The Mass-Flo throttle body...
The Mass-Flo throttle body fits nicely on the Edelbrock intake manifold and packages.
It's been a long time coming, but we finally are where we were supposed to be months ago with this series of articles. To refresh your memory, this series started out as a comparison of a modern EFI system stacked against an age-old Q-Jet on a chassis dyno and on the street. But that was before the SD-455 developed a rod knock on Ida Automotive's chassis dyno.
The Firebird owner, HPP contributor Melvin Benzaquen, seized the opportunity to work with RaceKrafter's Automotive Machine to upgrade the SD while it was being rebuilt. Cylinder-head work, along with a new crankshaft, connecting rods, and pistons, were on the list and accompanied the installation of a Comp Cams hydraulic-roller bumpstick.
Our camshaft choice, though light-years ahead of anything Pontiac offered in 1974, did throw a snafu into the project. The aftermarket valley pan that was required to clear the taller roller-valve lifters wouldn't accept the production-Pontiac intake manifold.
As a result, since the modified-for-EFI Edelbrock Torker II intake that was part of the Mass-Flo EFI kit fit with no problem, it was used for testing. This issue turned into an asset since it allowed a more modern carburetor to perform the A-B testing. Since HPP was pitting a modern EFI system against a carburetor, it was only fitting to use a state-of-the-art fuel-mixing device as well.
To this cause, Barry Grant stepped up with a 750-cfm Speed Demon mechanical secondary carburetor. In many ways, this would be a more accurate comparison since the Speed Demon would bolt right in place on the Edelbrock manifold, and thus, both fuel system theories would employ the same intake and eliminate any variables in flow or tuning characteristics. Finally, a real-world EFI-versus-carburetor test on a Pontiac engine!
Different Methods to Accomplish the Same Thing
Barry Grant supplied a 750-cfm...
Barry Grant supplied a 750-cfm Speed Demon with mechanical secondary operation for our test. Note the fuel rails and injectors are in place to block the holes in the modified-for-EFI Edelbrock intake.
Contrary to what many believe, an engine has no idea of how it's being fueled. The gasoline, along with the combustion event, can't tell if there's an EFI system or carburetor attached to the intake manifold. What does impact the engine is what happens to the charge-defined as the air/fuel mixture-on its way to the intake valve.
Carburetor: A carburetor is considered a wet-flow fuel-metering device. The gasoline is discharged through a booster venturi where it's atomized and emulsified (broken down into small particles and mixed with air, respectively). From there, the mixture undergoes a phase change from a liquid (albeit in small particles) to a gas. (not be confused with gasoline-a gas is defined as a liquid in its most rarefied form). The phase change requires heat and is called vaporization. For this to occur efficiently, it's important for the fuel to be atomized as much as possible and for the emulsification process to be thorough.
EFI: In contrast, EFI uses an injector to atomize the gasoline. This is accomplished through high pressure (in relation to a carburetor) that forces the fuel through either a small hole or series of holes in the injector tip. The fuel is then mixed with the air already in the manifold runner, and the vaporization event takes places. An EFI system is considered a dry-flow design since only air travels through the majority of the length of the intake-manifold runner.
What a carburetor does mechanically requires much more complexity to achieve alongside fuel injection. Add to that the reality that an engine does not breathe in a linear fashion-more rpm does not always mean higher cylinder-fill rates. A carburetor handles this by nature of its design. It provides fuel flow in lock-step with air throughput. EFI needs to monitor the airflow of the engine to then calculate the amount of fuel to create a desired air/fuel ratio. For practical purposes, there are only two methodologies that are accurate: speed density and mass airflow.
The Demon line of carburetors...
The Demon line of carburetors flows slightly more air than its competitors (the 750 actually flows around 875 cfm), so it was close in size to the amount of air the original Q-Jet would move.
In a true speed-density system, the EFI controller uses an engine-vacuum sensor (MAP) along with a complex algorithm to model the amount of cylinder fill the engine is incurring. This concept can be made to work accurately at the OE level since all of the variables can be quantified in the test laboratory.
The airflow (MAF) design measures the incoming air mass and employs a transfer-table-logic internal to the ECM that decides the amount of fuel the engine requires. It then manipulates the fuel-injector opening time to deliver that amount. MAF systems are much more forgiving to typical enthusiast engine modifications since they measure the engine's ability to breathe and care little about what happens afterwards. The Achilles heel of most aftermarket EFI systems is that they don't use the more accurate and forgiving MAF sensor like the Mass-Flo system does.