The 2016 model-year Shelby GT350 Mustang made its debut in late-summer 2015 to much fanfare. Representing the closest thing Ford has offered to a mass-produced factory racer, the anxiously-awaited car hit the showroom with a bang. There are a number of advances in technology showcased with this platform, as it features a 5.2-liter engine – code-named Voodoo – which is substantially different from the 5.0-liter Coyote engine found in the standard Mustang GT.
The use of the new engine prompted Kooks Custom Headers to develop a new exhaust system from the headers to the muffler tips. The setup featured in this article belongs to none other than HiPo Joe Charles’ GT350 was created in-house, on the car, and serves as the foundation for Kooks’ line of GT350 products, which are on the shelf at this very moment.
One major change in the Voodoo engine is found in the use of a flat-plane crankshaft. While not any sort of revolutionary development in and of itself (many four-cylinder engines use a flat-plane crank), the design presents unique challenges when selected for use in a V8 engine of this type. Ford’s powertrain engineers went to great lengths to counteract the unwanted harmonics in the production vehicle, notable issues that crop up with regard to noise, vibration, and harshness (NVH).
From Conception To Design
By virtue of its prominent place within the automotive aftermarket, Kooks has the ability to work closely with Ford’s engineering team – and well-placed race teams – on product development long before the vehicle in question is ready for prime time.
“We’ve been working with Multimatic in developing the exhaust for Jack Roush, Jr.’s Mustang that he’s used in road racing competition, so we knew about the issues that the car had,” says Chris Clark, Kooks’ director of sales and marketing.
“There are some vibration issues, but we got a jump-start on figuring that out. We had an idea that the chassis and the header configuration was close [to the Coyote engine in the standard S550], so we figured out to start with our 2015 S550 Mustang header as a base, and then we were going to build from there,” Clark said.
“This thing just keeps pulling; if it weren’t for the factory shift light you’d never be able to bang the gears. It’s unbelievable.” – Joe Charles, car owner
“We tried different primary lengths, different tubing diameters, and we even looked into going with a Tri-Y design at one point. Ultimately, we decided to use a four-into-one stepped-style header,” Clark continued.
In an attempt to retain the low- and mid-range torque available from the 5.2-liter engine, the decision was made to retain the primary tubing length on the short side to keep the torque values up. The step location was catered to the engine’s torque band.
“We felt that running a full-length big-tube 1.875-inch primary was going to cost us in the torque curve, whereas the stepped header gives us the best of both worlds. As the RPM climbs, the larger-diameter tube after the step helps to get the exhaust out of the cylinder head,” says Clark.
In its as-delivered form, the 2016 GT350 (and its big-brother, the GT 350R) arrive with dual-mode mufflers that offers drivers the choice of a subdued — like-a-Mustang-GT-but-better tone — or the all-out, ear-splitting insane howl of the uncorked pipes. This system is controlled by the driver using a selectable switch on the dash.
The OEM exhaust consists of an interesting 4-into-3-into-1 short-tube header configuration, which has been enhanced to work with the low backpressure this engine produces, thanks to the flat-plane crankshaft’s tendencies. In addition, the engine has a super-wide powerband – nearly 3,000 rpm separates the torque and horsepower peaks – which was also taken into account during Kooks’ design process.
“Keep in mind that we still have to play with the factory exhaust; we also made sure we still had room in there for catalytic converters for those people who need emissions-testing capability,” Clark said. “The idea is to design an exhaust system that will be the best on the street, at the track, and really give the system that’s going to make the most overall power for everyone – road racers, drag racers, or street-car guys.
The System In Action On Charles’ Car
The heavy-wall stainless-steel headers Kooks developed for the GT350 (PN 11542300, MSRP $1,449.99) utilize a 1.75 to 1.875-inch tubing step; the primary tube then feeds into the company’s traditional three-inch collector. Included with the headers are oxygen sensor extensions, Cometic header gaskets, and Stage 8 locking header bolts. Headers are offered with the option of ceramic coating in one of three styles: High Lustre Silver Calico, Cool Black, or Cool Gray (MSRP $371.31).
