Every aftermarket intake box on the shelf promises somewhere between 15 and 30 horsepower. We have dyno'd enough of them in Charlotte to say plainly: on a stock, unturned car, the honest number is usually 3 to 8 wheel horsepower, and it lands almost entirely at the top of the rev range where you rarely drive.
That is not a reason to skip an intake. It is a reason to buy one for the right job. On a turbocharged platform running a Stage 1 or Stage 2 tune, the factory airbox becomes a genuine restriction, and a well-designed intake is worth real power because it lets the tune do what it was written to do. On a naturally aspirated commuter with the factory map, you are mostly buying induction noise.
The design details that separate a good intake from a loud one: a fully enclosed airbox or a heat shield that actually seals to the hood, a smooth mandrel-bent tube with no sharp step at the MAF housing, a MAF bore that matches the factory diameter so the sensor scaling stays honest, and a dry filter if you drive through Carolina pollen season. Oiled cotton filters work fine but they will contaminate a hot-wire MAF if you over-oil them at service time.
The failure mode we see most often is the open-cone intake sitting in the engine bay with no shield. On a dyno with a fan blasting the bay it makes great numbers. In traffic on a 95-degree August afternoon it pulls in 150-degree air, intake air temperatures climb, and the ECU pulls timing. The car is measurably slower than stock in exactly the conditions where you would notice.
Our recommendation for most builds: pick a sealed-box intake from a brand that publishes its own dyno methodology, install it at the same time as your tune, and log intake air temperatures for a week before and after. If IATs stay within about 15 degrees of ambient at cruise, the install is doing its job. If they do not, the problem is the heat shield, not the filter.
