Power Valve Channel Restrictions The power valve lets additional fuel in when manifold vacuum drops under hard acceleration, such as when accelerating off a corner. The new adjustable fuel jets or "power valve channel restrictions" in the power valve mounting base in the adjustable metering block now allow tuning of the volume of fuel delivered, not just the timing of it. In a stock metering block, only the timing of the power valve actuation can be tuned by changing out the complete power valve to one of a different manifold vacuum rating. The number stamped on a power valve, such as "65," shows the manifold vacuum below which a power valve works/opens. In this case, a 65 means that, at all manifold vacuum below 6.5-inch Hg (Mercury), the power valve is working. A power valve times when fuel is delivered under acceleration-it is somewhat of a linear delivery, but it will flutter a little as it shuts off. The power valve in the adjustable 2-bbl metering block is stamped "45."

Poe explains the tuning utility of the adjustable power valve channel restrictions. "On the dyno, you can pull an engine and in the mid-range (say from 4,000-5,500 rpm and without changing the main jet-which can alter the fuel curve over the entire power band), you can adjust the power valve fuel bleeds and affect the fuel curve just in this mid-range band. That's a useful tuning aid." Changing the diameter of the power valve channel restrictions can lean or richen the desired fuel curve at the time the power valve opens. The tunable 2-bbl metering block comes with 0.042-inch screw-in restrictions.

Anti-Siphon Air Bleed Box-stock, the non-adjustable 2-bbl metering block has a fuel delivery quirk that is a race engine-tuning nightmare. At WOT (wide-open throttle), with high vacuum caused by the high velocity of the air in the venturis, fuel is pulled out from the accelerator pump discharge nozzles-in two unmetered solid streams. Not exactly the optimum condition for managed fuel metering required for racing, is it? The total air/fuel mixture is richened without control by this siphoning effect. Consequently, an anti-siphon air bleed is placed in the accelerator pump passage just above the main jets in the tunable 2-bbl metering block. Air introduced into the pump passage by this screw-in air bleed (stock is a 0.020-inch diameter hole) breaks the siphoning vacuum being pulled on the fuel, and stops the fuel from being sucked up into the venturii at high vacuum/WOT. A smaller size hole will have less effect on the fuel suction, and a larger one will have more influence.

Dyno-tune, Not Seat-of-the-Pants The tunable metering block makes the Holley 0-80787-1 2-bbl carb more usable. For example, there are many different engine combinations racing in the NASCAR Late Model series, and since Dodge came into the series with yet another engine block and cylinder head combination (see Circle Track Oct. '02 issue for an exclusive full description of their Late Model engine), a one-tune-fits-all carb was unrealistic.

According to Poe, "The carb needs to be tuned to the Ford, Chevy, or the Dodge, whichever it's on. It needs to be more accurately tuned if it's on a 5.7-inch rod engine with a greater duration camshaft, or a 6.25-inch rod engine, which is likely to have a much shorter duration camshaft." He continues, "This metering block, used on the class-legal 2-bbl carburetor, gives the adjustability to gain peak power out of any engine combination it's installed on."

But all this adjustability should not be taken lightly. "There are 19 holes in this metering block to be played with to make this carb work on any engine combination," clarifies Poe. "As long as Holley has a hole in the metering block, we can upsize it, or downsize it, but we can't add any more holes to it."

The PN 134-276 adjustable metering block gives a carb tuner or engine builder adjustability with 13 of these holes. The engine tuner should use a dyno to take full advantage of all the fuel metering adjustability to quantify any gains (or losses). Poe won't give up all his metering block wizardry, but he offers this tuning starting point: "The engine tuner/builder needs to calculate the amount of area now being added to the fuel channel. If there are now three drilled emulsion holes with 'X' amount of square inches of area (in their combined hole dimensions), and now there are five adjustable holes, then begin with the same amount of area spread out over the five holes now available." Essentially, duplicate the hole area in the adjustable emulsion holes of the new metering block to create a tuning baseline.

Poe says he's seen as much as "5 to 7" hp gain on a Chevrolet engine between a standard 7448 and the HP carb with its adjustable metering block. "In track testing, back-to-back, a good race 7448 as we used to run, compared to an HP with the adjustable bleeds, has yielded as much as 0.010 seconds or more. This was with a Late Model Stock with one of my Chevy engines," explains Poe.