Think of an engine as a set of lungs and the throttle body as a mouth. In theory, a larger throttle body will allow more air to be sucked in by the engine; just as a larger mouth would for a set of lungs. So how do you know what size is right for your Mustang?
With pre-2005 models, making the move to a 70mm throttle body is a great way to free up some ponies. V6’s may not reap the same rewards of making this upgrade as the engine, or lungs, don’t draw in the same amount of air as a V8 could in stock form. Newer more powerful V6s, post 2005, can benefit from a 70mm throttle body as they can draw in more air. In stock form, the V8s of the newer Mustangs can draw in even more air and are equipped with twin throttle bodies from the factory.
With the GT as a standard, twin 55mm throttle bodies sit on top. For a stock application, one may want to consider moving up to the twin 62mm set up if they’re after some additional power. Keep in mind, modifications to the engine and its tune can change everything. A more powerful engine can draw in more air and at that point, things change and even larger throttle bodies can be considered regardless of the factory displacement.
Moving away from the 4.6L engine, if you are driving a naturally aspirated 5.0L with some mods, moving up to a 90mm throttle body would be a solid choice for improved power and performance. A 90mm throttle body is a great step up from stock, giving both naturally aspirated and forced induction motors a healthy dose of air, helping to give you a strong power band.