Exhausting Rehab, Upgrading Our Fox Exhaust With BBK and Flowmaster

If there is one modification that is likely at the top of any Mustang enthusiast’s list, it’s an exhaust system. This can include everything from the mufflers to installing a complete header-back system. There’s good reason for that; any car guy, whether a Ford fan or not, knows the sound of a Mustang with some healthy pipes rumbling down the street.

Since exhaust mods are so popular within the hobby, we decided this was a good place to start for Project Rehab, our 1988 Mustang Coupe. When we brought Project Rehab home we found that it was wearing a poorly installed H-pipe, an old pair of aftermarket mufflers, that were also badly installed, and some rusty tail pipes that looked to be the work of a backyard hack. Needless to say, we were tired of choking on exhaust fumes during test drives, and scraping the H-pipe over the gravel on the driveway.

We put a call in to BBK Performance and Flowmaster Mufflers for some help. Both sent us popular components and parts that should help Project Rehab make a little more power, and sound much healthier.

Left: Someone had literally clamped a pipe to the old muffler, which looked like it had been cut off another car. That pipe was then clamped to a flow-tube which then attached to the H-pipe. All were leaking. Center Left: A common rookie mistake on installing exhaust on Fox-body cars is missing the hanger on the cross member. As you can see they either didn't care or didn't know. Center Right: Rust, holes, and leaks, the exhaust that was on Project Rehab had all three. Right: We'll add this mess to the scrap pile, about the only parts that were reuseable are the stock manifolds.

Mustang fans are no stranger to Flowmaster either. We’ve heard Chevy guys who were notorious Ford-haters admit that nothing sounds better than a 5.0 with a set of Flowmasters. Alex Ortega of Flowmaster seemed to echo that same thought, “The 5.0 Mustang is largely responsible for Flowmaster’s street car market being where it is today,” says Ortega. It was that signature sound that moved Flowmaster from being primarily a manufacturer of racing mufflers to help racers and tracks meet the sound restrictions of ever encroaching urban sprawl, and catapulted them into the lucrative street market.

BBK Performance and Flowmaster sent us everything we need to get Rehab’s exhaust in top condition.

Longer Tubes

We recommend a good high-flow cat back exhaust system to help improve performance even more. -Tim Gilpin, BBK Performance

Beginning in 1986 Ford started equipping the 5.0 HO engine with stainless steel dual exhaust, including stainless tubular shorty headers. While this was an improvement from the heavier cast manifolds, the shorties were still small in diameter, and choked by the factory four-cat H-pipes, and restrictive factory mufflers.

Long tube headers have been a favorite bolt on modification since the early days of hot rodding. Adding more torque by providing each exhaust port with less restrictive flow, and often deepening the tone of the exhaust note.

The BBK long tube headers are CNC mandrel-bent, welded by both hand and robot, and feature a three-inch collector and 3/8-inch thick flange.

Since Rehab is a factory equipped AOD car, we ordered a set of BBK long tubes specifically for that application. Our headers are part number 1531, Tim Gilpin of BBK tells us our headers feature, “1-5/8-inch primary tubes, three-inch collectors for maximum horsepower & torque and 3/8-inch flanges for added strength and durability”. All BBK headers are mandrel bent, and both robotically and hand welded. Our headers have a chrome finish, but a polished silver ceramic version is also available – part number 15310.

The BBK X-pipe matches the wide automatic headers, it is 2 1/2-inches with a x-shaped cross over to maximize flow and performance.

We also ordered BBK’s matching short X-pipe which is necessary due to the wider design of the automatic specific headers. The X-pipe is part number 1811, and features 2 1/2-inch diameter pipes, with an X-shaped crossover in the center for better exhaust pulse equalization and flow than the H-pipe it replaces.

Further Back

“We recommend a good high-flow cat back exhaust system to help improve performance even more,” says Gilpin. This is where Flowmaster comes into the Project Rehab picture. We ordered a Flowmaster 2 1/2-inch Super 44 Series cat-back exhaust in 409 stainless steel, part number 817213.

Since the system is made from 409 stainless steel, it comes with a lifetime warranty from Flowmaster. When asked if the use of stainless over mild-steel changes the exhaust tone Ortega answered, “We get that question quite a bit, stainless steel does not change the performance features or the sound quality of the exhaust. You simply get the benefit of the longer warranty.”

The 5.0 Mustang is largely responsible for Flowmaster’s street car market being where it is today. -Alex Ortega, Flowmaster

Our Super 44 series Mufflers are a two-chamber design and feature the company’s Delta-Flow technology. According to Ortega the Delta-Flow in the Super 44 improves the muffler’s scavenging ability. “The V-shaped Delta Flow plates generate a low-pressure area behind them, almost creating a suction effect,” says Ortega. This scavenges exhaust pulses much more rapidly, improving exhaust flow and horsepower. The Delta flow plates also improve the sound canceling abilities of the mufflers, without sacrificing that traditional 5.0 rumble.

Our system came with the muffler’s flow tubes already installed, manufactured as part of the muffler. Also included were 2 1/2 inch mandrel-bent over axle pipes, with 304 polished stainless tips that are subtly laser etched with the Flowmaster name.

Our Flowmaster system features Super 44 mufflers, and 2.5 inch pipes. The entire system is stainless steel, and the tail pipes are laser etched with the Flowmaster logo.

