Steve Baur
Former Editor, Modified Mustangs & Fords
September 22, 2006

Flowmaster's Mufflers arguably offer the definitive sound for the 5-liter Mustang, and for good reason. From the very outset of 5.0 frenzy, Flowmaster was one of the first companies to step up with a low-restriction muffler that offered a significant power improvement for the Fox-body Stangs.

The muffler's lack of traditional sound-deadening material or "packing" provided a unique metallic sound, and when combined with an off-road or catalytic converter-less H-pipe, it resulted in a deep, thunderous rumble that told people quite some distance away that there was a mean and nasty Mustang in the vicinity.

That was some time ago, yet Flowmaster remains one of the top players in the Ford performance-exhaust-system market. Recently, the company released its new 3-inch system for '87-'93 5.0 Mustangs, and we thought we should try it out. Listed as PN 17396, the system consists of a 3-inch crossover pipe that utilizes Flowmaster's Scavenging D-Port crossover, and bolts right up to aftermarket short-style headers or stock exhaust manifolds.

Super 40 Series two-chamber mufflers were constructed with Flowmaster's Gen II Delta Flow technology. The Delta Flow concept increases exhaust scavenging and prevents reversion into the engine. Three-inch-diameter tailpipes conclude the system components, and all of the tubing is 16-gauge, aluminized steel that is mandrel bent for the best flow characteristics. Optional polished stainless steel tailpipes are available for LX owners.

The science behind the Scavenging D-Port system is based on the principle that engines have exhaust pulses that travel down the exhaust pipes. Opening and closing of valves generate pulses of gas, heat, and sound frequencies, and the D-shaped ports provide a larger surface area for the exhaust pulses to scavenge or pull each other along as they travel the length of the collector. This provides a stronger, faster signal, resulting in an increased torque curve throughout the rpm range. Obviously a 3-inch-diameter exhaust system is not for everyone. If you have a 12-second Mustang with aftermarket heads, cam, and intake, you'll probably be better off with the standard 2.5-inch exhaust system.

These days, though, it's not uncommon for Mustangs to make 400, 500, or even 600 hp and be driven quite regularly. At these power levels, engines are moving a huge volume of air and fuel, so it may be time to step up.

To test the results of installing the 3-inch system, we chose a car that already employed Flowmaster's 2.5-inch off-road exhaust system and two-chamber 40 Series mufflers. The '89 Mustang featured a stock 302 short-block, Edelbrock Performer heads and intake manifold, and a Ford Racing E303 camshaft. Additionally, a Vortech V-1 S-Trim supercharger was huffing 12 psi of boost.We dropped in on the crew at RevXtreme in Tampa, Florida, for some dyno pulls on their Dynojet. After two runs of 389 and 391 hp, we arrived at an average of 390 hp to the tires. Torque averaged in at 387 lb-ft.

0610mmfp_01z Ford_mustang_fox_body Front_view
Are your pipes big enough? If not, Flowmaster has what you need with its 3-inch header-back system for 5-liter Fox-body Mustangs.
0610mmfp_02z Funneling_the_fumes Tailpipes
Retail prices for the 3-inch system range from $700 to $800 depending on what tailpipes you order, and the system includes a mid-pipe with D-port crossover, a pair of Super 40 mufflers, two-piece tailpipes, and all the clamps to put it together.
0610mmfp_03z Funneling_the_fumes Aluminized_tubing
The PN 17396 kit uses 3-inch, 16-gauge aluminized tubing that is mandrel bent for the best flow characteristics.
0610mmfp_04z Funneling_the_fumes Crossover_pipes
Flowmaster's Scavenger D-Port crossover pipe allows the exhaust pulses to merge to create a pulling or scavenging effect, thus increasing performance. The D-Port design also maintains a deep musclecar tone unlike other x type crossover pipes. Here we compare the 3-inch D-port to the 211/42-inch H-pipe setup.
0610mmfp_05z Funneling_the_fumes Mufflers
Using Flowmaster's Gen II Delta Flow technology, they can move the needed amount of air while maintaining a low interior resonance.

With the Flowmaster 3-inch system in place, we saw the horsepower average go to 400 and the torque to 379 lb-ft. Despite its large-diameter components, the system isn't any louder-at least to our ears-and the interior resonance is greatly diminished.

With the Mustang making 400 hp at the tires, or around 460 at the flywheel given a 15 percent drivetrain loss, the 1 5/8-inch shorty headers we're using may be a restriction at this point. The Flowmaster 2 1/2-inch setup could be just that good at this power level as well. If that were the case, the 3-inch system would probably be better suited to an engine making closer to 500 rwhp. Had we started with 1 3/4-inch shorty headers from the outset, the final results would have been more conclusive.

Flowmaster already has a 3-inch after-cat system for '99-'01 Cobra Mustangs, while a 3-inch system for '94-up Mustangs is in the works should the public provide a need. As we mentioned before, it's easy to make power these days. Is your exhaust capable of funneling the fumes?