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Carburetors or Fuel Injection - Carbs vs. EFI
So You Want To Build A Carbureted Mustang? Here Are Some Things You Need To Know.
It's Not uncommon for old-school auto enthusiasts to switch from fuel injection to carburetors. To many, carburetors are simple and practical in terms of installation and use. Carburetors mix the air and gas and don't need an electronic brain to tell them how to do it.
Using a carbureted induction is often the first choice for the budget-conscious builder, as there is no expensive EFI unit to purchase and tune. All you need are some basic parts to get going, and a few tuning passes often yield quick and impressive results. But we wondered if switching from EFI to carburetion, or building a carbureted combo from scratch, was really that simple. In the next few pages we'll answer those questions.
Before you venture forth with your project, consider that lopping off your EFI system is not an emissions-legal modification-unless your car was originally equipped with a carburetor, and in that case, you usually have to use 50-state-legal replacement parts. If you're looking at a strip-only project, then you don't have to worry about it.
That said, your project and your goals will ultimately determine the level of induction the engine will need in order to reach peak performance. You can almost always reach your goal with either EFI or a carburetor (or two), but there are some serious differences between the two systems, so the first thing to do is compare the components.
Many EFI engines that make over 500 hp require some type of enhanced EFI or a stand-alone engine-management system. This can cost well over $1,000 for a basic system and upwards of $2,000 for a fully optioned setup. You will also need a manifold, a throttle body, a mass air meter (save for Speed Density systems), fuel rails, injectors, a suitable pump and regulator, a good pressure gauge, and a feed and return system.
In contrast, a carbureted system requires only a pump, a feed line, a carburetor, and the manifold. Basic street/strip carbureted equipment costs far less than EFI parts, but once you get into hard-core racing parts, the price difference between EFI and carbureted components tends to get much closer.
One of the more important components to either system is the fuel pump. Electronically fuel-injected cars run at high fuel pressures, generally between 35 and 60 psi, whereas carburetors require anywhere from 5 to 9 psi. The high-pressure systems either employ a return line or an electronically regulated pump to control flow. There are in-tank and external pumps, and if you plan to make serious horsepower, you better plan to have a serious pump.
That said, it is best to get a fuel pump that is designed for a carbureted application when switching to carburetion. Aftermarket electric and mechanical fuel pumps are readily available from companies like Barry Grant, Holley, Aeromotive, and Mallory, and at times you can get away with the stock mechanical fuel pumps.
Keep in mind that if you choose to use a mechanical fuel pump, you may need a cam eccentric to make the pump work. The eccentric bolts on to the camshaft and manipulates the fuel pump's arm in order to produce the pumping action. An older block must also be used, as the EFI blocks have no provision for this pump. In high-performance applications, an electric pump is the better choice.
When switching your EFI Mustang to carburetion, you'll need a tank pickup from a pre-'86 Mustang. This is a drop-in piece that replaces your EFI pump and pickup assembly with a straight pickup and allows fuel to be drawn up and out of the tank to the feed line. Another option, aside from the factory pickup, is a "sumped" fuel tank or a fuel cell. A sumped tank has an extra well or sump located at the rear/ bottom of the fuel tank, and it keeps a deep pool of fuel waiting at all times. You can buy the tanks with the sump already welded in or you can buy the sump and weld it yourself.
Sumped tanks normally accept aluminum A/N fittings and braided stainless steel fuel lines to carry the fuel to the carburetor. Once you have the fuel flowing from the tank, you can get stock '85 fuel lines or pony up for stainless steel braided hose. The latter are more expensive (usually around $8-$10 per foot, depending on size), but the larger line will provide plenty of fuel for future mods. If using non-stock lines, be sure to route them safely away from the driveshaft, suspension, or bellhousing. The level of power you plan to make will dictate the size of the line. We suggest you contact your engine builder or carburetor manufacturer for fuel-line recommendations.
Once the fuel tank and line are installed, you can mount a fuel pump and filter. You'll need to install a low-pressure fuel filter to prevent grit and debris from getting into the regulator and carburetor. Filters come in all sorts of designs, and the one you'll select will probably come down to what your pocketbook can afford. Some feature reusable/replaceable elements, so look at the long-term cost as well. Often, the manufacturer will recommend a filter that compliments the pump.
An adjustable fuel pressure regulator and gauge are next on the list. Some fuel pumps are designed to operate at certain pressures and others require the fuel pressure to be regulated for optimum performance. Most regulators are made from billet or cast aluminum and can be had for about $50-$100. It's important to set the regulator with the engine running to get a true setting. Pressure will depend on the application, but it should generally be between 7 and 8 psi.
In most cases, the carburetor will not bolt on to your old fuel-injection manifold, so you'll need a new one of those as well. Most aftermarket manifolds are cast aluminum, and there are single-plane and dual-plane designs from which to choose.
Much like the various EFI manifolds, the single- and dual-plane manifolds offer different intake runner lengths and plenum sizes, which change the usable rpm range of the engine. Single-plane manifolds such as Edelbrock's Victor Jr. offer a powerband from 3,500 to 8,000 rpm. This is great for high-rpm horsepower, but low and midrange torque can suffer. So, if you have stock heads and a small cam, a single-plane might not be the best choice.
