Tom Wilson
February 1, 2001

Step By Step

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Flowmaster’s Force II System for ’86-’93 Mustang LXs represents the typical 5.0 after-cat idea perfectly. Its four basic pieces (two muffler and pipe combos plus two tailpipes) are the industry standard. With its new Delta-flow mufflers for reduced resonation at 2,000 rpm, the aluminized mild-steel mufflers, and polished stainless tailpipe tips, this Flowmaster kit combines traditional cost-containing technology with the latest sound-quality improvements.
A stock Mustang exhaust uses 21/4-inch-diameter tubing, which fits easily under the car but becomes restrictive to airflow as bolt-on power builds. Thus, almost all aftermarket systems use 21/2-inch-diameter tubing throughout, as seen in this collection of Flowmaster muffler and tailpipe ends. The larger tubing means more care is required to make the system fit around the rear axle, especially with aftermarket upper arms, Panhard rods, and so on.
Mustang tailpipes run up and over the rear axle. With many turns, this is an area where mandrel bends pay off. Mandrel bends are easy to spot. As shown here, the pipe does not neck down where the pipe makes a turn. Instead it remains at its standard diameter. Non-mandrel bends have a distinct choke-down in the bends. Interestingly, aftermarket tail-pipes don’t necessarily follow the same path as Ford tailpipes. So, guesstimating how they’ll fit is always more crystal ball than science.
Step one is cutting the tailpipes off the rear of the mufflers and fishing the tailpipes out of the chassis. A power reciprocating saw is ideal for this, as is a hoist. If working on jackstands, you’ll find you need a surprising amount of height for working room.
Next, unbolt the mufflers from the rear of the H-pipe. At the rear of each muffler you’ll find a hanger. Once the front of the muffler is unbolted, undo the rear hangers.
Once unbolted from the H-pipe and unhangered at the rear, the mufflers will fall out the bottom of the car.
Here’s one difference between the early Fox and the ’94-and-later cars. The Fox-body Mustangs use two 10mm bolts to hang the rear of the muffler. The ’94-and-later SN-95s use the heavy bent wire and a rubber isolator. This is why you’ll find different part numbers for early and late late-models. If you end up needing to change from one system to the other, you’ll have to scrounge the necessary hanger and bolts, or weld the wire hanger to the rear of the muffler.
There’s little mystery in getting the new mufflers in, although a helper or stand to hold the mufflers while you fiddle with the H-pipe-to-muffler hardware is mandatory. At this point, just snug up the muffler hardware, as you’ll need to adjust the mufflers in a minute.
Fishing in the new tailpipes is done from the rear. This is the moment of truth when you’ll find what hits where. Most of the time the new pipes go in without incident, but the more exotic your rear suspension, the more interesting tailpipe clearance can become. Once the tailpipes are in and mated to the mufflers, lightly snug the U-bolt clamps between the mufflers and tailpipes. Don’t tighten these clamps yet! Muffler clamps work by dimpling the two pipes together to form a gas-tight seal. At this point you will need to adjust the pipes, so just snug the clamps.
Our project car sported a Panhard rod, and this came close to the driver-side tailpipe. Kevin, the technician, fixed this situation the exact same way the factory does, by slightly dimpling the pipe. True, Ford dimples its pipes somewhat differently than this, but the results are the same. On a street car, a bit of hammer flattening doesn’t mean a thing power-wise, so while it looks barbaric, it’s the way to do the job.
Once satisfied the tailpipes were about where they needed to be, Kevin clamped his hanger pieces together with locking pliers. Then it was time to eyeball the entire installation and start adjusting the mufflers, tailpipes, and hangers until everything was straight and properly clearanced from the chassis.
Again, it looks straight from the Dark Ages, but a long pry bar is the tool when it comes to persuading a half-clamped exhaust system into the exact form you desire. This is another good reason to get the chassis way up in the air when doing this job on jackstands. Work to keep the mufflers up, not hanging down, and the tailpipes evenly spaced at the rear.
With the system positioned, the muffler-to-tailpipe clamps and muffler-to-H-pipe hardware are tightened. Kevin also whipped out the wire-fed welder on the tailpipe hangers to make them permanent. This job has to wait until the system is properly hung because once the welder has done its thing, adjustments turn into the cut, grind, and reweld variety.
Proper clearance can look funny from under the car while the suspension is at full droop. Here we see the minimal clearance Kevin fits between the upper control arms and the tailpipes. Because the car never sees full droop unless it’s on a hoist—or flying over a guardrail—this provides max arm-to-tailpipe clearance. If you leave clearance here while the car is on the hoist, the tailpipes will not hit the arms during normal suspension movement.
The larger Flowmaster mufflers just fit the chassis, as Kevin’s gloved finger shows. This is just enough clearance to keep the mufflers from thumping against the floorboard.
Kevin tack-welded the muffler-to-tailpipe connections to ensure the tailpipes would not rotate relative to the mufflers. This would cause clearance problems and leave the tailpipe tips looking lopsided.

