Fox Body Throttle Bodies - Facts & Fundamentals

Upgrading your 1979-1993 Foxbody Mustang’s throttle body isn’t nearly as exciting as slapping on a super charger, but it is something that will need to be done to increase airflow and support horsepower down the road. That last little part right there, “support horsepower down the road,” is very important. A throttle body (TB), on its own, will not increase engine power. Swapping a larger, aftermarket TB Rather, it will support horsepower by being able to flow the necessary (and increased) amount of air into a better flowing intake. The idea at play is to flow as much air as possible, as fast as possible, into the intake. The stock unit, a puny 58 mm, simply isn’t up to the task with anything other than stock parts.

When picking an aftermarket throttle body for a Fox Body, you have to be careful. Biggest is not always best. Too large a throttle body and the air will not flow fast enough, resulting in poor throttle response. Too small of a throttle body leaves power on the table. Instead, you want to create a funnel effect throughout the intake track. Air flow starts at the air filter, through the mass air meter (if equipped), throttle body, throttle body/EGR spacer, into the intake manifold, down into the cylinder head… so on and so forth. The funnel idea starts with as large an air intake as possible, and progressively diminishes in diameter so as to increase air velocity. An example of this would be a 76 mm mass air meter flowing into a 70 mm throttle body.

Stock 60 mm throttle body. You can see the ‘funnel’ concept even incorporated into the stock piece. Note how it gradually diminishes in diameter.

There are many different manufacturers and sizes of Mustang throttle bodies. Thankfully, despite the massive array of options, there are a key few that stand out. First and foremost, there are 3 very common throttle body sizes within the aftermarket Foxbody world. 65 mm, 70 mm and 75 mm. Which do you pick? Think back to the funnel concept. 65-70 mm work great on stock displacement motors (5.0 L/302 cu), whereas a 75 mm is better suited to a larger displacement or forced induction motor. Generally speaking, a throttle body larger than 75 mm, although available, will hurt power on most applications due to low air speed (again, consider the funnel effect).

EGR Spacer

One other aspect that must be considered when purchasing a new throttle body is the EGR spacer. The stock setup utilizes a 58 mm throttle body, connected to a 58 mm spacer, connected to the intake manifold. The spacer provides the ports and hook ups for the EGR (smog and emissions related stuff) equipment. If you are still running the EGR equipment, you will need an aftermarket spacer sized to match your new throttle body. Using, say, a 70 mm throttle body with the stock 58 mm spacer (provided they even mate properly) totally defeats the purpose. If you don’t run the smog stuff, you just need a plain old spacer (sometimes called ‘EGR delete spacer’). The only exception to this spacer talk is if you are using a Cobra intake manifold. The Cobra intake manifold has provisions for the EGR already incorporated.