Why Upgrade Your 9th Gen Civic Si with the AEM 21-678C?
Because the K24 wants to breathe, rev, and make glorious VTEC noises — and that factory intake is holding it back like a helicopter parent.
With the 21-678C, you get:
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Better airflow than the restrictive OEM airbox
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Sharper throttle response throughout the rev range
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Stronger high-RPM pull where the Si actually lives
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Deep, aggressive induction sound (yes, VTEC gets way louder)
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Perfect MAF scaling thanks to a properly engineered tube
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A gunmetal powder-coated aluminum intake that actually looks good
This intake is a staple mod in the 9th-gen Si community for a reason — it’s reliable, it works, and it pairs beautifully with exhausts and Hondata tunes.
AEM Air Intake Systems are designed to produce massive horsepower and torque gains as well as better engine sound when used as a replacement for the factory airbox and air filter.
Key Features of the AEM 21-678C
🌀 Mandrel-Bent Aluminum Intake Tube
Smoother airflow and a precise diameter to keep MAF readings stable.
🧼 AEM Dryflow Filter
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Washable, reusable
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No oil required (MAF-safe)
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High-flow design with excellent filtration
❄ True Cold Air Placement
AEM routes the filter behind the bumper where the Si pulls the coldest charge temps.
🔧 OEM-Plus Installation
Designed to use factory mounting points and sensor locations.
🎧 VTEC Symphony Enhancement
The intake sound alone is worth the price of admission.
AEM intake systems are designed, tuned, and dyno-tested to fit specific cars and trucks. These air intakes usually include an oversized AEM Dryflow synthetic washable air filter for outstanding air flow, filtration, and performance while allowing easy oil-free cleaning.
Performance Gains for the 2012–2015 Civic Si
With the 21-678C installed, owners typically notice:
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Faster throttle response
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Improved mid-range torque
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Stronger top-end power
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Much louder induction noise when VTEC engages
Combine it with a cat-back and Hondata FlashPro tune, and the Si wakes up dramatically.
Most AEM intake systems use mandrel-bent aluminum intake tubes, powder coated heat shields, and high quality hardware designed to reduce restriction and reduce intake temperature.
Daily Drivability
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No tune required (though recommended for maximum gains)
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Works flawlessly with factory sensors
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Dry filter reduces maintenance headaches
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Perfect for street-driven Si builds
This intake gives you character and performance without compromising daily reliability.
The design and components of each intake system vary so consult the installation instructions for more information on the contents of each part number.
Installation Notes
Installing the 21-678C usually takes 1–2 hours with basic tools.
You’ll:
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Remove the OEM airbox and inlet plumbing
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Route the intake pipe into the bumper area
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Mount the heat shield and Dryflow filter
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Reconnect the MAF and clamps
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Double-check all clearances
If you can change your own oil, you can install this.
AEM manufactures a variety of intake kit styles including short ram intakes, cold air intakes, dual-chamber intakes, Brute Force intakes for trucks and SUV’s, and Brute Force HD intakes for diesel applications.
Zach Rennie –
i have an 08 GSR and the stock tune was really rich, so getting a CAI helps lean out the AFR. Box is nice, better protected, and still produces that addicting whooshing sound from the extra air flow. without a tune I’m getting about 2 mpg’s better when i am behaving. By the way, if you are ever buying a part for your car, alwayyyys do a bit of extra research to make sure everything is what you want. Forums had this stuff everywhere. For $255 shipped, can’t beat it. install was tough because of some of the evo’s piping, but got it done in about 2 and a half hours and it was my first install for an intake. great product for a great car.
Cadence –
This cold air intake system is just perfect for my needs. I originally was interested in the concealed filter/powdercoated heatshield, but have come to realize that It’s worth the price for the bolt on simplicity of it, and is said to add about a 16-20 HP gain @ 4100 RPM (depending on your tune).All in all, I like the way the unit looks, it’s very aesthetically pleasing.The install was pretty average. Hate with a passion the rubber grommet material used to isolate the air box/intake module. It’s not flexible and sits poorly. That’s the only complaint I have, so far.
Amazon Customer –
Fantastic! All hardware was included in this kit for a complete install. Great quality product. Perfect fit on my evo x. Looks like it came from factory. Will highly recommend. Also it did not throw a check engine light. AEM did their homework on this one!
Julio A M Neto –
AEM is a really well know brand and this intake is what 9/10 Evo owners have on their X.There is around 20% more air inflow on the car, so you’ll notice a little more economic car after retuning the system and it’s totally worth for the price and added HP (around 20hp) and torque.The only downsite (less 1 star) is because of the plastic joint. The original Evo air entrance is supposed to fit this but the size for the hole is like 2mm bigger than it should be so it keeps getting separated from the original entry. Works fine on the beginning but then it comes off after some time.Even though, totally recommended! The best one out there
Patrick Buckles –
The design is kinda crappy! Why wouldn’t they just make the intake pipe one piece? It gives the results you want, and it looks great, just disappointed in the design of the piping. Guess that’s why I got it $230 instead the AMS $430
Jorel –
Great quality. I conducted a lot of time researching this and other similar products and by far this is the best one i found. Was delivered faster than expected.
Noturavgguy –
Instructions sucked. Eventually Got it installed and all worked as intended. It’s made for replacing OEM. Unfortunately, I didn’t have a stock box on my Evo. Had to custom make some parts to get it to fit. If you don’t have the stock box or mounting parts, it will be challenging.
Cameron Watson –
Works perfect and had everything packaged perfect. Now my Evo X can breathe how it should!
Dennis –
Teile nicht vollständig!
Es fehlt eine Dichtung ohne diese ist eine Montage nicht möglich!
Emiliano –
Tutto okay
Montato su Evo X (2008), tutti i tubi sono di lunghezza giusta, all’interno del kit c’è quasi tutto quello che serve. Peccato che siano necessari tutti i supporti originali, quindi nel mio caso in cui nella macchina c’era già installato un filtro a cono ho dovuto ricomprare i pezzi oem.