The choice for most street engines is a dual-plane manifold. The Weiand Stealth or Holley Street Dominator are both dual-plane units and work well in the idle-6,500-rpm range. A dual-plane manifold feeds four cylinders from one side of the manifold and four with the other to improve the signal in the lower rpm range. This helps torque and throttle response.
Carburetors are just as well sorted, with each brand offering different options depending on application. Options like electric choke, manual or vacuum secondaries, and four-corner idle circuits are just some of the things to consider. Driveability-wise, they've come a long way, and while some people don't mind the occasional quirks, EFI still has carbs beat on the street, especially in most blown applications.
We've proven in past tech articles that bigger isn't always better with regard to carburetor sizing, so be sure to check with the carb company concerning your application and intended usage before choosing. Selecting a carb that's too big is the single biggest mistake that people make. It's always better to have a carb that's slightly undersized, as it will increase throttle response and torque over a carb that is too big. Most 11- and 12-second Mustangs will do well with a 600-700 cfm carb, despite what your buddy thinks. Save the 850 for your 700hp race engine.
Igniting the gas of the carbureted engine requires some different components as well. Early Fox-body Mustangs used the Ford DuraSpark distributor, which was available on many late-model carbureted Ford V-8s. It has a simple three-wire hookup, but according to our own Yo, Ken columnist, Ken Miele, they may not be the best or most dependable setup to use.
"You can go to the junkyard and get a DuraSpark unit and ignition module for about $150, but you have no idea what you're getting," Miele says. Said distributors may have untold amounts of mileage and therefore wear, and oftentimes it's hard to find the associated DuraSpark ignition box you need to go with it.
Even if you find the DuraSpark ignition module, it may not be the best choice for a performance application. HP Performance's Tony Gonyon says, "The Ford ignition modules were known to go bad quite often, and there's more wiring involved than if you use something like an MSD box. Also, the dwell goes away at 5,000 rpm on the stock ignition module, so you're going to lose power."
If you're set on using the DuraSpark distributor, try to get a unit from the '83-'85 Mustangs, as Gonyon says the mechanical advance in these is better, and the '85 units already have the steel gear for use with hydraulic roller cams, if that's the direction you are headed. You should also consider complementing the DuraSpark with a capacitive discharge ignition box such as those from MSD or Crane. As with most projects, yours will likely get faster as time passes, and supplying it with a healthy ignition system from the get-go will be one less headache down the road. There are, however, a lot of good aftermarket distributors, and some that don't even require an ignition box. These work well with normally aspirated applications, but you may want to step up to a distributor/ignition box combo when using power adders.
The last part of the ignition equation is the coil or coil packs, depending upon application. Late-model EFI coils will work with your carbur-eted setup and offer plenty of power for the average performance street machine. They require a two-wire hookup, just like older and most aftermarket coils.
If you've read this far into the article, then you're pretty serious about going to carburetion, however, as easy as the swap may sound, you'll want to consider several things. While most people believe carbureted fuel systems are far more inexpensive than fuel injection systems, this is not always the case, especially if you're starting with an EFI car.
The price of intake manifolds is reasonable compared to EFI units, but throw in quality fuel pumps and carburetors and you can easily tack on another $400-500. A braided stainless steel fuel line is not cheap at $8-$10 per foot (depending on size), and you can sink a lot of money really quickly into aluminum fittings.
For a real-world experience in switching from EFI to carburetion, we spoke with Al Papitto of Boss 330 Racing, who recently switched to carburetion on his high-rpm modular engine. Papitto uses a Barry Grant Race Demon carburetor and Kris Starnes-modified Sullivan intake manifold, along with an Electromotive crank trigger ignition. "I was at the racetrack and saw these Comp Eliminator guys with carburetors who were running fast and never had any problems," he says. "Over the past couple of years, I've hurt the motor with EFI-related issues and grew tired of it."
Papitto had noticed uneven burn patterns in the combustion chamber that were indicative of mixture problems, and had always believed there was 40-50 hp in the engine if he could straighten that out. Three intake manifolds and various injector-targeting strategies returned no improvement, but switching to a carburetor did just what he was looking for, that being better mixtures and the 50 hp he estimated was there for the taking.
Papitto was quick to point out, however, that with any sort of power adder, EFI is the way to go. His 5.4-liter DOHC modular engine is normally aspirated and currently generates nearly 600 hp to the tires. Additionally, in a well-prepared drag race engine, the useable operating range is a window of about 1,500-2,000 rpm, an area in which a carburetor can, in many cases, equal or surpass EFI in performance.
EFI has its proponents, too, with Turbo People's Job Spetter Jr. making the statement that there is no reason to run carburetion any more. Spetter's wizardry with Accel DFI and FAST electronic engine management systems has propelled numerous EFI cars to victories in a number of drag racing categories.
Corroborating Spetter's statement are Street Warrior champ Darrell Peterson and Street Outlaw competitor Cale Aronson, both of whom run fuel injection on their normally aspirated racing powerplants with great results.
The long and short of it all is that an EFI-to-carburetion switch is not a decision that should be taken lightly. Do your research and have a definitive plan for your project's performance.