Jim Smart
December 1, 2008

Flowmaster pioneered the art of exhaust acoustics. We've all experienced that Flowmaster phenomenon at one time or another, hearing the throaty "warble" of virtually any V-8 through dual mufflers and instantly recognizing them as Flowmasters. The "Flowmaster sound" is the epitome of American muscle car power.

Flowmaster's legacy goes back to 1983, when Sprint Car racers in California were faced with tough noise legislation. Flowmaster founder Ray Flugger developed a muffler that would quiet racing engines without compromising performance. At first, Ray's exhaust silencers were downright ugly. He even welded handles to them to make them easier to carry during testing. They became known as "suitcase mufflers," but their design was ultimately patented, the result of years of testing in very modest beginnings.

Those first Flowmaster mufflers came from a small barn in Northern California. Today, the company produces mufflers and exhaust systems from its state-of-the-art manufacturing facilities. In fact, Flowmaster operates the only research and design facility of its kind, using a SuperFlow SF-7100 engine dynamometer for testing. Ray Flugger remains active in the company, keeping a close eye on the legacy he created nearly three decades ago.

Flowmaster offers several choices for the Mustang's 45 years of production. That means there's a Flowmaster muffler designed specifically for your Mustang. Which type should you choose and why?

First, you need to choose either an entire exhaust system or a pair of Flowmaster mufflers that will fit your Mustang's existing pipes. Flowmaster does entire exhaust systems for all Mustangs except the '74-'78 Mustang II. This means complete systems for '65-'73 classics, '79-'04 Fox-bodies, and '05-'09 cars.

There are two basic Flowmaster American Muscle exhaust systems available with 2 ¼- or 2 ½-inch pipes for classic Mustangs. Two important points: They will not fit convertibles, nor are they compatible for the '65-'66 GT valance panel with trumpet tips. They employ late-model-style exhaust hangers. If originality doesn't matter, they offer an improvement over original equipment because they isolate noise and vibration better. If you must have original pipes and hangers, go with one of Virginia Classic Mustang's factory original-style exhaust systems and fit it with a pair of Flowmaster mufflers. We're going to show you both ways.

Quick {{{Mustang}}} Reference Guide
To Flowmaster Mufflers
Muffler TypePart No.Specifics
Series 40 Two-Chamber424432 ¼" in/out Offset Both Ends 19" Overall Length
Series 40 Two-Chamber424412 ¼" in/out Offset In/Center Out 19" Overall Length
Series 40 Two-Chamber425432 ½" in/out Offset Both Ends 19" Overall Length
Series 40 Two-Chamber425412 ½" in/out Offset In/Center Out
Series 40 Delta Flow9420412" in/out Offset In/Center Out 23" Overall Length
Series 40 Delta Flow9424412 ¼" in/out Offset In/Center Out 23" Overall Length
Series 40 Delta Flow9424432 ¼" in/out Offset Both Ends
Series 40 Delta Flow9425432 ½" in/out Offset Both Ends 23" Overall Length
Series 40 Delta Flow9425412 ½" in/out Offset In/Center Out 23" Overall Length
Series 40 Delta Flow 4098424412 ¼" in/out Offset In/Center Out 23" Overall Length
Series 40 Delta Flow 4098424432 ¼" in/out Offset Both Ends 23" Overall Length
Series 40 Delta Flow 4098425412 ½" in/out Offset In/Center Out 23" Overall Length
Super 44 Two-Chamber9424482 ¼" in/out Offset Both Ends 19" Overall Length
Super 44 Two-Chamber9425482 ½" in/out Offset Both Ends 19" Overall Length
Series 50 Delta Flow9420512" in/out Offset In/Center Out 23" Overall Length
Series 50 Delta Flow9424512 ¼" in/out Offset In/Center Out 23" Overall Length
Series 50 Delta Flow9424562 ¼" in/out Offset In/Center Out 19" Overall Length
Series 50 Delta Flow9424532 ¼" in/out Offset Both Ends 23" Overall Length
Series 50 Delta Flow9325562 ½" in/out Offset In/Center Out 19" Overall Length
Series 50 Delta Flow9325512 ½" in/out Offset In/Center Out 23" Overall Length
Series 50 Delta Flow9325532 ½" in/out Offset Both Ends 23" Overall Length
NOTE: These mufflers apply strictly to Mustang. Three-inch pipe mufflers are not included because they are not recommended due to space concerns.

Complete Flowmaster Exhaust Systems
Complete Flowmaster exhaust systems are available from Virginia Classic Mustang for classic and late-model Mustangs. If you're doing a classic, there are four basic systems designed to fit: two for '65-'66 and two for '67-'70. All are from the transmission crossmember back. If you have a convertible, you're out of luck because these systems are designed for hardtops and fastbacks only. Late-model systems (not covered here) are from the catalytic converters back (known as "cat-back" or "after-cat") with the H-pipe, mufflers, tailpipes, and all attachment hardware.

Kit No.Specifics
172732 ½" with Series 40 Delta Flow Mufflers and polished stainless tailpipe tips.
173342 ¼" with Series 50 Delta Flow Mufflers and polished stainless tailpipe tips.
172812 ½" with Super 44 Mufflers with polished stainless tailpipe tips designed for the {{{GT}}} valance.
172822 ½" with Super 44 Mufflers with turndowns for non-GT valance.

Delta Flow
How do you improve on a winner? Flowmaster calls it Delta Flow. Born from the Delta Force racing muffler, the new street muffler offers exciting technology that provides superior scavenging over conventional mufflers. Delta Flow technology uses additional deflectors to provide a cleaner, faster scavenging path for hot exhaust gases to virtually eliminate turbulence. Here are the benefits of Flowmaster's new Delta Flow muffler.

Resonant tuning chamber captures and cancels specific sound frequencies, which reduces resonance inside the cabin.

Power chamber creates a negative pressure (suction) that helps scavenge exhaust gases. This means more power, better low-end torque, and to some degree improved fuel economy. It also reduces exhaust system heat.

Low pressure balance chamber blocks power-robbing atmospheric pressure from adversely affecting exhaust system function.

Delta Flow Technology provides advanced noise cancellation by separating and recombining exhaust pulses at precise phase shift. In other words, quieter operation.

H-Pipe Or X-Style Crossover?
Why should you tie both sides of a dual exhaust system via a crossover pipe? Aside from how it affects sound, it is also important for performance. The crossover balances cylinder bank exhaust pulses on V-6 and V-8 engines. When you separate cylinder banks, you get a popgun sound from dual exhausts. Mixing it up allows the banks to complement one another. It also improves scavenging.

Boyd Butler of Pro Motorsports Engineering pioneered the X-style crossover-a crisscross pipe design that smoothes exhaust flow across cylinder banks. Without question, and consistently proven in stock car racing, it frees up horsepower and torque. The X-style crossover does its best work at high rpm.

Should you install an X crossover on your Mustang? If you're going racing or you like the bizarre, buzzy sound and high rpm of an X, then yes. If you're going cruising or building a show car, it's your option. The X-style crossover does improve performance, so you can expect some improvement in power.