Borla has developed new exhaust systems for Miatas that allow for free breathing, are quieter and comply with Spec Miata rules.

It seems a bit curious at first why Borla would decide to build an entire exhaust system for one class of race car. The company offers a variety of mufflers that people could use to fabricate their own systems, so why bother crafting an entire system?

Very simply stated, people asked them to build it.

Borla’s sales and marketing manager, David Borla had been asked to speak at the Professional Motorsport Circuit Owners and Operators Convention at the Performance Racing Industry trade show in December 2011. One of the topics at hand was car noise, and how it has come to the forefront at different tracks around the country.

“When they started building these racetracks, they put them in the middle of nowhere. Over the last 30 or 40 years, developers have started to develop land closer and closer to the racetrack,” Borla said. “This whole thing came up because noise has become an issue at racetracks and Miatas are very popular. As manufacturers, it costs quite a bit of money for us to invest in developing an exhaust system. Unfortunately, we can’t go out there and make custom exhaust systems for various different racecars.

“For our part, and anybody who builds exhaust systems, it’s application specific, and I had a bunch of guys coming up to me and talking about various cars and things to get into,” he added. “Why did we start with the Spec Miata? It’s very popular.”

Borla’s project manager Alvin Tolosa began by rounding up three vehicles for testing and development: an NA 1.6, an NA 1.8 and an NB 1.8, all of which use different systems (disclosure: my car was one of those used for development purposes).

After getting test vehicles, the first step in development was to establish baseline dyno figures with their existing exhaust systems. Then it was a matter of establishing priorities for its system, and Borla actively sought racer input first and looked at the rulebook before it bent the first pipe or laid the first weld. The new system would have to comply with class rules, achieve as good or better power targets, be close to or less than the same weight as other systems on the market, attenuate sound levels better than others, put out a good sound and deliver it at the right price point.

The Borla system complies with the rules and dyno figures show it produced more torque and horsepower throughout the rpm range on an NA 1.8 than a Springfield Dyno system, even with the auxiliary rear muffler in place. What’s more, the 2.25-inch Borla system outflowed a 2.5-inch system on the NB 1.8 used for testing, and it made the most power with the auxiliary rear muffler in place.

Borla’s primary NA system weighs 13 pounds. For comparison’s sake, the Springfield Dyno system weighs 15.5 pounds and the Mazdaspeed system weighs 19.5 pounds. Adding the auxiliary muffler to NA cars bumps the exhaust system’s overall weight to 21 pounds. For NBs, the weights are 15 pounds for the primary system and 23 pounds with the rear muffler installed.

Borla’s R&D facility in Oxnard, Calif., developed the new racing exhaust systems for Miatas.
Borla’s R&D facility in Oxnard, Calif., developed the new racing exhaust systems for Miatas.

Prices for the Borla systems are a bit higher than competing products, so racers will have to decide for themselves whether they want to shoulder the additional cost.

Power and weight targets achieved, Borla then focused on attenuating sound, a subject the company has been investigating for decades. Using it’s patented multicore technology, Borla was able to produce a deeper exhaust note that registers lower decibel readings. The auxiliary muffler reduces noise even further, another 2 to 3 decibels (dB A).

Borla’s patented multicore muffler technology.
Borla’s patented multicore muffler technology.

Other factors come into play in measuring noise that are beyond the company’s control, such as temperature, humidity, barometric pressure, altitude and surrounding vegetation, but that’s another story.

To a racer who grew up with the conventional wisdom that larger-diameter exhaust pipes are better, it almost seems counterintuitive that the company’s multicore mufflers would outflow a simple straight-through design, but Borla pointed out that its mufflers will often outflow a straight piece of pipe, with no perforations or packing like you would find in “glass pack” mufflers. Believe it or not, a pipe too large in diameter for the engine application can often create more back pressure than it relieves.

“A 1.8 liter, naturally aspirated engine does not need 2.5-inch exhaust pipe to perform well,” Borla explained. “Pipe diameter that is too large creates turbulence, which can create backpressure, too. The gases travel through the exhaust pipe on outside of the pipe. The larger the pipe, the more surface area you have. When you have more surface area, you actually create more friction. The exhaust gases actually have friction as they go against the wall of the pipe, so you’re creating more friction by not directing the gases back. They’re bouncing all over the place and creating what’s called turbulence, and you’re creating backpressure.”

The new systems are equipped with custom brackets and hangers necessary to fit them to the Miata’s stock mounting locations.
The new systems are equipped with custom brackets and hangers necessary to fit them to the Miata’s stock mounting locations.

The principle is similar to putting your thumb over the end of a garden hose to speed up water flow. The idea is to get exhaust gases out of the tailpipe as quickly as you can, and to strike the balance where you increase exhaust velocity without creating backpressure. A large exhaust pipe on a small engine might exhibit some advantage at peak power, but much is lost lower in the torque curve.

“Where you lose the power is mostly in the low end and the midrange,” Borla said. “When you get up to the really high rpm, sometimes a large pipe will increase peak horsepower. But you’re trading all that low-end torque and all that low end power to just make a little bit of peak horsepower.”

Heat also plays a role, Tolosa added. As exhaust gases cool, they slow down. That’s one reason people cover their exhaust systems front to back with header wrap tape, which is not permitted under Spec Miata rules. Larger pipes lose heat more quickly, which can add back pressure. According to Tolosa, Borla achieved the balance with its multicore technology, mandrel bends and smooth transitions from pipe to muffler.

The new Miata systems are manufactured using mandrel bends for smoother transitions and improved exhaust gas flow.
The new Miata systems are manufactured using mandrel bends for smoother transitions and improved exhaust gas flow.

Even though the product has been released, Borla is still doing testing. Tolosa was planning a trip to Mazda Raceway Laguna Seca in October, where he was going to sit with officials in the sound-measurement booth between turns five and six. As much as the company needs to please the racers, it still needs to pass muster with tracks that have sound restrictions. Mazda Raceway Laguna Seca’s sound limit on most days is 92 dB A, and there really is no appealing a black flag for exceeding the limit.

“I think we’ll probably be testing this for a long time, and we may have to tweak the product here and there depending on what’s going on at different times of the year and in different parts of the country,” Borla said. “We’re well aware of that. It’s just part of the game.”

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The new systems can be fitted with an auxiliary muffler at the rear for additional sound attenuation. The second muffler is tucked up and in far enough that it won’t be damaged by bump drafting or incidental contact.
The new systems can be fitted with an auxiliary muffler at the rear for additional sound attenuation. The second muffler is tucked up and in far enough that it won’t be damaged by bump drafting or incidental contact.

Part Numbers and Prices

1990-1993 1.6, Primary System

Part No. 12665

$449.08

1990-1993 1.6, Auxiliary System

Part No. 60537

$379.44

1994-1997 1.8, Primary System

Part No. 12668

$449.08

1994-1997 1.8, Auxiliary System

Part No. 60537

$379.44

1999-2005 1.8, Primary System

Part No. 12667

$469.61

1999-2005 1.8, Auxiliary System

Part No. 60538

$379.44
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Image courtesy of Brett Becker