How to Incorporate Exhaust Cutouts into Your Custom Exhaust System Design

What Are Exhaust Cutouts?

Exhaust cutouts are valves or flaps installed in the exhaust system that allow you to bypass certain parts of the muffler or catalytic converter. When open, they direct exhaust gases straight out, creating a louder sound and potentially better performance. When closed, the system operates normally, reducing noise and emissions. These devices come in several forms: manual cable-operated cutouts, electric motor-driven butterfly valves, and even electronically controlled wastegate-style units. The core principle is simple—divert exhaust flow around restrictive components when you want extra volume or flow, and seal them off for daily driving or legal compliance.

Cutouts can be placed at various points in the system, most commonly in the downpipe (before the catalytic converter), in the mid-pipe (before the muffler), or directly after the headers. The location heavily influences the sound character and performance gains. For example, a cutout placed immediately after the headers will produce a raw, aggressive tone with maximum flow increase, while a cutout after the catalytic converter will still pass through the converter but bypass the muffler, offering a milder change.

Benefits of Using Exhaust Cutouts

Exhaust cutouts offer a unique blend of performance and versatility that fixed exhaust systems cannot match. Here are the primary advantages:

Variable Exhaust Note: Open cutouts produce a deep, aggressive tone that can be turned off with a flick of a switch. This allows you to enjoy a race-car sound on the track or a quiet ride in residential neighborhoods.
Power Gains: By bypassing restrictive mufflers and sometimes catalytic converters, cutouts reduce backpressure, which can increase horsepower and torque, especially at high RPMs. Some manufacturers claim gains of 5–15 hp depending on the vehicle and existing exhaust restrictions.
Enhanced Exhaust Flow: Open cutouts improve scavenging and reduce exhaust gas reversion, particularly in turbocharged vehicles where free-flowing exhaust is critical for spool times.
Ease of Installation: Many bolt-on cutout kits are designed for simple DIY installation, often requiring only a saw and clamps or welding equipment. Electric cutouts come with wiring kits that plug into switched 12V power.
Cost-Effective Upgrades: Instead of buying multiple exhaust systems, a single cutout gives you two exhaust personalities. This is especially beneficial for those who daily drive their performance vehicle.
Emissions Compliance: When closed, the exhaust system functions normally, passing through the catalytic converter and muffler, so you can pass emissions tests or avoid attention from law enforcement.

Key Considerations Before Installing Exhaust Cutouts

While cutouts are an exciting modification, they require careful planning to avoid legal fines, poor performance, or safety issues. Below are critical factors to evaluate:

Legal Regulations

Laws regarding exhaust noise and emissions vary widely by region. In many areas, it is illegal to operate a vehicle on public roads with an open exhaust cutout that bypasses the muffler or catalytic converter. Some jurisdictions also have specific decibel limits. Always check your state or country’s vehicle codes before installing. The EPA prohibits tampering with emission control devices on federally regulated vehicles, which may include cutouts placed before the catalytic converter. Consider using cutouts only for off-road or track use if you are uncertain.

Component Quality

Not all cutouts are created equal. Low-cost cutouts often use thin stainless steel, plastic gears, or weak seals that fail after a few cycles. Invest in units from reputable brands such as Flowmaster or DMH Performance, which offer heavy-duty construction, stainless steel bodies, and high-temperature seals. Electric cutouts should use sealed motors to prevent moisture damage.

Placement and Fitment

Proper placement is essential for both sound and performance. Most enthusiasts place cutouts in the downpipe before the catalytic converter for maximum flow, but this requires careful routing to avoid ground clearance issues or heat damage to nearby components. For daily drivers, placement after the muffler is simpler but yields less gain. Measure your existing exhaust pipe diameter (typically 2.5 or 3 inches for LS swaps, 2.25–2.5 for V6s) and choose a cutout with matching inlet/outlet.

