For decades its been known that if you want low end torque and street or offroad performance you want to run a dual plane intake manifold with your carburetor for better carb signal, and with a combination of long and shorter runners, a wider flatter power curve. However, with the advent of modern electronic fuel injection this has to be reexamined.
Carburetors work using the venturi effect that pulls in fuel based on the airflow past the venturi. So if one venturi has more airflow than the others that one will pull more fuel in with it. Most performance EFI uses a speed density approach and injects fuel from a fuel table based on engine RPM and load, which it calculates from manifold absolute pressure (MAP), throttle position, air temperature, etc. This means each injector injects the same amount of fuel (unless you have a powerful sequential multiport system with individual cylinder tuning that you take the time to tune each cylinder).
What happens in reality, especially with throttle body style fuel injection, is that if there are different runner lengths and different plenum volumes for a group of cylinders, those cylinders will get the same fuel even though they likely need something different.
Take for example a common V8 with the very popular(for good reason) dual plane RPM “air gap” style manifold. This was being used with a performance self learning 4150 throttle body fuel injection with high pressure fuel injectors. This manifold also has the opening in the divider directly under the throttle body to help equalize the plenum volume for each group of cylinders.
However, when you look at the spark plugs(about 6,000 miles of use, ignore the copper anti-seize on the threads), the difference becomes obvious. First they are all rich, this was being done to attempt to cover up the low speed just above idle hesitation at low throttle (~5%) only. Furthermore, there is a definite difference between the upper and lower planes of the dual plane intake. The upper plane plugs on the intake are marked with a sharpie…2,3,5, and 8. They are definitely richer. The lower plane plugs are what’s left…1,4,6, and 7, those are closer to optimal but still rich. This shows the cylinders were not getting even fuel distribution.
A single plane intake manifold was installed, and the engine ran smoother at low RPM and the off idle hesitation at low throttle was gone. Fuel use at idle previously was 9 lb/hr and idle vacuum was 12~13 inHg at 850 RPM with the IAC open to 27 counts. Previously it would not idle consistent at 800 RPM and was even worse if ran leaner. After the intake swap to single plane the fuel was only using 6 lb/hr and vacuum was 15 inHg with the IAC only open to 5 counts (butterflies were not moved for comparison purposes). So the engine was using less fuel and air to idle, if you moved the idle RPM to 850 the IAC was still at only 15 counts. This means it was idling more efficient, likely due to the more even fuel distribution, even though the dual plane intake should have a stronger low end.
The engine ran even better at lower RPM (even though it was running relatively well before that), and top end(above 3k) was stronger of course. Overall the engine was happier with the single plan intake over the dual plane intake with the performance throttle body fuel injection. This may not always be the case because there are many factors that affect whether the cylinders are using more or less fuel, especially cam specs as more valve overlap can exacerbate the problem by confusing the EFI with mixing intake air in with the exhaust at lower RPM and tricking the 02 into reading lean, causing it to richen the mixture.
In addition, a factory EFI engine with a dual plane style intake manifold doesn’t seem to exist. Even the tried and true GM TBI 5.7L of the late 80s/early 90s had a single plane intake manifold despite the fact that it was a low RPM truck engine. If you are wondering about your EFI with a dual plane setup, remove your spark plugs and have a look, more than likely the fuel distribution is not even. Whether or not engine performance or driveability is effected enough for you to want to swap to a single plane is up to you. Many times the improvement in fuel distribution and the resulting smoothness will overcome the potential bottom end torque loss.