance at a low RPM then the engine tended to be too lean at high RPM. If jets large enough for good high RPM performance were installed then the engine was too rich at low RPM. What was needed was a way to use smaller jets for good low RPM performance yet be able to add the extra fuel needed for good high RPM running. Enter the vacuum actuated enrichment device.
The power valve is basically a vacuum actuated valve that is closed when manifold vacuum is high (at idle and low RPM) and that opens when manifold vacuum is low (when the throttle is opened). One side of the power valve consists of a rubber diaphragm that is exposed to manifold vacuum via a passage in the main body and throttle body. The diaphragm is attached to a conical ‘needle’ that fits into a ‘seat’ on the fuel-bowl side of the power valve. A spring positioned between the needle and seat holds the two assemblies apart in the absence of a vacuum applied to the diaphragm.
When an engine is idling the diaphragm ‘sees’ a high manifold vacuum. The diaphragm responds by moving in the direction of the vacuum, which overcomes the force exerted by the spring and pulls the needle into the seat, thus closing the valve. As the throttle is opened and manifold vacuum falls, at a given point the force exerted by the spring overcomes the declining manifold vacuum and pushes the needle away from the seat, opening the valve. Fuel from the fuel bowl flows through the valve and through two small holes located beneath the valve- the power valve restrictions- then flows into the main fuel wells, joining the fuel supplied by the main jets. With a power valve you can use smaller main jets, which provides good low RPM performance and drivability, yet when the throttle is opened and manifold vacuum falls, the power valve supplies the extra fuel needed for acceleration and high RPM running.
You want to use a ‘window’ type of power valve for racing. This type of valve has two rectangular openings that fuel flows through when the valve opens. The area of the openings is far greater than the area of the power valve restrictions, so the power valve itself doesn’t control the amount of fuel that enters the main fuel wells. Therefore, when you install a different number power valve, you aren’t changing fuel flow at all.
Then what are you changing? The manifold vacuum at which the valve opens. Power valves come in various opening points because different engines will have differing degrees of manifold vacuum at idle. Since you don’t want a power valve to be open at idle- otherwise, what’s the point of having one- you need to be able to choose a power valve with an opening point that’s lower than your manifold vacuum at idle. For example, if you had 8.0 inches of manifold vacuum at idle and installed a 10.5 inch power valve, the relatively strong spring of the 10.5 power valve would hold the valve open at idle- not a good thing. In this case you would want to install a power valve rated at something less that 8.0 inches. How much less? A good rule is 1.5 to 2.0 inches less. In this example you would install a 6.5 power valve. If you had 12.0 inches of manifold vacuum at idle then you might start with a 10.5 power valve and so forth.
While racing engines usually accelerate their best with a power valve that opens relatively early, i.e. an higher number power valve, the opening point of a power valve can be used as a tuning tool. You might want to begin with the power valve you initially installed, say a 10.5, and make a baseline run. You might then drop two numbers to an 8.5 and make another run to see if acceleration has improved. If it does, drop two more numbers to a 6.5 and try it again. When you think you’re getting close to the optimum opening point then start changing one number at a time until you find the best power valve opening point for you combination.
If in the testing process you install a too-low number power valve you’ll likely wind up with a huge bog at corner exit. What’s happening is that the power valve is opening too late- the engine needs the extra fuel for acceleration before the manifold vacuum drops low enough for the power valve to open and supply it. If this happens just install successively higher number power valves until the problem goes away and maximum acceleration is restored.
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