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Fox Body Catalytic Converters - Aftermarket Fundamentals

Written By: Connor MC

From street use to track-only Mustangs, catalytic converters have a role to play and affect your exhaust setup depending on your goals.

American Muscle

Facts about Fox Body Catalytic Converters 

First off, the term ‘catalyst’, in the scientific world, is used to describe a process where a certain chemical reaction is exposed to a third-party reagent to speed up the reaction process, WITHOUT the catalyst getting involved. Here’s an easier way to think of this. Imagine Tim is at work, and his job is to unload incoming trucks. On this day, Tim’s boss is watching him. In the presence of his boss, Tim works faster, getting more work done, despite the fact that the boss has not said anything or is actually directly helping him. Right… back to cars.

  • Catalytic converters change harmful engine emissions into less harmful emissions before shooting them out the tail pipe
  • With older muscle cars, it is common to remove or replace the stock catalytic converters to increase horsepower
  • High-flow catalytic converters reduce harmful emissions while decreasing resistance and increasing lost horsepower

What do Fox Body catalytic converters do?

Aftermarket Fox Body Catalytic Converters Installed

The purpose of a catalytic converter is to change harmful engine emissions into less harmful emissions before shooting them out the tail pipe. After combustion, the main remaining products are carbon dioxide, nitrogen, water (vapour), carbon monoxide, hydrocarbon compounds and nitrogen oxide(s). The first 3 – nitrogen, carbon dioxide and water vapour – are harmless (well, we are aware of the potential environmental effects of CO2). It is the last 3 products that pose health risks. Thus, it is the job of the catalytic converts to take carbon monoxide, hydrocarbon compounds and nitrogen oxide in, and turn them into something else. After passing through a properly functioning catalytic converter, carbon monoxide and hydrocarbons will be turned into carbon dioxide, and the nitrogen oxides (NO and NO2) will ripped of their nitrogen atoms and released as nitrogen gas. So, if we were to analyze what is coming out of the tail pipes, all we should mainly see is carbon dioxide, water vapour and nitrogen gas.

Just to clarify (and without going to deep into the chemistry of it), don’t think that any elements mysteriously disappear inside the catalytic converter. Rather, the elements are just rearranged (by interacting with specific chemicals contained in the converter) into less damaging by-products (carbon monoxide becomes carbon dioxide by attaching another oxygen atom).

Catalytic Converters and Your Fox Body Mustang

All Foxbody Mustangs came equipped with catalytic converters. What type and how many converters? Well that depends on the year. 1979-85 Foxes were equipped with a single conventional-type catalytic converter. This conventional type dealt solely with hydrocarbons and carbon monoxide in the exhaust stream, reducing them to acceptable levels. The last generation of Fox Mustangs received a serious upgrade in emissions equipment. 1986-93 V8 Mustangs all came with a grand total of 4 catalytic converters in the stock H-pipe– 2 three-way converters (TWCs) followed by 2 conventional converters, located downstream of the TWCs. The TWCs regulate the big 3 (hydrocarbons, carbon monoxide and nitrogen oxides) by running the exhaust gas over a ceramic honeycomb coated with a platinum/rhodium catalyst. The conventional converters utilize a honeycomb consisting of palladium/platinium as the catalyst to reduce hydrocarbons and carbon monoxide emissions. The functions of the conventional and three-way converters essentially overlap, however TWCs also work scrub clean nitrogen oxides and also operate at lower temperatures.

Why do people remove their Mustang’s catalytic converters?

High Flow Catalytic Converters

Quite often in the world of performance cars, the topic of catalytic converters comes up. With older muscle cars (like the beloved Foxbody), a common occurrence amongst owners seems to be to run the car without any cats. Why do people do this?

Well, it is a matter of horsepower. The engine makes power by burning fuel and air, but it also requires some power to push the exhaust out. That is a key phrase right there – the engine has to push the exhaust out. Whatever power the engine is making, unfortunately, some is used simultaneously to push the exhaust out of the engine, into the headers, through the cats and mufflers and ultimately into the atmosphere. In this scenario here, the catalytic converters are a source for resistance, and resistance robs power. So, by removing the cats, the overall resistance is lessened and should require less engine power to push the exhaust out. 

Now, interestingly enough, on stock cars, the factory ECU is actually calibrated to work with a certain ‘back pressure’ (the resistance of the exhaust system, measured in inches of water displaced, in H2O). On a stock 5.0 Fox, the factory backpressure is 2.6 in H2O for AOD equipped vehicles and 2.3 in H2O for 5-speed transmissions. Because the ECU requires some backpressure to operate properly, Ford does not recommend removing the cats or even the mufflers on a stock 5.0 Fox Body Mustang (swapping them out for better ones is entirely different, however).

Are high-flow, aftermarket catalytic converters worth the upgrade?

Ceramic High Flow catalytic Converters

Having discussed factory stock catalytic converts, let us look at the opposite end of the spectrum – high-flow catalytic converters. The job of a high-flow cat is no different from a regular cat – they are there to reduce or eliminate harmful emissions flowing from your Mustang’s exhaust system. However, as previously mentioned, a catalytic converter in the exhaust stream presents an additional resistance the engine must push against, thus wasting some power in pushing the exhaust out. A high-flow cat is simply a catalytic converter that has been engineered to provide less resistance than a stock cat, and thereby regain lost rear wheel horsepower.

Should you choose to go with high-flow cats, than you will want to consult with your tuner and have the computer retuned. Running a high-flow cat will mess with your Mustang's O2 readers and adjustments will need to be made accordingly.

What are the pros and cons behind running catalytic converters on your Fox Body Mustang?

Let’s start with the pros. As stated numerous times above, the purpose of a catalytic converter is to take certain pollutants and turn them into less harmful emissions – emissions less harmful for both yourself and the environment. Secondly, in the process of emission reduction, they take care of what otherwise could be some very stinky and nauseating gas fumes. On the downside, they provide additional resistance, which means slightly less power hitting the pavement. 

Should I run catalytic converters on my Fox Body?

Overall, I think it is more beneficial to use cats, particularly for street cars. A very common complaint amongst owners with catless cars is that while driving the interior stinks of exhaust fumes, which can cause nausea and if not properly ventilated, carbon monoxide can build up inside the cabin. With ever stringent emissions standards and subsequently, emissions testing, it seems to be that high-flow cats are the best way to go. They will put some power back on the table, but won’t compromise overall driveability. Plus, the environment will thank you too.

Fitment includes: 1979, 1980, 1981, 1982, 1983, 1984, 1985, 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993, GT, Cobra, LX, SVO