If you're looking to wake up your K24, installing a full race downpipe 9th gen si is probably the single best bolt-on mod you can do for the money. If you've spent any time on the forums or in Facebook groups, you know that the 2012–2015 Civic Si is a bit of a polarizing car. People love the torque of the 2.4-liter engine, but they miss the high-revving craziness of the old 8th gen. The biggest thing holding the 9th gen back from the factory isn't the lack of displacement; it's how the car breathes.
Honda did something interesting with the K24Z7 engine—they used an "integrated exhaust manifold" or a "headifold." Instead of having four separate exhaust runners coming out of the head that you can swap for a nice long-tube header, everything merges into one single exit right at the cylinder head. This makes the downpipe the most critical piece of the puzzle. If that pipe is restrictive, your engine is basically trying to exhale through a straw. That's where the Full Race piece comes in.
Why the Stock Setup Sucks
The factory downpipe is a chunky, heavy piece of hardware designed to keep the car quiet and the emissions ultra-clean. It features two catalytic converters and some pretty narrow piping. For a daily driver, it's fine. But for anyone who actually wants to feel the VTEC kick and hear the engine sing, it's a massive bottleneck.
When you swap out that restrictive stock unit for a full race downpipe 9th gen si, you're instantly clearing the path for exhaust gases to escape. Because the 9th gen doesn't have a traditional header, this downpipe is essentially doing the job of both a header and a downpipe. It's the first thing the exhaust hits after leaving the head, so the quality and diameter of this specific pipe matter more than almost anything else in your exhaust system.
What Makes Full Race Different?
There are plenty of brands out there making parts for the FB6 and FG4 chassis, but Full Race has a reputation that's hard to beat. Their downpipe is a full 3-inch (76mm) design. A lot of people worry that 3 inches is too big for a naturally aspirated car, thinking they'll lose backpressure. But here's the thing: you don't actually want backpressure; you want exhaust velocity. On the K24Z7, the 3-inch diameter has been proven time and again to produce the best power gains across the entire rev range.
The build quality is also top-tier. They use T304 stainless steel, and the welds are honestly like art. One of the best features is the v-band connection. If you've ever wrestled with traditional two-bolt or three-bolt flanges that always seem to leak or blow out gaskets, you'll appreciate the v-band. It's a secure, leak-free seal that's way easier to take apart if you ever need to work on the car later.
The Sound: No More Rasp
Let's talk about the sound, because that's a huge factor for most Si owners. The 9th gen can sound "raspy" or "tinny" if you pick a cheap downpipe. You know that sound—the one that makes a $25,000 car sound like a lawnmower.
The full race downpipe 9th gen si has a much deeper, more refined tone. It's definitely louder than stock—don't get me wrong—but it's a "mature" loud. It gives the K24 a growl that sounds expensive. When you're just cruising on the highway, it's relatively tame, but once you cross that 5,000 RPM mark and VTEC engages, it screams in the best way possible. It pairs perfectly with the Full Race 3-inch cat-back exhaust, which is widely considered the best-sounding setup for this generation of Civic.
Real-World Power Gains
You're probably wondering about the numbers. On a bone-stock 9th gen Si, just bolting this on might get you a little bit of a bump, but the real magic happens when you tune it. With a proper downpipe and an intake, most guys are seeing gains of 15 to 20 wheel horsepower. That might not sound like a huge number on paper, but it's the area under the curve that matters.
The mid-range torque increase is what you'll actually feel when you're merging onto the freeway or pulling out of a corner. The car feels lighter on its feet and much more responsive. The "rev hang" that plagues the stock 9th gen also becomes much less noticeable once the engine can breathe properly, though you'll still want a tuner to completely kill that off.
The Installation Headache (And How to Handle It)
I'm not going to lie to you: installing a downpipe on a 9th gen Si can be a bit of a pain in the butt. It's not that the part doesn't fit—the Full Race fitment is actually spot on—it's just that Honda tucked everything way back against the firewall.
You're going to be working in a tight space. You'll definitely want to soak the bolts in PB Blaster or some kind of penetrant a day before you start. The heat shields are usually the most annoying part to remove. There are a bunch of small bolts holding them on, and they always seem to be in the most awkward spots.
But once those shields are out of the way, the full race downpipe 9th gen si drops right in. They include a defouler for the secondary oxygen sensor, which is supposed to help keep the Check Engine Light (CEL) away, but your results may vary depending on how sensitive your ECU is.
Do You Actually Need a Tune?
Short answer: Yes. Long answer: Absolutely yes.
If you run a 3-inch downpipe without a tune, the car is going to run lean. The ECU can only compensate so much for the massive increase in airflow. Plus, without a tune, you're almost guaranteed to get a CEL for "Catalytic Efficiency" because you've removed the heavy-duty stock cats.
You really need to budget for a Hondata FlashPro alongside this mod. Once you have the FlashPro, you can recalibrate the fuel maps to account for the downpipe. This is where you actually get your money's worth. A tuned 9th gen with a Full Race downpipe feels like a completely different car than the one that rolled off the showroom floor. It feels like the car Honda should have built from the factory.
Longevity and Build Quality
A lot of guys buy the cheapest downpipe they can find on eBay, only to have the welds crack six months later or the piping start to rust out. The Full Race unit is built to last the life of the car. Since it's made from high-quality stainless, it handles heat cycles much better than cheaper mild steel or low-grade stainless options.
It also looks great. Even though it's tucked away behind the engine where you can't see it most of the time, knowing you have a high-quality part under the hood gives you some peace of mind. The thick flanges mean it won't warp over time, which is a common cause of exhaust leaks in high-performance applications.
Final Thoughts
If you're serious about modding your Civic, the full race downpipe 9th gen si is pretty much the gold standard. It addresses the biggest weakness of the K24Z7 engine and sets the foundation for every other mod you might want to do down the road. Whether you eventually go with a bigger intake, an RBC manifold swap, or even a supercharger, this downpipe will be able to handle the flow.
It's one of those rare parts where you actually get what you pay for. It sounds great, fits well, and provides a noticeable jump in performance. Just make sure you have your FlashPro ready to go and maybe some extra band-aids for your knuckles during the install. It's a weekend project that you definitely won't regret once you hit the road and hear that first VTEC crossover.