In those prehistoric times prior to fuel injection, regulating fuel pressure on a hot carbureted car (race or otherwise) came to down to using one, two, or more deadhead regulators. Done deal, but those deadhead regulators came with their own issues.

For example, regulator creep—where the fuel pressure slowly creeps up—wasn’t uncommon. Deadhead regulators put an added load on the fuel pump too. Essentially, the fuel pump produces (for example) 15 pounds of pressure. The pump hammers the regulator check valve with those pressures and it has nowhere to go. This obviously increases the temperature of the fuel, sometimes to the point of vapor lock.

So basically, what you’re faced with is the potential for inconsistent fuel pressure numbers, added fuel heat, and the potential for engine flooding. It also tends to shorten the life of both the fuel pump and the regulator(s).

Bottom line here is a deadhead regulator is nothing more than an adjustable restriction in the fuel line, although they still have their place in some applications.

Meet the Bypass Fuel Regulator

Then poof! Fuel injection became popular and with it, bypass regulators became common.

Here, the regulator takes in all of the fuel the pump produced, regulates it to the required pressure, and returns the un-needed fuel to the gas tank. For a street car application, this makes for a much more reliable, consistent setup. It keeps the fuel cool, it eliminates pressure creep, and it increases the life of the fuel pump and the regulator.

So far so good. It’s easy to see how positive the benefits are for fuel injected applications, but they also apply directly to carbureted cars.

And that’s the whole purpose of this article.

Plenty of inline electric fuel pumps can produce 100 PSI at full bark. Plumbing in a bypass regulator (complete with a return line to the tank) and setting the pressure at 7 to 7.5 PSI allows the fuel to circulate just like an EFI application. With a car that launches hard, you have considerable fuel available to the regulator to counteract the G-forces.

Plumbing a Bypass Fuel Regulator

What about the plumbing? Many fuel system component manufacturers recommend a -10 AN fuel line (or larger) from the tank to the pump. And from the pump to the regulator, they typically recommend a -10 AN line. The return line can be -8 AN but a -10 AN line will also work.

Here, plumbing bulk can become an issue (hence the -8 AN return line size). What you don’t want is the return line spraying fuel and aerating it near the pickup inside the fuel tank or cell. Some manufacturers recommend the return be submerged in the tank. With this, you must use a fuel safe hose or metal tubing. And with the return line submerged, chances of aeration are lessened.

The ideal plumbing arrangement in a big power street-strip application has the bypass regulator mounted in a return line positioned shortly after the carburetor (see the accompanying photos). This allows the fuel to flow through the carburetor before it reaches the regulator. Fuel pressure is still regulated, however. In many cases, you’ll need to step down the fuel line size before the carburetor. It’s simply not practical to plumb -10 AN hose directly to the carb bowls. Here, -8 AN is usually the practical limit.

Engine Performance with a Bypass Fuel Regulator

When you compare a bypass system such as this to a deadhead setup, you’ll find it produces a more stable and much smoother fuel pressure curve. Because the supply of fuel is constantly circulating throughout the fuel system, the bypass regulator can respond much quicker to changes in the engine load. This means the fuel bowls are always full and it also lessens the chances of lean out conditions, particularly on a race track. Of course, the other benefits found on fuel injected cars also apply: The fuel isn’t heated, the pump isn’t overworked, the regulator isn’t getting hammered on and off, and there is no pressure creep.

In my own car (shown in the accompanying photos), the components incorporated all came from the SummitRacing.com, and they include a Weldon A-600-A inline electric fuel pump, a Weldon 2040-2810-A-15 regulator and a mix of -10AN and -8AN Earl’s Performance fuel line. The tank is equipped with a Holley 16-107 Hydramat with a 1/2-inch NPT pickup (adapted to -10 AN). There are bulkhead fittings welded to the tank: a -10AN supply, a -8 AN return and a -8 AN vent. I use a non-vented gas cap.

The Weldon pump will easily pull a prime and as a result, the pump does not need to be mounted lower than the fuel-level in the tank. This makes for cleaner plumbing on a street driven car. You can read my earlier story for details on the Weldon pump.

