Ask Away! with Jeff Smith: Building a Fuel System That Can Handle Both Carbureted and EFI Setups

I’m building a street big block Chevy 496 that will make around 650 hp. I’m trying to decide whether to go with EFI or still run a carburetor. But before I decide, will this change the type of fuel pump I use? Is there a fuel pump that will work for both EFI and a carburetor if I decide to go with EFI at a later date?
T.D.

Mechanical or engine-driven pumps do not have the capacity or pressure to work with any EFI system so this leaves the discussion to electric pumps.

The short answer is that you could use a rear-mounted electric fuel pump with a separate fuel pressure regulator and that could be designed to feed a carburetor with low pressure (around 5 to 6 psi) in what is typically called a dead head design with no return. But this would not work well with an EFI system, so you are better off to design a system that is compatible with both.

If you are considering EFI, but want to stick with a carburetor for now, then our recommendation is to convert to an in-tank electric fuel pump with the capacity to feed a high-pressure fuel injection system at 43 to 58 psi. This will require a fuel pressure regulator that includes a return that routes fuel back to the tank. This is different from what is called a dead head regulator where fuel is just plumbed to the regulator (for a carbureted application) and no fuel is returned to the tank. It’s best to install a full return system for either carbureted or EFI as this style system offers better control.

As for a fuel pump recommendation, it would be best to position the pump in the tank so that conversion does not have to be made later when you move to EFI. Aeromotive for example, offers a really nice Phantom in-tank pump conversion kit using a 340 liter-per-hour pump that’s capable of feeding up to 1,000 hp carbureted and 850 hp when running EFI. (We’ve listed all the part numbers for the Aeromotive parts in the chart near the bottom of this post.)

You didn’t mention what kind of car this Rat motor is in, but Aeromotive also sells complete bolt-in fuel tank assemblies with a Phantom pump already installed. There are several companies, like Holley for example, that also sell these conversion kits.

Later, if you decide to upgrade to EFI, this will require changing to a regulator that can control the higher pressure of 43 to 58 psi. This will require a different return style pressure regulator. Aeromotive might be able to make this easier with an X1 fuel pressure regulator that can be converted from low pressure carburetor style to high pressure EFI with just a conversion kit.

For example, this AEI-13304 carbureted Aeromotive regulator comes equipped with a larger seat diameter of 0.313-inch to allow plenty of volume at low pressure. With EFI, you can retain this regulator but use an Aeromotive Kit to convert this regulator to a 35-75 psi regulator. The conversion kit comes with a high pressure spring along with a new diaphragm and seat arrangement that decreases the seat diameter to 0.188-inch to better control the pressure and flow.

All this adaptability is not inexpensive as the regulator and the subsequent conversion kit will be well in excess of $300—but then the cost of two high-quality regulators could be even more.

So either way, it won’t be inexpensive but that’s the price of running two different induction systems.

It’s also important to note that while fuel mileage will probably be better with EFI, there will be no horsepower improvement with the addition of a throttle body EFI. As long as the carburetor is not a restriction to airflow, there will be no power advantage to a throttle body EFI. That question often comes up but there is no power advantage to the EFI assuming that both are tuned properly.

Aeromotive Parts List for EFI/Carbureted Fuel System Conversion