Installation Skills

Manual cutouts require only basic mechanical skills and a position to mount the cable. Electric cutouts demand wiring knowledge—you must run a fused power wire, install a switch or remote receiver, and ground the unit. If you are uncomfortable with wiring or exhaust welding, seek professional installation from a custom exhaust shop. A poorly welded cutout can cause exhaust leaks, rattles, or even fall off.

Noise and Vibration

Open cutouts can produce deafening noise levels that may be harmful to hearing or cause driver fatigue over long drives. Additionally, the cutout valve and actuator can introduce buzzing or rattling if not properly secured. Use high-quality rubber mounts or add a flexible section to dampen vibrations.

Design and Placement Strategies

Choosing where to place your cutout depends on your goals—performance, sound, or both. Below are the three most common placement strategies with their trade-offs.

Pre-Catalytic Converter (Downpipe Cutout): Placing the cutout before the cat yields the largest performance gain because it eliminates the most restrictive component. Expect a raw, aggressive sound that may include a distinct “trumpeting” note. This configuration is illegal for road use in many areas and may set off check-engine lights (due to downstream O2 sensor readings). Use only for track or off-road applications.
Mid-Pipe Cutout (Before Muffler but After Cat): This is the most popular street location. The exhaust passes through the catalytic converter (reducing emissions) but bypasses the muffler. Sound is louder and deeper than stock but still retains some refinement. Performance gains are moderate, typically 5–10 hp on normally aspirated engines.
Axe Exhaust Cutout (After Muffler): Placed just before the tailpipe tip, this cutout bypasses the muffler entirely. It’s easier to install and legal in many areas because the cat remains in line. The sound becomes more aggressive without being overly loud. This location is ideal for those who want a quick switch without major fabrication.

Diameter Considerations

Match the cutout diameter to your existing exhaust piping or one size larger if you are building a new system. A 3-inch cutout on a 2.5-inch pipe can cause turbulence and reduce flow benefits. Conversely, a 2.5-inch cutout on a 3-inch system becomes a bottleneck. Use reducer couplings if needed.

Cutout Valve Types

Manual cable-operated cutouts are the most affordable and reliable—no electronics to fail. Electric cutouts offer convenience with a remote or dash-mounted switch, but the motors can seize if exposed to heat or moisture. “Electronic wastegate-style” cutouts use a solenoid and vacuum diaphragm (common in OEM applications) but are more complex to integrate into a custom system.

Step-by-Step Installation Guide

Follow these general steps for a typical electric exhaust cutout installation. Always refer to your specific manufacturer instructions.

Plan Placement: Locate a straight section of exhaust pipe at least 12 inches long where the cutout will fit without interfering with suspension, driveshaft, or fuel lines. Ensure you have clearance for the actuator.
Measure and Mark: Use a piece of string to trace the pipe’s circumference. Mark the cut length using a tubing cutter or soapstone.
Remove the Pipe Section: Cut the exhaust pipe at your marks using a reciprocating saw (for steel) or cut-off wheel. Deburr edges. For welded systems, you may need to unbolt or remove the exhaust section.
Test Fit the Cutout: Slide the cutout (with its matching flanges or clamp-on adapter) into place. Check alignment and ensure the actuator clears the floor pan and heat shields.
Secure the Cutout: Weld the cutout in place for a permanent, leak-free connection (TIG or MIG welding on stainless steel is recommended). Alternatively, use high-strength exhaust clamps with anti-seize compound. Verify the seal by running the engine with the cutout closed—listen for hisses.
Reconnect the Exhaust: Attach the remaining pipe sections using band clamps or weld them. For a cleaner look, use V-band clamps at the cutout flanges.
Wire the Electric Unit: Run a 12V fused wire from the battery to a relay, then to the cutout motor. Install a switch inside the cabin (or use a wireless remote module). Ground the motor to a clean chassis point. Test operation: open/close multiple times.
Final Inspection: Check for exhaust leaks by holding a rag near joints while the engine idles. Ensure the cutout does not contact the underbody or exhaust heat shields when vibrating.

