Electric exhaust cutouts are a unique and time-tested way to have the best of both worlds where exhaust systems are concerned. An electric exhaust cutout allows your vehicle to run quietly and normally most of the time, but you can open it at the flick of a switch to make your car loud and fast in seconds. Electric exhaust cutouts can be costly to purchase, but making one yourself is a less-expensive option if you have the materials, tools and some fabrication skill.
Cut with a grinder or saw five rectangles from a 1/16-inch stainless-steel plate. Cut two that measure 12 inches by 3 inches (the sides of your box) and three that measure 12 inches by 6 inches (the top and bottom of the box) Cut two more rectangles that measure 3 1/4 inches by 6 1/14 inches to act as the end plates.
Center the large ends of your slip-fit adapters on the end plates and weld them in place. Cut the disc of end-plate metal inside the adapter tubes using a jig-saw, then smooth the joint between the end plates and the inside of the adapter tubes using a die grinder or rotary tool.
Weld the box together with the adapter side of the end plates facing outward. Smooth the welds and trim the excess from your end plates using a grinder. You'll end up with a rectangular box measuring 12 inches by 6 inches by 3 inches. Scribe a rectangle on the bottom of your box; it should measure a half-inch from the sides and front and 1 inch from the back. Cut this rectangular plate out of the box's bottom using a cut-off wheel.
Attach your hinge to the inside back of the box with the hinge mechanism facing up. You can drill holes and attach it with bolts or weld it, whichever you prefer. Attach the other side of your hinge to the rectangular cut-out so that the "door" hinges inward. You may need to do some grinding and finishing with the die grinder to get everything working smoothly.
Cut another plate from the center of the third 12-inch-by-6-inch plate. The rectangular cut-out should measure 1 inch from the sides and 1 1/2 inches from the front and back. Hold it against the bottom of the box; this plate will act as a flange for the "door" to rest against. Drill holes that are 1/8-inch larger than the bolt heads in the door and bottom so you can service the assembly. Weld the flange to the bottom of the assembly.
Fabricate a bracket to mount the power-window servo to the front of the cutout, orienting it so that the actuator rod faces backward toward the door hinge. Weld a half-inch-tall, 3-inch-long "fin" to the bottom of the trap door, placing it just ahead of the hinge and orienting it so that it runs front-to-back. Push your door-lock servo all the way down, then fabricate a linkage rod from the servo actuator tip to roughly the center of the "fin."
Extend the servo and hold the actuator rod in place to open the trap door while you do so. Play with different rod positions on the fin until you find one that opens the door all the way when the servo is extended and closes it when the servo draws in. About 1 inch away from the hinge should work for most motors. Drill a hole in the fin and insert your actuator rod.
Connect the servo to a 12-volt power source to check the function. If everything is working correctly, the door will swing all the way open when the servo extends. You may choose to cover the inside of the flange with high-temperature silicone RTV to seal it.
Spray your exhaust pipes with white spray paint before cutting them. Then drive the car for a couple of full-throttle quarter-mile runs. Go under the car and look at the paint; you'll notice that some of it has burned away. The point where the paint burns stop and the white paint starts is the ideal location for your cut-out. Mark a line around the pipe where the paint stops. Measure 12 1/2 inches rearward from that line and make another line.
Cut that 12 1/2-inch section out using a reciprocating saw or grinder. Slide an exhaust clamp over the front and rear tubing extensions on your cut-out, then slip the cut-out tubing extensions over the cut pipe ends to fill the gap between them. Tighten the hose clamps with a ratchet and socket.
Install an eyelet connector onto the black ground wire coming out of your servo. Drill a 3/32-inch diameter hole in the front of your cut-out. Hold the eyelet connector up to the hole and drive a self-tapping screw into the hole with a drill and nut-driver drill bit to secure the wire to the cut-out. Use a butt connector to connect the servo's power wire to a 15-foot length of red wire.
Connect the other end of the red wire to a switch inside the car. There are many ways to mount and wire a switch, and various types of switches. No matter what kind of switch you choose or where you choose to install it, connect the power-wire extension to the power-out terminal on the switch. Connect the power-in terminal on the switch to a 12-volt power source such as the fuse box or battery.