“The idea is to design an exhaust system that will be the best on the street, at the track, and really give the system that’s going to make the most overall power for everyone,” – Chris Clark, Kooks Custom Headers
The Tuner high-flow catalyst stainless-steel X-pipe system is also available (PN 11543200, MSRP $899.99), as is an off-road stainless setup (PN 11543100, MSRP $749.99). Each of the X-pipe systems features 2.750-inch outlets to mate properly with the OEM exhaust setup, and will require the OE system to be cut to match up correctly with Kooks’ products.
For those owners who wish to replace the entire exhaust system – as Charles did here – the company is also producing a complete 3.000-inch rear muffler section exhaust system (PN 11546300, MSRP $1,399.99), which includes Kooks mufflers and works with the factory quad tips to retain the OE appearance when viewed from the rear of the car.
Customers also have the option of selecting one of the three complete kits that Kooks has available, the Off Road Comp system (PN 1154F310, MSRP $3,599.97), the Tuner Catted Comp system (PN 1154F320, MSRP $3,749.97), or the Green catted Comp System (PN 1154F330, MSRP $4199.97).
Clark says installation of the entire system isn’t much different than any other Coyote-equipped car – it’s not the easiest task as it’s time-consuming, but it’s not especially challenging.
Active Exhaust – Or The Lack Thereof
One concern that has been noted by various sources with this engine configuration are the aforementioned harmonics, which can play havoc with speed parts if they aren’t developed properly.
Although Kooks has built its exhaust products with heavy-gauge tubing, they also made provisions to use some of the factory mounting arrangement to retain the vibration reduction measures Ford engineers built into the car.
There are two areas where Kooks chose to mimic the factory configuration in order to lessen vibration concerns; through the use of the tuned OE-style dampers just ahead of the catalysts, and just aft of the catalysts, where sections of flex pipe are installed into the X-pipe to assure that unwanted vibrations don’t damage expensive pipe sections.
Put simply, this system from Kooks doesn’t have it — but the car thinks it does.
“If you disconnect the valves for the actuators, you will not be able to control your driving modes,” says Clark. “The system requires a complete closed loop – what happens is that the car needs to see one rotation of the mechanism in each direction, or it won’t give you access to the driving modes. We made a custom bracket on top of the mufflers that acts as the valve, and allows the system to see that full rotation in order to allow mode control and damper adjustment.”
Each muffler has this bracket to connect the factory valve; although you’re technically fooling the system, you want to hear the tone of your new exhaust without any compromises. Another interesting note is that Kooks reuses the factory GT350 exhaust tips, which are welded to the Kooks dual-exit muffler tubing.
In testing, the car made 466.72 horsepower to the tires in stock configuration. With the headers and X-pipe installed, the power shot up to 481.8 horsepower, a gain of 15 horsepower at the tire.
Finally, once their own muffler system was installed to go with the new headers and Green catted X-pipe, the car pumped out a whopping 491.0 rwhp – a gain of 24.3 ponies to the tire – with no tune help to maximize the car’s performance with the improved breathing. The car also produced 20 lb-ft more torque with the full exhaust system.
“The exhaust system is freaking awesome,” says Charles. “It sounds awesome and makes awesome power. The biggest thing in my opinion going into this project was that I wanted more horsepower, and we found power all over the board, especially peak power. This thing just keeps pulling; if it weren’t for the factory shift light you’d never be able to bang the gears. It’s unbelievable.”
As proven from the impressive gains found on the dyno, the Kooks GT350 exhaust system just flat out works. Charles found even more power once he got the car home, installed a cold-air kit, and set the car up on Tim Matherly’s dyno at MV Performance for a proper tune with the CAI installed. He wouldn’t share how much more power they found from that session, however. We heard the car recently at the NMRA/NMCA All Star Nationals in Georgia, and all we’ll say is that Charles’ claims of “it sounds awesome” were backed up. Solid sound, solid engineering, solid performance.
“What matters to us is that we can build a header that isn’t going to break on people,” Clark said.