For manual cable cutouts, skip the wiring step and instead mount a pull cable through the firewall to the driver’s seat area. Lubricate the cable with graphite powder.

Control Systems: Manual vs. Electronic

Your choice of control system affects everyday convenience and reliability. Here’s a breakdown:

Type	Pros	Cons
Manual Cable	Simple, no electronics, very reliable, low cost	Requires physical cable routing; may be difficult to reach; cannot be automated
Electric Switch	Easy to install with a dash switch; allows remote operation	Motor can fail from heat/corrosion; wiring adds complexity; requires power
Wireless Remote	Convenient from outside the vehicle; modern & clean install	Battery in remote dies; RF interference possible; higher cost
Smart Controller (e.g., Bluetooth)	Programmable automation (open at certain RPM, closed at idle)	Expensive; app dependence; more points of failure

For most street builds, a quality electric unit with a waterproof motor and a simple toggle switch offers the best balance. If you plan to take the car to the drag strip, a manual cable is more bulletproof.

Maintenance and Safety Tips

To keep your exhaust cutouts functioning safely and reliably, incorporate these checks into your routine maintenance:

Monthly Inspection: With the engine cold, inspect the cutout valve for carbon build-up, rust, or debris that could prevent full closure. Operate the valve open and closed several times to confirm smooth movement.
Check for Exhaust Leaks: Use a soapy water spray on all joints and the cutout flange. Bubbles indicate a leak that must be sealed immediately to prevent harmful fumes from entering the cabin.
Lubricate Moving Parts: Manual cables and electric motor shafts should be lubricated with high-temperature grease or dry-film lubricant every 5,000 miles.
Battery and Wiring: Ensure the fuse is the correct amperage (usually 15–20A) and that no wires are chafing against the chassis or hot exhaust components. Replace any corroded connectors.
Test Before Track Use: If you plan to use the cutout at a drag strip or autocross, test it the day before. A stuck closed valve can starve the engine of flow on a wide-open throttle pass.
Emissions Test Mode: If your area requires periodic smog checks, always keep the cutout closed and sealed. Some cutouts may leak enough to fail a visual inspection even when closed.

Safety is paramount: never run an open cutout in an enclosed space (garage, tunnel) because exhaust contains carbon monoxide (CO). Also, be aware that extremely loud exhaust can damage hearing—wear earplugs during extended open-pipe driving.

Performance Testing and Tuning

After installation, verify that the cutout is actually improving performance. A baseline dyno run with the cutout closed, followed by a run with it open, will quantify gains. On a normally aspirated V8, expect a gain of 5–12 hp at peak, with torque improvement in the midrange. On turbocharged engines, gains can be even larger because reduced backpressure helps spool the turbo faster. However, be careful—too large a cutout on a turbo car can cause boost creep or wastegate instability. Monitor air-fuel ratios (AFR) with a wideband O2 sensor; if the AFR becomes too lean after opening the cutout (due to reduced backpressure altering fuel trim), you may need to retune the ECU.

If you notice no power gain or a slight loss, the cutout may be too small, improperly placed (causing flow turbulence), or your engine may already have a free-flowing exhaust that leaves little room for improvement. Consider upgrading to a lower-restriction cutout such as a “full bore” design that uses a full 304 stainless steel gate rather than a butterfly valve that partially obstructs flow even when open.

Final Thoughts

Incorporating exhaust cutouts into your custom exhaust system is one of the most rewarding modifications you can make—it gives you the ability to flip a switch between a quiet, legal daily driver and a snarling track machine. The key to success lies in choosing the right placement, using high-quality components, and adhering to local laws. Whether you opt for a simple manual cable setup or a Bluetooth-controlled electric system, proper installation and maintenance will ensure years of trouble-free switching. Remember that with great power (and great noise) comes great responsibility—use your cutouts wisely to avoid unintended consequences.