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Auto Repair Questions - What's Your Problem?

A Question Of Restriction
Bill Roth, Adams, WI: Why do you need a large-diameter fuel line of 31/48 or 11/42 inch to supply a fuel regulator that has a 71/432-inch (0.219-inch) hole feeding your carburetor? That small hole is even smaller if you subtract the pin diameter that goes through the hole to unseat the check ball. It seems like a regulator would kill your volume.

Jeff Smith: Great question, Bill, and you're absolutely right. Pressure and volume are inversely proportional. That means that as pressure increases, volume decreases. The classic example is a garden hose. Open up the spigot valve, let the water run out of the end of the hose, and you have lots of volume but little pressure. Restrict the end of the hose with your thumb, and the pressure increases in the line but the volume decreases.

All pressure regulators are designed to limit the amount of fuel pressure that is applied to the carburetor inlet bowls. This is because most high-volume pumps can create more line pressure than a typical Holley can tolerate. Generally, the maximum fuel pressure a Holley can stand is between 6 and 8 psi. Excessive pressure will overpower the needle and seat and flood the bowls. This usually results in fuel shooting out the vent tubes-not good. Recent testing by the guys at Aeromotive indicates that even 8 psi is too much pressure, creating bubbles and foam in the float bowl, causing erratic fuel delivery from the carburetor.

But to get back to your question, Bill, the original Holley blue regulator (PN 12-803, $26.95 at you mentioned does, in fact, have a very small, 71/432-inch (0.219-inch)-diameter ball restrictor. I'm guessing here, but I think the regulator was sized that way because it was only designed to feed a single Holley dual-feed carburetor with two needle and seat diameters of between 0.097 and 0.100 inch, so a 0.219-inch-diameter ball would not be any more of a restriction than the combination of the two needles and seats. I checked this theory by computing the area of a 0.219-inch-diameter ball, which is 0.0376 inch. The flow area of two 0.100-inch-diameter needles and seats comes out to only 0.0157 inch, or less than half the area of the regulator.

Where you can get into trouble is when you use this single small regulator to feed a pair of Holley carburetors. Then the regulator might be a restriction, even though the area calculation indicates that it would not. The newer lineup of larger carbureted pressure regulators offers less restriction, such as Holley's PN 12-707 ($145.50,, which has an internal flow diameter of 0.437 inch, or a flow area of 0.149 square inches, roughly four times that of the 12-803 regulator.

That's a long way around to answer your question, Bill, but to drill down to it, you need a good pump to ensure that there is both plenty of fuel volume and pressure heading into the regulator, so that even with a 15-foot-long feed line between the rear-mounted pump and the front-mounted regulator there is sufficient fuel volume and pressure to feed the carb under a high-g launch.

A common misconception is that a larger-diameter fuel line, like 11/42 inch, from the pump forward will create a greater pressure drop due to its larger area when attempting to pump fuel forward under a high-g, dragstrip-style launch. The reality is that the diameter is not the culprit-it's the length of the line. Let's use an example to test that statement.

Assume that we have a large swimming pool full of water and we submerge two lengths of 10-foot-long tubing vertically. One is 11/42 inch in diameter and the other is 31/48 inch in diameter. The pressure of the weight of a column of standing water 10 feet deep is the same at any point on the bottom of the pool, including the base of both our 31/48-inch and 11/42-inch lines. If we reduce the depth of the pool from 10 feet to 5 feet, the pressure at the bottom of the pool will be lower because we reduced the height of the standing column of water. The point is that the diameter of the fuel line is not important, but the length is critical. Increasing the length of the fuel line that the pump must push the fuel through creates a greater pressure drop under acceleration. This requires additional pump pressure to overcome the static load of a long fuel line. In this case, longer is not better. That's why you'll see many race cars running a small fuel tank (assuming it's legal) in the front of the engine compartment with a very short feed line running to the carburetors. This system uses the g-force of acceleration to help the fuel pump deliver fuel to the regulator, instead of the pump having to fight against the force of acceleration. Even with a rear-mounted tank and pump, it's still wise to mount the fuel pressure regulator ahead of the carburetor, so that the reduced pressure is using the force of acceleration to help push the fuel from the regulator to the carburetor.

To boil it all down, a shorter, larger-diameter fuel delivery line to the regulator is best, aided by a relatively low fuel pressure into the carburetor to prevent aeration of the fuel in the float bowls. While this is also subject to continuing debate, most manufacturers point out that a full-flow return system is superior to a deadhead system where the fuel pump bangs up against the regulator. But I'll wait for another letter to talk about the inside info on full-flow systems. It's good stuff.

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