[StanJ]

"Power valve" (or enrichment valve) circuitry can be used to tailor the shape of a carburetor's fuel delivery curve in several ways, including mid-to-high load enrichment (thus allowing "cleaner" idle and low-load running) and also as a high-load "lean out" for situations where class rules require the use of a carburetor with venturi sizes substantially smaller than what would normally be required to satisfy the engine's airflow demand (I'll be happy to explain the "how and why" details of this to anyone who wants to know, but otherwise won't bore all of you with them here).

In most road-racing and oval-track applications, sharp part-throttle response and fuel mileage are very important, so we utilize power valves (along with a couple of key additions that I'll cover in a moment) in the carburetors that we build for these applications in order to keep un-needed fuel out of the cylinders (and off the sparkplugs) until throttle demand requires it.

As for drag racing carburetors utilizing a power valve only in the "front" (or primary) metering block, the main reason has to do with fuel movement inside the rear float bowl when these cars launch. To understand what happens here, picture in your mind's eye a Holley (or Demon) metering block and float bowl on the secondary (rear) side of a drag carb. With the vehicle "staged", the level of fuel in the bowl would be --depending on float adjustment -- just to the bottom of the sight plug opening and essentially parallel with the ground; covering the jets...and the power valve fuel inlet, if one were installed. However, when the car leaves the starting line, the combination of acceleration forces and chassis roll (think wheel stand here) cause the fuel in the rear bowl to "slosh" violently to the back of the bowl and away from the metering block...uncovering first the power valve fuel inlet (which is less that .625" below the "static" level") and then the jets themselves. Under those conditions of course, fuel delivery into the engine is cut more-or-less in half, resulting in a huge "bog" that drops the nose of car back down until the jets and PV are again covered by fuel...at which time the whole process often repeats itself until the driver aborts the pass or until the car finally reaches sufficient speed so that it's decreased rate of acceleration no longer causes the jets to be uncovered. "Jet extensions" -- which have become practically universal in drag racing carbs -- solve part of the problem by allowing the jets to feed from the rear of the float bowl...but keeping the power valve fuel inlet covered is much more difficult due to the design of the float mechanism. Since drag racing is generally not a "part throttle" sport, most competitors solve the remaining issue by simply not utilizing the power valve circuitry in the rear of the carburetor (installing a plug if necessary to block it off) and installing larger jets in the rear metering block to compensate for the lost fuel flow.

As I mentioned earlier, road racing and oval-track competitors need the part-throttle fuel curve benefits that power valves make possible, and for them the "fuel slosh" issue mentioned above wasn't really a serious problem until about 15 years ago when advances in chassis/tire technology drastically increased forward bite out of the corners. Addressing the problem here wasn't quite so simple, since these cars can't afford the total disruption of fuel delivery under deceleration that drag racing-style jet extensions cause either. Our solution involves a baffle that we fabricate and install over the power valve fuel inlet which keeps it covered with fuel during high lateral loads, along with a modified float and a reshaped float bowl interior that is much more resistant to fuel movement away from the jets. Together, they work extremely well...and we have even used this technology successfully on some quicker (sub 10 sec.) drag cars.