The metal hanger bars, are already welded to the mufflers and tailpipes to slide into the factory locations. The output side of the mufflers has been expanded slightly, and a Z-shaped notch added to allow the included band-clamp to close over the muffler and tail pipe once assembled. This provide more uniform sealing than traditional horseshoe shaped clamps.

How Loud?

Prior to our installation we broke out the decibel meter and checked the car cruising, at idle, and at wide open throttle. We found 95 db cruising at 70 mph, 82 db at idle, and 96 db at wide open throttle. With our new system installed we now have 86 db at idle, 98 db cruising at 70 mph, and 100 db at wide open throttle. Not that dramatic of a difference in terms of the in-car volume.

The 2 1/2-inch diameter pipe is ideal for our project car. Ortega says that this size tubing is excellent for applications with power outputs ranging from 250-450 hp. “Above 450 hp you really need to consider three-inch exhaust, because 2 1/2-inch can start to choke the exhaust down,” says Ortega.


Years of rot, rust, and dirt, as well as bad installation methods had plagued our exhaust system. We started in the back and worked our way forward. We found the bolts for the tail pipes weren’t coming off, so we broke out the sawz-all and hacked them out. The muffler bolts took some persuading with an impact gun, but dropped right out, the same held true for the full-length H-pipe.

We had previously removed what was left of the factory smog/air pump tubing, it must be removed for this job. We then removed the factory exhaust manifolds, and the oil dipstick and tube, followed by the starter.

The driver’s side header comes in from the bottom. This requires removal of the steering shaft, which will run between two of the header tubes. Be sure to do the driver’s side header first. We found it easier to drop the k-member about six-inches, giving us plenty of room to install the new header.

Following BBK’s instructions our next required piece of work was to remove the steering shaft. This is accomplished by removing the pinch bolt from the rack side, and then the retaining bolt on the column side. We then loosened our power steering rack mounting bolts and slid the rack forward. This allowed the steering shaft to be removed.

BBK recommends installing the driver’s side header first, both sides require that the engine mounts be disconnected from the K-member and the engine jacked up with a hoist. We had an old and faulty set of instructions that showed installing the passenger’s side first, which we did, then subsequently struggled with the driver’s side. BBK is now correcting all their instruction sheets to ensure this doesn’t happen to anyone else. Ultimately we found it was easiest to drop the K-member approximately six-inches and install the driver’s side header from the bottom. Doing this eliminated the K-member interference problem we were having during install. Once in place, and with the K-member back in position we found a tight fit without any interference.

With the headers bolted in place we reinstalled our steering shaft, and proceeded underneath to the X-pipe. The X-pipe went on smoothly, sliding into the factory hangers on the transmission cross-member, something that had escaped the previous exhaust installer.

Left and Center Left: The passenger side header goes on second. The starter and smog tubes must be removed for adequate clearance and the engine jacked up with a hoist or a jack and block of wood under the oil pan. Center Right and Right: You can see the headers are a tight fit, but once secured we encountered no clearance problems. Now is a good time to install new oxygen sensors, make sure you install the passenger side sensor before bolting the header to the cylinder head.


Ortega told us to plan 90 minutes to two hours for our cat-back install. With wrenches in hand, we had the system in place in less than an hour. With our tail pipes checked for straightness, we fired the car up for the first time and listened to that sweet rumble. A quick check for leaks and we had the car on the road.

Top Left: The X-pipe fits to the ball and socket design headers, there are no gaskets to burn up and leak. Top Right: The Flowmaster cat-back system went on quick and easily, the z-shaped notches in the muffler outlets allow the tailpipes to easily slide in place and then be clamped using the hardware provided. Bottom Left: Since all of our rubber hangers were either destroyed or damaged we replaced them with new pieces. Bottom Right: The complete system installed on the car.

We headed back to Automotion in Louisville, Kentucky to strap Rehab back on the rollers and see if we’d made an impact. For years now we’ve seen and read a wide variety of numbers touting what an exhaust system can do for a car. We were anxious to find out what quality parts would do for Rehab.

On the dyno we were rewarded with 229.3 hp, and 308.7 ft-lbs of torque to the rear tires, a respectable gain of 8.1 hp and 12.2 ft-lbs of torque over our previous baseline of 221.2 hp and 296.5 ft-lbs on the same dyno, with similar weather conditions just two weeks prior. It’s interesting to note that the gains were steady and significant throughout the RPM range, meaning they didn’t just come on at peak, this equates to a useable improvement in power throughout the engine’s operating range that should make a real performance difference.

The solid lines represent our best dyno pull of 229 hp at 308 ft-lbs of torque. A gain of 8 hp and 12 ft-lbs. The dashed lines are our previous best run before this installation.

With new pipes installed it’s time to move on to the next part of Rehab. We’ll be back on the rollers and hopefully at the track soon enough, for now though we’ll be enjoying those sweet new pipes, and that classic rumble you can only get from a pushrod 5.0.


About the author

Don Creason

Don Creason is an automotive journalist with passions that lie from everything classic, all the way to modern muscle. Experienced tech writer, and all around car aficionado, Don's love for both cars and writing makes him the perfect addition to the Power Automedia team of experts.
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