Custom Exhaust Builds: 7 Essential Rules to Follow

FMU Custom Exhaust

Custom Exhaust Builds: 7 Essential Rules to Follow

So you’ve decided to ditch your factory exhaust and go custom. We back that decision 100%! Whether you’re paying a well-known race shop or you’re a person who possesses fabrication skills, adhering to the following rules will help ensure your next exhaust has great sound and freely flowing horsepower.

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 we know what sounds not just good, but what sounds “great”. 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 all the difference. Internal testing has shown that power and sound go hand in hand. Every single thing we’ve learned along the way has been noted and utilized in the design of every future exhaust system we produce. Regardless of what the aftermarket manufacturers 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:

    RULE #1 – ONLY BEND IF NECESSARY

The easiest way to understand this rule is to comprehend this: only bend the pipes if the frame of the vehicle dictates that you absolutely must. The straightest path between two points is the path of least resistance and a lack of resistance directly translates into increased power output.


FMU Custom Exhaust

FMU Custom Exhaust



    RULE #2 – PROPER PREP IS ALWAYS THE MOST PROPER

In order to achieve great looking welds, pieces must be placed close to each other with the smallest gap 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.

FMU Custom Exhaust

FMU Custom Exhaust



    RULE #3 – Back-Purge Everything

We further minimize excess weld slag by fully back-purging all of our welds with inert gasses which prevents oxidization of the inside of the pipe. Oxidization creates a pile of slag that adds turbulence which cuts down on power and alters the sound. Back-purging also adds an extra bit of strength and rigidity to the weld positively affecting the system as a whole.

FMU Custom Exhaust

FMU Custom Exhaust



RULE #4 – THE BEST MATERIALS MAKE THE BEST POWER AND SOUND
The metal we use for fabrication is a brand 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.

FMU Custom Exhaust

FMU Custom Exhaust



    RULE #5 – DRONE ISN’T SOMETHING YOU HAVE TO PUT UP WITH

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 claim that drone is a byproduct of the power increase a free-flowing exhaust system delivers. However, this is not entirely true. While drone may 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.

FMU Custom Exhaust

FMU Custom Exhaust



    RULE #6 – 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 clarity, 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 has never outperformed or sounded better than a high-quality, straight-through resonated muffler design. That’s why we choose to use a straight-through design in all of our designs.

FMU Custom Exhaust

FMU Custom Exhaust



    RULE #7 – 90 DEGREE INTERSECTIONS ARE A NO-NO

If a clean tone at high RPMs is the desired result, blending both cylinder banks together in a V-configuration is a must. At FMU, we hate exhaust rasp with a passion. To us, the high pitched, tinny, ripping noise is a sign the design is not optimal. That’s why we created 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’ve run numerous A/B tests of exhaust systems on many types of vehicles. We’ve 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, crnoise-canceling noise-canceling chamber that can best the performance and sound of a straight through, resonated, stainless steel exhaust system. Our reasoning is this — the 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. However, If you are looking for the absolute best, the further you are from a 90 degree angled intersection of two pipes, the better.

FMU Custom Exhaust

FMU Custom Exhaust

If you have questions about exhaust systems or why we build them in a specific way, drop us a comment! Make sure to follow our Facebook page so you can stay up to date on all our projects! Facebook – Fluid MotorUnion

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