If You Want it Done Right...

The performance after-market for Hyundai’s Genesis Coupe is not very good. Some staunch proponents of the platform will argue otherwise, but they are wrong. They’ll say that there are many options for parts such as the numerous exhaust kits, but they fail to appreciate that most of those options are shit.

Source: WebCarStory 
Source: WebCarStory 
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Oh, sure, there are some very solid bolt-ons for the GenCoupe. Some of the finest names in after-market performance make parts for the car, including Cusco and ACT. The trouble is that the Genesis Coupe tuning scene is hit-and-miss. You can get an amazing clutch/flywheel combo and differential, but if you want good engine mounts or a trick exhaust system you’re out of luck.

In our opinion, no manufacturer of significant performance-oriented car parts can claim their products are high-quality without being actively involved in motorsport. When it comes to exhaust systems, there are “premium” options for the Genesis Coupe, with ARK Exhaust being at the top of that list both in terms of cost and popularity. While their kits are shiny and their marketing is slick, nobody is out there regularly pounding on their gear in a race car.

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Contrast that with companies like Kooks Headers or Vibrant Performance, who do a significant volume of business with race teams. Both those companies make street-oriented products and their experience with racing shines through in the quality of everything they make.

So when it came time for us to scratch the performance exhaust itch on Project Hoondy, we just couldn’t bring ourselves to settle for a ho-hum off-the-shelf solution. Installing such excellent parts on the car so far has led us to have high standards for any further upgrades. This left us with only one solution: a custom exhaust system.

Happily, when it comes to the art of custom automotive fabrication, exhaust systems are among the most common and least costly expressions of the craft. There are numerous vendors who offer top-quality, stand-alone exhaust components that, with a little know-how, anyone can assemble into a quality exhaust system kit.

Freed from the constraints of the GenCoupe aftermarket, we had much greater control over what we wanted our exhaust system to look and sound like.

That far-reaching freedom meant we had to set a realistic goal. While a full exhaust replacement with headers would have been nice, we weren’t looking to drop $3000+ on this project. In addition, we don’t think headers are necessary for us to reach our eventual performance goals. Furthermore, close examination of the OEM exhaust system suggests there are far more gains to be had in some parts of it than others. In fact, we think that Hyundai expected many buyers of the Genesis Coupe to modify its exhaust and intentionally cheaped out on certain parts of it because of that.  

Looking under the car, we see that the front half of the OEM exhaust is quite nice:

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Down-pipes made from good quality stainless tubing with smooth, large-radius bends leading to the secondary cats. The different parts are mated with thick, stainless flanges. There are some nice flex-joints installed right behind the primary cats as well. Lovely.

The rear half of the OEM exhaust setup, in stark contrast, is just gross:

Source: Leral
Source: Leral
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Those excellent down-pipes are fed into the nastiest Y-adapter we’ve ever seen which feeds a single pipe that is way too small, a crummy little resonator, and a typical back-pressure-inducing trash can muffler. Then, adding insult to injury, some absolutely gigantic fake exhaust tips. Disgusting.

It seems obvious to us that Hyundai assumed “cat-back” exhaust kits would be quite popular on the Genesis Coupe and spent as little money as possible on the rear half of the OEM exhaust in the expectation that it would frequently be scrapped anyway. Any GenCoupe owners who don’t want to modify their exhaust probably also don’t care about and/or will never notice the restriction in performance and fake tips. Hyundai’s strategy here contributes to a lower sticker price on the car for all parties concerned (or a better profit margin) without pissing any owners off. We think it was honestly a pretty smart move on their part.

So it looked like limiting the scope of our exhaust build to a custom cat-back kit wouldn’t hurt us too badly performance-wise. A quick check on the tube diameter math backed this up.

Contrary to popular belief, bigger is not always better when it comes to exhaust pipe tube diameter. Sure, with turbocharged or supercharged cars it is difficult to have too big of an exhaust system. However, when dealing with naturally aspirated engines you need a careful balance between engine power and exhaust tube diameter.

Source: MotoIQ
Source: MotoIQ
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A compromise needs to be made between back-pressure and exhaust gas velocity. Mike Kojima and Sara Forst did a great job of explaining the concept in this excellent MotoIQ article. In essence, if you use too small of an exhaust pipe you will have overly high back-pressure, which robs top-end power, but if you use too large of an exhaust pipe you get low exhaust gas velocity which reduces mid-range torque. Keeping exhaust gas velocity high while maintaining low back-pressure is the name of the game and it’s a tricky game to play if you don’t understand the rules.

In order to determine the exhaust sizing needs for your particular engine you can complete complicated equations involving engine displacement, exhaust gas temperature, fuel mass, and other variables. However, there is an easy and reliable shortcut that gets you to the heart of the issue.

Even experienced exhaust fabricators will agree that a simple horsepower-to-CFM conversion will give you accurate enough information to get you in the ballpark of what exhaust pipe diameter is right for your car. The exact conversion factor varies slightly, but it is generally accepted to be between 2.0 and 2.2 CFM of exhaust flow per horsepower.

From there you just need to determine how much CFM a given size of exhaust pipe supports and select the one that is closest to the needs of your platform. In general, good quality mandrel-bent exhaust pipe will flow 115 CFM per square inch of cross-sectional area. From there it is simple math to figure out what your car needs.

The Hyundai Genesis Coupe with the 3.8 liter V6 engine has 2.25 inch down-pipes as standard. We decided to check the flow of these pipes to see if changing them might yield increased performance.

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2.25 inch exhaust pipe has 3.55 square inches of cross-sectional area. Multiply that by 115 CFM per square inch and you get 408 CFM. Multiply that by two (two down-pipes) and you get 816 CFM of total exhaust flow. divide that by 2.2 to get back to horsepower and you get 371hp. The BK1 Genesis Coupe V6 makes just over 300 horsepower, so a dual 2.25 inch exhaust setup has more than enough flow to support the V6 engine even with copious power modifications. Going up to 2.5 inch down-pipes would give us enough flow for 463hp, which we’ll never hit while staying naturally aspirated. Going to 2.5 inch down-pipes would gain us slightly reduced back-pressure, but would drastically reduce exhaust gas velocity, costing us mid-range torque that we definitely don’t want to sacrifice.

Therefore, we can conclude that Hyundai knew exactly what they were doing when designing the OEM down-pipes, which means sticking with a 2.25 inch diameter for our custom cat-back exhaust system is definitely the way to go.

With this information in hand, we could confidently select individual components to build our custom exhaust. However, there was a dizzying variety to chose from. Tune in next week to see what brands won the race to be a part of Project Hoondy!

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