So you want to build a custom exhaust you say?

So you’ve decided to ditch your factory exhaust and go custom. Whether you are paying a well known race shop, or you are a person who possess fabrication skills, these rules will help make a great sounding, horsepower freeing exhaust system.

If you don’t already know, we consider ourselves to be somewhat of exhaust connoisseurs. After hearing thousands of systems across many makes and models, it’s safe to say I know what sounds “good”. We have been through a great deal of trial and error with various cars using our Dyno Dynamics dynamometer, and we’ve found it’s the details that make the difference. Testing has shown us that power and sound go hand in hand. Every single thing we’ve learned along the way has been absorbed and utilized in the design of our exhaust systems. Regardless of what the aftermarket may want you to believe, it really isn’t that hard to understand what works and what doesn’t once the header ends and the exhaust begins:


-Only bend if necessary
The easiest concept to understand is to only bend the pipes if you absolutely must. The straightest path between two points is the path of least resistance, and that lack of resistance translates directly to power output.



-Proper prep is proper
In order to get a great looking weld, the pieces must be as close to each other with as little gap as possible. This is why we spend a large amount of time processing the raw pieces of pipe, so that they match up as perfectly as possible prior to the actual welding taking place. This is also great for exhaust flow, as there are no overlapping edges (and thus, no excess weld penetration) that can lead to turbulent exhaust flow, cutting down on power and altering the sound.


-Back-purge everything.
We further minimize excess weld penetration slag by fully back-purging all of our welds with inert gasses. This prevents oxidization of the inside of the pipe. Oxidization creates a pile of slag that adds turbulence, cutting down on power and altering the sound. Back-purging also adds an extra bit of strength and rigidity to the weld and the system as a whole.


-The best materials make the best power and sound.
The metal we use for fabrication is a new piece of high-grade stainless steel that looks beautiful from the outside and provides a uniform flow on the inside, maximizing the total gains possible.


-Drone isn’t something you have to put up with.
A great example is a term that is often thrown around called “drone”. Drone is a sound resonance that can creep in at a given rpm and throttle position. The drone will resonate with the materials of the vehicle causing an annoying hum or buzz in the vehicle’s cabin. Many purveyors of budget systems may say that drone is a byproduct of the power increase that the free flowing exhaust system delivers. This in not entirely true. While drone may still be present to some degree on a well designed system, a good fabricator should be able to tell you with confidence that it will be at a negligible level.


-Never use chambered mufflers.
While it’s true that having the pressure drop from the chamber can add depth to low-end exhaust tone, that extra low-end depth typically comes at the price of top end sound cleanliness — often adding a hollow, whooshing noise. During our extensive work with custom exhaust setups, we’ve found that no matter what level of technology was used in the design of the muffler’s chamber, it never outperformed or sounded better than a high quality, straight-through resonated muffler design. That’s why we chose to use that straight-through design in our own handiwork.



-90 degree intersections are a no no.
If a clean tone at high RPMs is the desired result, blending both cylinder banks together on a V-configuration motor is a must. Here at FMU, we hate exhaust rasp with a passion. To us, the high pitched, tingy, ripping noise is the sign of a design that is not optimal. That’s why we designed a smooth, high flowing X-pipe that allows the high-speed pressure wave from the exhaust pulse to be blended properly with the wave from the opposing bank. We never use balance tubes, H-pipes, or resonance chambers. During our years of fabricating, we have done a lot of exhaust testing on all manner of vehicles. We have seen countless exhaust designs, using all sorts of cited science and laboratory testing. There has never been an instance where we have seen a baffle, crossover pipe, or noise cancelling chamber that can best the performance and sound of a straight through, resonated stainless steel exhaust system. Our reasoning is thus — those three components we mentioned change the direction at or near 90-degree angles, which causes friction and turbulence, leading to power loss and sound degradation. We never use components in our exhaust that will drastically alter the laminar flow of a fluid, and exhaust gas acts as a fluid while in the exhaust system. Again, we are not saying there aren’t great kits out there that use these marketing bells and whistles mentioned, because there are, and plenty of them. If you are looking for the absolute best; the further you are from a 90 degree angled intersection of two pipes the better.



If you have any questions about building a custom exhaust for your car don’t hesitate to give us a ring at 815-230-2900. We’re conveniently located just outside of Chicago in the Naperville/Plainfield area at 11965 S. Aero Dr. Plainfield, IL 60585. E-mail better? E-mail us at