What's the advantage of a twin throttle body versus single? Obviously there's increased flow, but what exactly does that effect?

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2003 Ford Mustang SVT Cobra
4.6L DOHC V8 - 6 speed manual
520rwhp - 525rwtq at 16psi on stock blower
2002 Ford Mustang GT
4.6L V8 - 5 speed manual

You get a faster throttle response.

more cubic feet per minute and increased throttle response which means more horsepower, torque, better fuel mileage. Also, the chrome one looks cool

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Keep feeding me your denial and hate, from that I will create

More throttle response is about all you will get. Maybe a couple hp. Depends on what size tb you get.

sniper.. more throttle response does not mean hp and torque, and especially not more mpg.

yeah, it only helps to add horsepower if your engine is properly tuned.

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2003 Ford Mustang SVT Cobra
4.6L DOHC V8 - 6 speed manual
520rwhp - 525rwtq at 16psi on stock blower
2002 Ford Mustang GT
4.6L V8 - 5 speed manual

or if your TB is too small for the engine or you have a highly modded engine with a stock TB

yeah exactly.

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2003 Ford Mustang SVT Cobra
4.6L DOHC V8 - 6 speed manual
520rwhp - 525rwtq at 16psi on stock blower
2002 Ford Mustang GT
4.6L V8 - 5 speed manual

I know in some applications it is really useful to have twin throttle bodies to limit the amount of intake tubing around the engine bay. For example the Nissan 300zx TT has a twin throttle body setup on either side. This allows them to route the boosted air to smaller side mount intercoolers on either side of the engine and then have the air come to throttle bodies on the same side of the engine as the intercooler. That way you don't have intake tubing wrapped around the engine gathering more heat, more space in the engine bay to work with for fans and the rad, and in the case of this application the removal of the need for a large front mount intercooler. It doesn't really give you much more of a power boost, it's more convenience, especially on a V-platform engine.

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2001 Chevrolet Monte Carlo SuperSport

there is more air going into the engine which increases power a little bit

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Keep feeding me your denial and hate, from that I will create

You will have better air flow but you have to so much fuel in the engine

There is really no advantage to a twin throttle body, no disadvantage either. Depending on how the system was designed, one throttle body can do the job of two. The example of the 300z is perfect, but the car has 2 turbos. By running the system they have, it allows for efficiency, not meaning maximum power. 80's Ford trucks used a system that had two tubes running next to each other with two valves, almost like having two throttle bodies, but then after that the air mixed together. Why not just design the one to be large enough to equal the volume of the 2?
Your engine will only be able to pull in air to it's capacity, unless there is forced induction. Allowing the engine to breath easier, will make it to where your engine will work more efficient. Less work to produce the power. Does not mean that your engine will make more power with twin throttle bodies. Some engines will take and split the air intake into 2 sides for each bank. Some will have the 2 throttle bodies to do so. But in the end, why have all that extra parts when one can do the job as well as 2 could?