Right after fitting a K&N air filter, the next thing Max Mustanger does is hang an aftermarket muffler and tailpipe kit under his ride. It’s all about sounding cool, of course, but as the power levels rise, there is some gain from an after-cat system—so why not?

What is an after-cat system, and what’s involved in getting one on your car? Late-model exhaust systems have three main parts: the headers, the H-pipe—which contains the catalytic converters and crossover pipe that forms the middle leg of the “H”—and finally the after-cat, which is itself built up from the two mufflers and the two tailpipes. The entire system is built from low-grade stainless steel, meaning it will last more than 100,000 miles before rusting out along a seam or in the lower front of the mufflers. So, if you’re waiting for the mufflers and tailpipes to wear out so you can justify buying new ones, you may have a long wait!

At the Dearborn assembly plant, Ford welds the tailpipes to the mufflers, then hangs the entire exhaust under the chassis before fitting the rear axle. This means that changing the after-cat requires either removing the rear axle (kinda dumb), or simply cutting the tailpipes off the mufflers. All aftermarket exhaust kits come with separate mufflers and tailpipes for easy installation.

Also easy are the legalities. Because the after-cat is downstream of the catalytic converters, the EPA and your state’s environmental police are unconcerned if you change the mufflers—at least from an emissions-control standpoint. Officer Bob may still want to discuss noise pollution on an individual basis if you give him a reason to, but sound-only traffic stops seem rare in this day of seismic-bass sound systems.

Fitting an after-cat is one of those jobs that can be done in the driveway, but it’s definitely easier in the pro’s installation stall. The old system comes off easily enough provided you have a few sharp blades for your saw—the stainless steel will rapidly dull low-quality saw blades. Installing the new system is also fairly easy, but you must be careful to get everything lined up and properly clearanced from the chassis, and make sure the tailpipes are evenly spaced and not twisted. You’ll really want a helper, and a few extra support stands will surely help. The bolt-on hardware of the quality aftermarket kits will do the job, but you’ll find the pros are loathe to give up tacking things together with a smoke wrench.

Our installation illustrated here is an archetypal Mustang install. We procured our complete Flowmaster Force II system (PN 17203) for ’86-’93 Mustang 5.0 LXs with polished stainless tailpipe tips from mail-order giant Brother’s Performance Warehouse ($309.99). Then we had it installed down the street at American Muscle Racing ($75—such a deal!), and then for our magazine purposes, had the before-and-after rigs dyno tested on Brother’s Dynojet.

Also typical with an aftermarket installation, not everything went according to the instruction sheet because there aren’t that many dead-stock Fox Mustangs around. Our test car was a ’93 Cobra, which had been using a stock ’94-and-later after-cat, which uses a slightly different attachment system for the mufflers, and the tailpipes hang from slightly longer hangers. No big deal, and it was all taken care of in minutes by cutting off the old hangers and welding them to the new system. But it is indicative of how installs tend to go, and how access to a bench vice, a power saw, and especially a welder can save the day. Follow along for the details.

Horse Sense: Ford uses two different-size mufflers on Mustangs to help cancel the V-8 resonance that gives the dreaded 2,000-rpm drone. Aftermarket systems use the same-size mufflers to contain costs. And while the modern name-brand gear keeps the drone to a minimum, watch out for bargain or race-only systems. You’ll soon be sticking foam in your ears on long trips otherwise.