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Mustang GT350 Restomod 4

Less pressure, more power and a cleaner look

Having covered the mods on the drivetrain, chassis, and interior of Dan Holm's GT350-style conversion of a '67 Mustang, it's time to take a look at the tweaking directed at the exhaust system. As with many of the other upgrades Dan chose, there were more than a few ways that the exhaust could have been handled, allowing for personal expression of taste and preference. Choices made can affect a myriad of aspects, such as the peak horsepower, powerband width and range, exhaust tone, aesthetics, cost and durability. We'll address all of these factors starting from the exhaust headers and working back to the tailpipe tips.

Headers

The tuned, ceramic-coated exhaust headers are equal-length for improved scavenging, a design chosen for performance, looks, and durability—and yet are still not the most expensive on the planet. Their modest-sized runners (individual tubes) allow for much better flow than the stock cast-iron manifolds, while still maintaining a wide powerband. Larger tubes would result in a reduction of low-end horsepower/torque and slightly more horsepower at peak rpm, which Dan deemed less-than-ideal for the car's occasional street use.

The ceramic coating makes for a more durable header and also improves performance. This outer layer, which was not commercially available back when the original GT350 was produced, protects the metal from heat extremes, exhaust moisture, and other corrosive by-products. The fact that they look "cool" (and will for a long time) is icing on the cake. The performance value results from the fact that the headers carry more heat out of the engine for improved combustion.

Gaskets

Holm also took advantage of another advancement in technology by using a heavy-duty, leak-proof gasket. There is no longer any cause to complain that short-lived header gaskets are reason enough to avoid the use of headers. (Note, however, that they do need to be re-torqued periodically.) Just aft of the header collectors is a pair of custom-fabricated exhaust "cut-outs." With the caps removed, they don't necessarily add anything to the actual on-track performance against the stopwatch. Yet Holm enjoys the raw, straight-outta-the-pipes cacophony surrounding the car, as it reverberates within his Shoei racing helmet. All that's needed to transform the exhaust from "street-subdued" to "track-truculent" is the removal of the three wing nuts and the blocking plate they secure on each side.

X-Pipes

Next, we move to the center section of the exhaust where one of the new-style X-pipe systems has been installed. This system employs an X-shaped junction with a smaller-than-the-pipe equalization hole at the intersection. Internally, it looks more like two U-shaped bends connected at their respective bottoms, so that the exhaust is not actually trying to cross itself (like the old-style X configuration). Exhaust equalization is still achieved, and without a separate equalization tube.

What this configuration results in, practically speaking, is excellent exhaust flow without the "boom" that sometimes accompanies EQ tube systems. Not only is the boom reduced, but also the overall noise level is surprisingly low (even with dump-spouts instead of tailpipe extensions, as was done in this car). Again, this approach is a plus for street use.

Finally, we move to the mufflers and dump-spouts that exit right behind them. Flowmaster three-chamber street mufflers were used for their trademark tone, quieter operation, and legendary durability. Using dump-spouts rather than tailpipes was done for several reasons, all practical, but in different ways. Obviously, with performance on the track in mind, weight savings are achieved by eliminating the tailpipes. Also, any durability problems that might be associated with having tailpipes on a racecar have been bypassed. Tailpipes' slight increase in backpressure also rob the rear wheels of peak horsepower (using the cutouts would help to all but eliminate this problem, however).

Holm also likes the look of a dump-spout system, and while some may disagree with him, they are perfectly welcome to put tailpipes on their cars. He doesn't find the extra interior noise associated with dump-spouts to be intolerable on his car—part of the reason he chose the X-pipe system with the three-chambered Flowmasters. Due to the complexity of this particular installation, as well as the choice to opt for a clean look by eliminating many clamps and welding many of the joints instead, Holm enlisted some professional help from an experienced installer/fabricator (your author hereof). So while Holm's approach may not be to everybody's taste, it does suit his street/track usage and keeps a lid on the overall budget. As he wisely points out about buildup mods, "You've got to know when to stop." But in the meantime, he's also enjoying his Mustang's show and go.