When it comes to the debate between carburetors and electronic fuel injection, you will never be able to declare one a definitive winner. At best, you can say, under a given set of circumstances one will make more power, or one will require more or less adjustment than the other. Because the fact is, for every test showing one will make more power, there’s another that will show the opposite result.
To that end, Keith Dorton of Automotive Specialists had a very unique EFI system come through the shop and wanted to know if that particular system would make more or less horsepower than a traditional four-barrel Holley carburetor. It just so happens, that Dorton had just built an identical engine with a four-barrel that was slated to run on the dyno, making for a solid apples-to-apples comparison.
Both engines are identical 400 cubic-inch small-block Chevrolet engines spec’d out as Dorton’s pump gas street engines. The engine package starts out with a cast-iron Dart SHP block, with four-bolt mains, to serve as a bulletproof foundation on which to build the rest of the engine.
From there, a Molnar 3.75-inch-stroke forged crankshaft, made from 4340 steel is bolted into the mains with coated ACL main bearings. Then a set of 6.00-inch Molnar H-beam rods, machined from billet 4340 steel, with ARP2000 rod bolts are affixed to the crank, again with coated ACL bearings.
From there, a set of 4.125-inch MAHLE pistons with a 1.2mm ring package are added. The slight dish in the crown of the pistons makes for a pump-gas friendly 9.7:1 compression ratio with the 65cc combustion chamber volume of the AFR Eliminator cylinder heads. With fully CNC-machined ports and combustion chambers, a 2.05-inch intake, and 1.60-inch exhaust valve to complement the 195cc intake runner.
The hydraulic-roller valvetrain in the two engines was also identical. Starting with a Crane Cams billet-steel camshaft with 236 degrees of duration on the intake and 244 degrees on the exhaust — both at .050-inch of tappet lift — Dorton then added a set of Crane tie-bar lifters. Moving up to the top end, a set of Crane aluminum 1.5:1 roller rockers makes for .548 inches of lift at the intake valve, and .558 inches at the exhaust valve.
The Carbureted Combination
Topping off the carbureted is an Edelbrock Victor Jr single-plane intake manifold and a Holley 750cfm Brawler carburetor (to start, anyway). Ignition is handled by an MSD Pro-Billet distributor and capacitive discharge box. After the initial dyno runs, which amounted to 536.7 horsepower and 516.3 lb-ft of torque, it was clear to Dorton and the team that the engine needed more carburetor.
In order to provide the additional airflow, a Holley 830cfm double-pumper was swapped on, and the dyno fired up once again. This time, the results were more in line with what Dorton expected of a 400-cube small-block as the computer showed a significant gain, with final numbers for the carbureted combination being 557.2 horsepower and 531 lb-ft of torque.
Getting Creative with Fuel Injection
This fuel-injection system is about as far from “typical” as you can get. It is a Borla individual throttle body system, but not the traditional small-block Chevrolet setup. Rather, this setup is one that uses four two-barrel throttle bodies. Designed by Johnson’s Hot Rod Shop in Gadsden, Alabama, the kit uses an obscure 4×2 intake manifold and four Borla Induction 3004-series throttle bodies which are a direct replacement for Weber D, C, and F carburetors.
Each throttle body has twin 44mm throttle bores and a single 36 lb/hr injector per cylinder. This is significantly smaller in bore diameter than the traditional Borla small-block Chevy-specific kit’s 50-58mm bores, meaning right out of the gate, this EFI setup will be at a disadvantage when it comes to the peak power comparison. Additionally, the intake manifold used isn’t readily available, as it was supplied by the customer and was originally found at a swap meet.
“I’ve never seen a manifold like this,” says Keith Dorton. “It’s a 4 x 2-barrel setup, with no names on it. It came from a swap meet, years ago, so we cleaned it up and made a few changes. We used the underneath of it as a balance chamber. We’ve got holes drilled in each runner, so it still thinks it has a common plenum. That way our MAP sensor is getting a good reading.”
Besides the obvious (different intake manifold and induction), the main difference between the two otherwise-identical engines is the ignition. Dorton switched out the Pro-Billet distributor for an MSD crank trigger sandwiched between the ATI Super Damper and the crank pulley. There’s also an MSD cam sync in the distributor hole, to monitor the position of the camshaft.
To control the EFI. Dorton used all F.A.S.T. electronics. The self-tuning XFI engine control unit is supplemented by the XIM ignition module and an individual-coil ignition setup. After a quick engine break-in on the dyno and a few check runs to make sure the fuel maps were correct, a couple of power runs were made.
Right off the bat, the results were disappointing, with the graphs only showing 475 horsepower and 505.9 lb-ft of torque due to a lack of airflow. However, Dorton wasn’t willing to just accept those numbers and call it a day. Each throttle body has a protective screen underneath each pair of air horns to prevent debris from entering the engine. Made from .020-inch wire, the screens posed a significant intake restriction and were removed before making more pulls.
With the screen removed, the numbers were much better, with the engine now producing 512.4 horsepower and 516.0 lb-ft – a solid 36.5 horsepower and 10 ft-lb increase over the previous runs. While the peak numbers are below those of the carbureted engine, there’s more to the story than peak numbers here.
“Surprisingly, [the EFI setup] made more bottom-end with the same components. The throttle bodies outperformed the four-barrel up to 4,550rpm. After that, the increased airflow capacity lets the carb walk away. It made about 20 more horsepower at 3,500rpm than the four-barrel, but was substantially down at 6,000,” says Dorton.
“We have to realize that this is going into a street car, taking off from a stoplight. It needs to make power from idle on, so we worked hard to make this thing really good and drivable with off-idle acceleration. I was kinda surprised by how well these four throttle bodies did compared to the four-barrel. This is going to make such a good driving combination, which is surprising given the short runners.”
While this test gives the peak power win to the carburetor, the EFI combination did post higher numbers under certain conditions, and as Dorton points out, this 4×2 EFI setup certainly wins when it comes to style points. So this isn’t a definitive blow for either side of the argument, it certainly does create some unique data points.