ITBs with a charged engine

Question

Are there any benefits of running independent throttle bodies with a super/turbocharged engine, as opposed to running a single throttle body on the same engine?

If so, what are the benefits (and how extreme are they)?

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Static characteristics of engine

Hypothetically, the engine is as follows:

• 2000 cc
• 4 cylinder inline
• DOHC 16v
• Single turbocharger

The turbocharger is, hypothetically, as follows:

• Inducer diameter of 80 mm
• Exducer diameter of 120 mm
• Wastegate at 8 psi

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Variable characteristics of engine

The single throttle body will be a manifold going from four 20mm chambers to a single 40mm chamber in a 4-1 fashion, with a butterfly valve on the 40mm chamber to give throttle control.

The independent throttle bodies will be 20mm chambers each with a butterfly valve inside to give throttle control.

Answer 1

Yes there is a large benefit of response and efficiency and if not for the cost would likely be seen in more factory equipped vehicles other then the high performance production models (like BMW M3).

The combination of independent throttle bodies, plenum and turbo compared to placement of the single throttle body has a direct affect on ready air flow. A single throttle body is located prior to the plenum resulting in a brief pause for airflow rushing in on opening.
Independent throttle bodies have a no wait state on opening. The turbo with its plenum, which if properly designed and matched to the throttle bodies, will maintain a full air state in the plenum whether the throttle is open or closed. This pre filled state will result in a no wait rush of air on throttle opening with a very crisp response.
The difficulties is in matching, design and machining. The machining of the throttle bodies to fit the application. The design of the plenum to allow proper airflow of equal pressure and volume to each throttle body and of course tuning ECU control.
I will clarify a bit independent throttle bodies can suffer in mid performance if not designed with long intake tubes or a proper plenum to keep pressure stable, while they gain in quick and rapid response over the single throttle body the addition of a matched plenum and turbo to keep stable pressure will provide maximum air across full range of operation. The only other solution close to ITBs is to keep the pressure up with the single throttle body by moving it to the input side of the turbo. This has issues of it own on higher performance engines.

Answer 2

At high RPM and wide open throttle, there is no significant differences between either one. Here a single throttle body might have a slight advantage since you have less turbulence in the intake runners caused from throttle plates and stuff.

The biggest, most noticeable advantage you will feel from using ITBs is when you are driving on low throttle or idle with something like a max effort high RPM camshaft that has tons of overlap. The ITB throttle plate will help prevent the intake charge from being contaminated with exhaust, making the engine run smoother, and the race cams will sound less like its going to die at idle.

For a single throttle body and a crazy cam, the intake plenum will be under vacuum under low throttle openings. Then when your intake valves open, the exhaust will sometimes flow backwards into your intake plenum, causing the engine to run rough from messing with the air fuel mix.

With ITBs and a crazy cam, even at low throttle openings your forced induction intake plenum will be at a higher pressure than the exhaust, then no exhaust will flow out of the throttled intake runner to contaminate the intake charge of another cylinder.

So basically ITBs are only really needed if you need to drive around on the street with a camshaft spec that is not normally streetable.

There are other minor stuff you can do with ITBs like using it to tune the direction of air flow into the cylinders, and improve throttle response like already mentioned, but the race cam low throttle performance improvement is going to be the biggest advantage. And the more extreme your camshaft, the bigger the advantage.