The needed volume is based on fuel requirements to make a certain amount of horsepower. Fuel volume is going to directly dictate what size fuel lines are required to reach a given horsepower range. In a carbureted setup, the pressure is going to affect the float levels, which will change the air/fuel mixture. If the fuel volume is correct for your setup, there shouldn’t be any changes in pressure from idle to part throttle to wide open throttle. The key here is having more fuel pump than you need, and regulating it correctly. In a dead-head–style regulator, only the regulated fuel goes to the carb, and what is left sits as a reserve in the fuel line. If you don’t have enough fuel pump, the reserve will be depleted at WOT, and you’ll see fuel pressure drop. The same goes for a return-style fuel system, but in this type of system, the fuel reserve is constantly circulating.

Return vs. Returnless

You might be saying, “What difference does it make if my fuel system is a return style or returnless style system?” Well it can make a big difference. A returnless fuel system consists of one fuel line that feeds the carb or fuel rails. The regulator controls pressure, and the stream of fuel deadheads at the rails or carb. While this is an effecting way to deliver fuel, it does have some downfalls. The fuel backs up behind the regulator (the regulator acts as a restrictor) and the fuel waiting in the line moves much slower on its way to the engine, absorbing heat as it goes. While it may not seem like a big deal, heated fuel has a different specific gravity than cooler fuel, which will change the mixture. Beyond that, hotter fuel is more susceptible to pre-ignition (detonation), although higher-octane fuels can remedy this problem.

A return-style fuel system keeps the flow of fuel through the entire system constant, keeping the fuel cooler. This will give you a denser air/fuel charge, and will keep the fuel pump cooler, as cooler fuel is constantly passing through it. Constant circulation of cooler fuel will also result in less vapor lock issues than a dead-head–style system, as fuel sits in the lines and carb absorbing heat from the engine.

An Outlaw drag car with three stages of nitrous has very different needs than a Street Stock with a small carb, and there are options for everything in between

How Regulators Work

At a glance, a fuel-pressure regulator is a metering device. It controls the flow of fuel to the your given method of fuel delivery, be it a carburetor or fuel injection. The body of the regulator houses a number of parts that are all critical to the performance of the regulator, including a spring (or multiple springs), a diaphragm, a seat, and the hardware to adjust pressure and hold it all together. While the type of parts can be pretty standard across the board, the quality of the parts can be the difference between a part that lasts, and a part that doesn’t.

Inside a regulator, the adjuster sets the spring pressure on the diaphragm, which controls fuel flow. The spring and diaphragm are what control the pressure. Keep in mind, fuel pressure is dynamic—it changes with throttle application. Keeping fuel pressure at a constant number is important, but so is how quickly the regulator reacts to changes in pressure. As the throttle opens and the reserve of fuel in the fuel bowls or fuel rails enter the engine, the regulator needs to open up to replenish that supply. The key is to replenish it without causing a spike in fuel pressure. Quick spikes in pressure, be it high or low, will change the air/fuel mixture causing a rich or lean condition. Even though this may be a momentary condition, it still adversely affects power and driveablity, as throttle response is affected.