If you've been scouring the forums looking for a way to wake up your VQ or KA engine without making it sound like a swarm of angry bees, you've probably seen a lot of people mentioning isr resonated test pipes. It's one of those parts that almost every 350Z, 370Z, or 240SX owner considers at some point because, let's be honest, the factory catalytic converters are huge, heavy, and they do a great job of sucking the soul out of your exhaust note.
But here's the thing: jumping straight to a set of cheap, straight-through test pipes is usually a recipe for instant regret. You'll get the volume, sure, but you'll also get that horrific metallic rasp that makes your car sound broken rather than fast. That's where the resonated version from ISR comes into play. It's designed to give you the flow of a straight pipe while using a built-in resonator to filter out the garbage frequencies that nobody wants to hear.
The Battle Against Exhaust Rasp
If you've ever heard a G35 or a Z with a cheap exhaust setup, you know exactly what "rasp" is. It's that high-pitched, tinny rattling sound that happens right as the RPMs climb. It's the enemy of a good sounding car. The reason it happens is that when you remove the factory cats, you're losing a massive amount of dampening. The exhaust pulses are hitting the walls of the thin metal piping, creating those harsh vibrations.
The beauty of isr resonated test pipes is that they include a small, perforated core wrapped in packing material—essentially a mini-muffler—built right into the pipe. This doesn't really restrict the airflow much, but it does a fantastic job of catching those high-frequency sound waves. The result is a deeper, much more "mature" growl. You still get the volume increase you're looking for, but it sounds deliberate and tuned rather than just loud for the sake of being loud.
Build Quality and What's in the Box
ISR Performance has a bit of a reputation for being the "bang for your buck" brand. They aren't trying to be a high-end boutique shop that charges $800 for two pieces of pipe, but they also aren't some nameless eBay brand using mystery metal. These test pipes are usually made from high-quality SUS304 stainless steel. They look good, the welds are consistent, and the flanges are thick enough that they won't warp the first time they get hot.
When you get your hands on them, you'll notice they feel substantial. The gaskets and hardware included are generally decent, though some guys prefer to swap in OEM Nissan gaskets for that extra peace of mind. One thing that's really nice is that ISR includes the O2 sensor bungs, and on most models, they're extended or angled to help keep that pesky Check Engine Light (CEL) away. It's not a 100% guarantee—modern ECUs are pretty smart—but it definitely helps.
The Installation: Get Your PB Blaster Ready
Look, I'm not going to lie to you and say this is a thirty-minute job. If you're installing isr resonated test pipes on a VQ-powered car like a 370Z or a G37, you're about to meet the infamous "demon bolts." These are the bolts that connect the catalytic converters to the headers. They are notoriously difficult to reach and usually rusted shut from years of heat cycles.
You're going to want a lot of extensions, some swivel joints, and a massive amount of patience. However, once you get the old cats out, the ISR pipes bolt right up. The fitment is actually surprisingly good. You don't usually have to worry about the pipes hitting the crossmember or the heat shields, which is a common headache with some of the cheaper knock-off brands. If you're doing this on a 240SX, it's a much easier process, but the same rule applies: soak everything in penetrating oil the night before.
How Does It Actually Feel?
Once they're on and you've cleared the smoke from your fingerprints burning off the pipes, the first thing you'll notice is the throttle response. Catalytic converters are basically honeycombs that the exhaust has to fight its way through. By switching to isr resonated test pipes, you're giving that air a straight shot out of the engine.
The "butt-dyno" will definitely feel a difference in the mid-to-high range. The car feels like it breathes easier, and that sluggishness you sometimes feel when flooring it at 3,000 RPM tends to disappear. On a naturally aspirated setup, you might gain anywhere from 5 to 10 horsepower depending on your other mods. If you're tuned, those numbers go up. But even without a tune, the way the car pulls through the rev range feels much more linear and aggressive.
The Sound Experience
This is why most people buy them. With the isr resonated test pipes installed, the idle stays relatively civil. It's a bit deeper, maybe a little throatier, but it won't wake up your neighbors at 6:00 AM. But when you get on it? That's where the magic happens.
The tone is much more "hollow" in a good way. It sounds like a race car. Because the resonators are doing their job, you get to hear the actual character of the engine rather than just the vibration of the pipes. If you have a cat-back exhaust already, these test pipes are going to amplify whatever sound that exhaust has. If you have a quiet exhaust, it'll make it medium-loud. If you have a Tomei Expreme Ti or another single-exit cannon, well prepare to be the loudest person at the meet.
The Smell and Other Trade-offs
We have to talk about the elephant in the room: the smell. When you remove your catalytic converters, your car is going to smell like raw exhaust. If you're sitting at a red light with the windows down, you're going to smell it. If you're backing into your garage, you're definitely going to smell it. For most enthusiasts, it's just part of the "race car" experience, but if you're planning on taking your grandmother to church in this car, she might have some questions.
Also, it's worth noting that these are technically for "off-road use only." Depending on where you live, you might not pass emissions with these on. In some states, it's a visual inspection failure; in others, the sniff test will get you. Most guys just swap their stock cats back on once a year for the inspection, but it's something you should keep in mind before you go tossing your old parts in the scrap bin.
Why Choose ISR Over Others?
You've got options when it comes to test pipes. You could go with high-flow cats, but those are expensive and still restrict some flow. You could go with non-resonated pipes, but you'll probably hate the sound after a week. Or you could go with a high-end brand like Motordyne, which is fantastic but costs twice as much.
The isr resonated test pipes sit right in that sweet spot. They provide about 95% of the performance and sound quality of the top-tier brands for a fraction of the cost. They're durable, they fit well, and they solve the biggest problem of the VQ and KA platforms: the rasp.
Final Thoughts
If you're ready to take your build to the next level, adding a set of isr resonated test pipes is probably one of the most satisfying "bang-for-your-buck" mods you can do. You get the power, you get the aggressive sound, and you don't have to deal with that embarrassing trumpet-like noise every time you hit the gas. Just make sure you're ready for a bit of a fight during the install, keep your old cats just in case, and get ready to fall in love with your car's exhaust note all over again. It's a transformation you'll hear (and feel) the second you leave the driveway.