How To Go From A Carburettor To EFI


Author: Bradley Iger | 04/06/2021

While there is certainly still a welcome place for carburettors in racing and classic vehicle restorations, considering the numerous advantages offered by EFI it has found a welcome adoption by vintage vehicle owners who love driving their cars. While old-school cool certainly has its own virtues, fuel injection can do things that a carburettor's mechanical systems simply cannot. “The ease of use is there – you don’t have to fetch your screwdriver to make an adjustment, and the sky’s the limit on what you can change,” explains Holley Performance fuel injection systems engineer, Matthew Lunsford. “And the computer can make changes as-needed while you’re driving down the road.”


And that capability translates to benefits across the board – easier starting, better reliability, and improved drivability in all types of weather and elevations. “It will start in Antarctica, or in the middle of the desert,” Lunsford points out. “And the ECU has the ability to measure your engine load and make fuelling changes based on the elevation you’re at.” And let’s not forget about the drastic improvements in emissions and fuel economy, too.


Sniper Stealth 4500 kit

Modern throttle body-style systems like this Sniper EFI are designed to be compact and very easy to work with, and essentially serve as a carburettor replacement.


The advantages of EFI aren’t exclusive to your weekend cruiser, either. “In the motorsports world, the ability to review your data after a session or a pass down the drag strip and make corrections based off of what the engine is actually seeing is awesome. You have to have an external data logger to do that with a carburettor, whereas with EFI, most of the time the ECU itself can take care of that.”


But one side effect of all the choices available in EFI retrofit systems today is that it can be tricky to determine which one makes the most sense for your application, and what parts you’ll need to make it all work together. Fortunately we’ve got Lunsford here to break it all down.


EFI Styles

MPFI manifold ports

Most modern, and many aftermarket, intake manifolds have built-in provisions for mutli-port fuel injection systems, as seen here.


Builders have ostensibly two different design approaches they can take when it comes to an EFI retrofit – throttle body injection (TBI) or mutli-port fuel injection (MPFI). “With TBI you’re using the intake manifold to distribute the air and the fuel,” Lunsford says. “But with MPFI, you’re only using the manifold to distribute the air – you inject fuel above the intake valve of each port.”


And that means that TBI is going to be the easiest and least expensive way to make the switch, with a small trade off of some of the capability that MPFI has. TBI units, such as Holley Sniper EFI, basically serve as carburettor replacements – you don’t even have to change your intake manifold to use one, and that really simplifies things and keeps the cost down. “The drawback is that, since the intake manifold is distributing both the fuel and the air, it can still have distribution inefficiencies. But they’re usually not as bad as they would be with a carburettor,” he says.


Though it’s more costly and complex, Lunsford tells us that a multi-port setup is a better approach to EFI conversion in almost all applications. “You’ll typically see much better fuel distribution, and a better-running engine as result. There’s a reason why all OEMs moved to multi-port systems roughly twenty years ago. It’s the best EFI out there.”


What You'll Need



Although throttle-body injection keeps things simple by incorporating all the fundamental EFI system components into one carb-sized unit, you still need to make sure that the rest of the vehicle systems are ready for the upgrade.


“The fuel system is a crucial part of any EFI conversion,” Lunsford says. “Carburetors are pretty resilient to dirty fuel and potentially fluctuating pressures, but fuel injection is not. The injector orifices are very, very small, so if you get debris in there, it can clog it or cause other issues because an injector opens and closes based on an assumed amount of fuel pressure. So having good fuel filters before and after the fuel pump is very important.”


With a TBI system like the Sniper EFI series, you’ll also need a fuel pump that can flow at least 255 litres per hour or more, depending on the horsepower your engine is making. EFI systems traditionally also run at pressures between 43 and 60 psi to maintain a consistent volume of fuel for the injector and help with atomization. Many EFI systems, such as Sniper, incorporate a fuel pressure regulator in the design but often an external fuel pressure regulator is needed. That’s a stark difference from carburetted systems, which normally operate at about 5 to 7 psi.


EFI Fuel Pump

EFI systems require much higher fuel pressure than carburettors do, so you’ll need a high-pressure fuel pump to support that. Those can be installed in-tank or externally mounted, depending on the application’s requirements.


Most EFI systems also require a return line that runs from the engine back to the fuel tank, but Holley offers return-less solutions that don’t require one. “You can get away with a return-less system as long as it truly does deliver high pressure to the fuel rails,” he says. But keep in mind that you’ll still need a fuel line that supports EFI fuel pressures to deliver the fuel from the tank to the engine.


Good wiring is also key. “It’s just as important as the fuel system,” Lunsford points out. “You’re converting car that’s fifty or sixty years old, and there’s no way they could have known what kinds of electronics we’re using today. Grounds are a big deal – you have to make sure that the engine block is grounded directly to the battery, that the chassis is grounded directly to the battery, and that you have ‘clean’ power – you can’t have the power split between a bunch of fuses and that sort of thing. At the end of the day, a beautiful install isn’t worth much if the wiring is bad. And a lot that comes down to using the right tools for the job and automotive-grade wire.”


A return-style system requires a regulator, which can be in the throttle body in some instances, or an externally-mounted design like this one.


Multi-port injection setups have the same fuel and electrical system requirements, more or less, but also throw a few extra elements into the mix that make the conversion a bit more involved. “The difference is that you’re going to have to change your intake manifold, and add injectors and fuel rails,” Lunsford explains.


Ideally, these components are selected based on the requirements of a specific build in terms of engine output and desired powerband characteristics, but there are ways to take some of the guesswork out. “There are kits out there, and a lot of modern intake manifolds can be adapted to accept fuel rails. Injector sizing really just comes down to how much horsepower the engine makes and the type of fuel you want to run. E85 requires more fuel than pump gas does to make the same amount of power. So it comes down to the specific combination you’re running. There isn’t really a good rule-of-thumb here, but Holley offers injectors that will support engines making anywhere from a hundred horsepower (or less), to 2000 horsepower (or more), so we probably have what you need.”


Injectors and Rails

Along with an MPFI-compatible intake manifold, multi-port systems require injectors and rails which are sized appropriately for the engine.


And whether you’re going the TBI or multi-port route, some ignition upgrades are worth considering as well. “Depending on how deep you want to go with your EFI control, you may need an ignition box like an MSD 6A,” Lunsford notes. “With that, the EFI system can also control the timing of the engine – now you have control of fuel and spark. Instead of using the traditional weights and springs in a distributor to adjust the timing curve, you can hop or your laptop or use the hand-held that comes with your ECU to make adjustments, and that can also be used to control additional features related to the ignition system.”

Getting Up And Running


Sniper EFI hand-held

Systems like Sniper EFI will walk you through the process of establishing a baseline calibration for the engine to get you going, after which the readings from the oxygen sensors will allow the system to fine-tune things as needed on its own.


Once everything is back together, bringing the car back to life with a new Holley EFI system is a fairly straight-forward proposition.


“If you purchase one of our Sniper or Terminator X products, it comes with what we call a ‘wizard’ in the hand held. You answer some basic questions about the engine specs – things like how many cylinders it has, what type of camshaft you’re using, the displacement and so on, and the wizard builds a baseline curve that’ll get the engine started and driving.”


From there the EFI system relies heavily on the exhaust system for fine-tuning, so you need to make sure the exhaust has no leaks. The oxygen sensors are providing the ECU with information about how the fueling in that engine is doing – if there’s a leak, it won’t get an accurate reading. And from those exhaust readings, the system will add or subtract fuel as needed until it meets the target air/fuel ratio for that particular combination.