Does the system work? You bet. Fuel pressure remains constant (I have an easily removable fuel pressure gauge); I don’t fight surging float levels and even in 90+ degree heat, there are no vapor lock issues. A bonus is, pump noise is much lower than the constant moan I was accustomed to hearing with deadhead arrangements.

fuel tank with hole cut for fuel pump
A bypass fuel regulator needs a return line. No secret. With my car, three bulkhead fittings were welded to the backside of the tank: one for the feed line, one for the return and one for a vent. The big hole in the tank top is where the Hydramat is inserted. (Image/Wayne Scraba)
line fittings welded into a new fuel tank
The -8 AN bulkhead fitting on the left is the return while the -10 AN bulkhead fitting on the right is the feed line. (Image/Wayne Scraba)
absorbent fuel mat for a fuel system tank
This is the Holley Hydramat used inside the gas tank. It measures 15 x 15 inches, and the center fitting is 1/2 inch NPT. The Hydramat is actually a wicking agent. It draws fuel into it, and it serves as the fuel pickup. Each corner of the Hydramat makes use of a rare earth magnet for fastening purposes. The magnets work incredibly well too—from personal experience, don’t use a metal screwdriver in an attempt to move them! (Image/Wayne Scraba)
fitting on a fuel tank mat
There are any number of ways to plumb the internals of the fuel tank. I chose to use a stainless -10 AN adapter fitting. (Image/Wayne Scraba)
fuel hose section with fittings
Fuel resistant hose connects the bulkhead fittings at the back of the tank to both the Hydramat and the bypass. The internal hose for the bypass simply lays on the tank floor. (Image/Wayne Scraba)
electric fuel pump under a classic car
A key with this setup is the electric fuel pump. As noted in the article, it’s a Weldon A-600. This pump can physically pull a prime, and in conjunction with the Hydramat, it never runs out of fuel unless the gas tank is bone dry. The ability to pull a prime means the pump can be mounted above the level of the gasoline in the tank. (Image/Wayne Scraba)
bracket with fittings for a fuel line
The fuel feed and return lines route to the front of the car, ending in this bulkhead plate located beneath the battery tray. (Image/Wayne Scraba)
plumbing fuel lines for a carburetor
From below the battery tray, a new set of lines routes to the carburetor and the fuel pressure regulator return port. You can’t see it here, but behind the alternator, the -10 AN fuel hose steps down to -8 AN by way of an inline male reducer fitting. This allows the line to be compatible with the -8 AN Earl’s carburetor fuel line. (Image/Wayne Scraba)
big block chevy engine in a vintage Nova
With the air cleaner installed, this is what the regulator and carburetor plumbing looks like. There’s a closer look (with the air cleaner off) in the next photos. (Image/Wayne Scraba)
fuel lines inside a engine bay of a classic car
The pressure (feed) line routes here (lower fuel line): Fuel runs directly to the carburetor bowls by way of the Earl’s carburetor line. (Image/Wayne Scraba)
fuel rail for a carburetor system installed
Once fuel goes through the carburetor, it is routed back toward the regulator by way of this 180 degree hose end. Typically, bent tube fittings such as this exhibit less turbulence than hard (abrupt) 90 degree examples—although we don’t have much choice at the carb bowl fittings. (Image/Wayne Scraba)
inserting a hose into a fuel pressure regulator
Fuel flow from the back of the carburetor is routed here to the Weldon fuel pressure regulator. (Image/Wayne Scraba)
inserting a fitting into a fuel pressure regulator
It is difficult to see here, but at the bottom of the regulator, a 90 degree bent tube -8 AN fitting directs the fuel from the regulator back to the return line, eventually ending up at the gas tank. (Image/Wayne Scraba)
fuel pressure gauge resting on a table
As noted in the article, I don’t use a permanently mounted fuel pressure gauge. Instead, I temporarily use this Autometer Pro Comp fuel pressure gauge to adjust and check the fuel pressure. Once complete the regulator fitting is covered with an AN cap (you can see it in the previous photo). With a bypass system, changes in the pressure aren’t common because there’s no pressure creep. (Image/Wayne Scraba)

Keywords
fuel pressure, Fuel Regulators
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Author: Wayne Scraba

Wayne Scraba is a diehard car guy and regular contributor to OnAllCylinders. He’s owned his own speed shop, built race cars, street rods, and custom motorcycles, and restored muscle cars. He’s authored five how-to books and written over 4,500 tech articles that have appeared in sixty different high performance automotive, motorcycle and aviation magazines worldwide.