Project S550 – 2015 Mustang GT Introduction


The accelerometer does a good job of tracking how “spirited” your drive was.


Ford took numerous key lessons from the 2013 Boss 302 and incorporated them into the 2015 S550’s power plant. The engine uses larger intake and exhaust valves, similar to the Boss, as well as a completely new cylinder head casting with revised high-flow ports to provide a more direct path to the valves for less restrictive intake and exhaust flow. These changes have resulted in a horsepower rating of 435 with 400 lb-ft of torque. This new Coyote engine also has a rebalanced, forged steel crankshaft to support higher-RPM operation and a new deep-sump oil pan that will aid in reliability as we begin to build those larger power numbers later on.


The updated 5.0 liter Coyote Engine produces 435 hp with 400 tq.


The biggest changes to the S550 Mustang platform are the suspension upgrades, which were included in all 2015 Mustang models. IRS now comes standard on all models; while select models in previous generations received this upgrade, this model year marks the first time this feature has been available on all trim levels. The front suspension also received some notable changes with a new double-ball-joint front strut that allows for larger brakes without the need for super-high offset wheels. With our performance pack option, the car actually handles quite well right out of the showroom. The stiffer front springs, the higher-rated sway bar in the rear, and the braces definitely provide more confident driving over the non PP-equipped cars we’ve driven.

First Mod

Not wanting you to feel short changed by a stats-only write-up, we’ll leave you with a couple modifications we did to Project S550 the day after it arrived. First, the stock GT is very quiet – in fact, way too quiet for us. So, the first thing we did (and the first thing that most Mustang owners do) was bolt on an after-market exhaust. While it isn’t really doing much for our performance at this point in our build, we did want a more aggressive exhaust note. We turned to our friends at Borla and they recommended their S-type Cat-Back exhaust, which falls smack in the middle of the sound range. They also offer a Touring model, which is quieter than the S-Type, and the ATAK (Acoustically Tuned Applied Kinetics) model, which is louder. Since this is a daily driver and our neighbors might not appreciate the 6 a.m.cold starts from the ATAK (and since we still want to be invited to the neighborhood BBQs), we agreed that the S-Type was the best solution. The Borla system uses stainless steel construction, an X-pipe center section, and is a direct bolt-on replacement. As an added benefit, their system is also 30 pounds lighter than the stock unit, and features 2 1/2 inch mandrel bent piping. While the stock system also is built with 2 1/2 inch piping it bottle necks to just over 2 inches at the resonator. Stock Mustangs, like ours, have seen a respectable 8-12 hp increase with this system.


The heavy stock resonator compared to the Borla X pipe.
The 4” Black exhaust tips not only fill the cutouts in the rear valence much better, but they also complement our black Performance Package wheels nicely.

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