Sep 182016

A proper exhaust system is not about drawing attention from painted titanium big tip and high pitch farting noise. There are sciences in it.

Modern factory exhaust system on most performance oriented vehicles has been fairly well designed for optimal performance. Horsepower gain from aftermarket performance catback exhaust system alone, for instance, on Honda S2000, Mazda RX8, and Subaru BRZ are in the hardly measurable single digits.

Nowadays exhaust system on a modern vehicle is often not the primary modification that yields the most gain for the buck. What it does, however, the system opens the door to support modifications that would increase engine flow such as ECU reflash, bigger cams, and cranking up the boost on force induction engines.

The other not as obvious benefit is in weight reduction. Most aftermarket catback system uses lighter metal that is prohibited by the accountant of the car manufacturer. Weight reduction is also more drastic when converting dual mufflers to single.

Function of Catback Exhaust

The exhaust system’s primary function is to evacuate post combustion exhaust gas from cylinder head and getting it away from the car. The hot exhaust gas leaves the exhaust port containing the energy of sound waves. Collecting immediately to the cylinder head at the exhaust port is the header. Header design worth its own article particularly between a naturally aspirated and a turbocharged system. For the sake of keeping this article in focus. We will emphasis the portion of the exhaust system after the catalytic converter, also known as Catback Exhaust.

The first component in exhaust system design is the tubing size. The tubing is sized to support the engine’s exhaust gas flow measure in Cubic Feet per Minute (CFM) during its peak output. Generally speaking, 2 to 2.1 CFM per horsepower is the constant used to size the tubing with the least amount of resistance.

Muffler is to lower the sound amplitude of the sound waves while maintaining the velocity of the exhaust gas and letting that velocity exit the system at the most efficient means. There are two types of passive mufflers in the market: Reflection and Adsorption.

Reflection mufflers contain baffles, chambers, and even tuned ports to allow the sound energy to bounce around cancelling each other out to achieve the noise suppression effect. However, the reflecting energy creates high pressure in muffler that takes away the exhaust energy which reduces exhaust velocity exiting the system. The muffler is then a restrictor in the system. The benefit of reflection mufflers is for achieving the quietest system where optimal efficiency and performance are not the primary focus

Adsorption type of mufflers are as known as straight-through. These mufflers inlet and outlet are aligned in a straight-through pattern. The muffler contains perforated pipe and adsorption packing material filling the core of the muffler. As the exhaust pulsation energy passes through the muffler, the perforation allows the energy to pass into the packing material which adsorbs the sound energy to lower the amplitude. Due to it being a straight through design, there is no change in exhaust velocity exiting the system. The adsorption mufflers are the most popular design in optimizing performance.

Exhaust tips affect the acoustic of the system. To dampen sound, smaller tip or longer tip will make muffler work harder while sacrificing a little bit of air flow but this will provide more mellow sound. On the other hand, if the tip is bigger and shorter, the fast air flow from the exhaust will rapidly expand the atmospheric air and also allows cold air reversion that causes the backfiring sound.

Finally, the exhaust system cannot change the core sound character of the engine. The character is set by the compression ratio, firing order, and various different aspects of the engine design. The system can manipulate the tone but cannot drastically change it.

Mountune High Flow Exhaust System

In our FiST, we have installed the Mountune MP215 kit (here). It increased engine output from 197HP to 215HP, and torque is raised from 202lb-ft to 236lb-ft. This power gain is the result of increasing the amount of air pushing through the engine from the intake side. It becomes a logical step to improve the flow at the exhaust end. Conventionally three components can be upgraded. The header, downpipe, and exhaust.

We looked into Cobb, Ford Performance, Milltek, in addition to Mountune. With the MP215 kit, our preference is to stick with Mountune for powertain performance upgrades. Then out of the blue, Mountune was running a 20% off discount on their High Flow Exhaust system (Product Code: 2364-CBE-AB). Without a second thought, our order was placed. For more information,

What is Mountune High Flow Exhaust System?

  • 5” AISI 304 Stainless Steel (EN58E) mandrel bent tubing
  • Dual 3.25 double walled, polished stainless steel tips
  • Rear silencer with internal Helmholtz chamber design
  • CNC TIG welded
  • OE Type mounting bracket
  • Limited lifetime warranty

What is in the box?

The system arrived in a nicely packaged box. It comes in three separate pieces: midpipe, slip fit elbow pipe, and the muffler. Also in the box are two quality flat band clamps, and a graphite-type of gasket for the midpipe and downpipe connection.

How does the Mountune High Flow Exhaust System better than stock?

In terms of flow and tubing size, using the previously established 2.0 – 2.1 CFM per 1 hp constant. FiST’s stock 197 hp peak output is via 2.25” piping size. The peak CFM would be about 394 – 414. A 2.25” 16-gauge tubing flows approximately 408 CFM. Thus, the stock pipe size sits well in between the range. It is an ideal match for the fact that the engine displacement is only 1.6L and being a street driven vehicle the engine doesn’t operate at full boost peak output at all times.

In our case, the stock system becomes the bottleneck with the MP215 kit since the stock tubing leaves little room for gain. At 215 hp, the peak CFM now increases to 430 – 452 which is well beyond the limit of the stock system.

The Mountune system has 2.5” tubing size; a 24% increase. Using the same calculation, 2.5” tube can flow 509 CFM, the Mountune system is capable for 242 to 255 hp; a performance figure that we consider as the absolute limit for a front-wheel drive without ruining the drivability.

For weight reduction, the Mountune system is about 2 lbs lighter than stock, and mostly come from its silencer-less midpipe. An insignificant reduction but it’s a decent amount consider the bigger diameter tubing uses more material and stainless steel are generally heavier. The beauty of stainless steel in addition to the excellent property in corrosion resistance, it also has lower efficiency of thermal conductivity which enhances heat retention. The general rule of thumb for performance is the hotter the exhaust gas, the better it flows.

Knowing that the tubing size and material in the Mountune system can be proven by science. What about the muffler design? As we have discussed earlier, muffler design plays a major function in sound and performance. Helmholtz chamber is utilized in the Mountune system. Most commonly, Helmholtz resonator muffler are those with an external chamber attach to the exhaust pipe. The Helmholtz chamber is integrated inside of the muffler in the Mountune system. This type of muffler is almost like a hybrid between reflective and adsorption muffler. The exhaust flow remains unrestricted from the straight-through design yet the core cavity where adsorption material is packed in contains various small reflective chambers to reflect incoming exhaust pulse to achieve sound cancelling effect. The best design in keeping exhaust flow velocity high and the exhaust note in check.

Exhaust note comparison video is a click away at the bottom of the second page.

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