Stock air box

[WeeYari]

Originally Posted by NJ Drive

Does anyone know why Toyota uses a double filter system in the air box?
The filter that isn't removable and attached to the upper half of the air box seems redundant.

"While normal air filters serve the purpose of removing particulate matter, such as dirt or debris, from the air entering the air intake manifold, the normal air filter provides little help in preventing evaporative emissions from the intake manifold from entering the atmosphere. Such emissions are a particular problem when the engine is turned off and the intake manifold is hot. This source of air pollution has received considerable attention from the federal and state governments with the goal of reducing such emissions.

To that end, the California Air Resources Board (CARB) which effectively sets the maximum air pollution exhaust standards for all motor vehicles sold in the United States, has passed a regulation which requires that a hydrocarbon trap filter be connected in series with the air intake of most motor vehicle internal combustion engines.

Designs of such a hydrocarbon trap air filter has resulted in a flat-panel hydrocarbon trap filter relatively permanently secured to a downstream section of a conventional OEM closed air box, to inhibit tampering, with a conventional flat panel air filter releasably secured to an upstream section of the closed air box."

[CrankyOldMan]

Originally Posted by WeeYari

"While normal air filters ...

Designs of such a hydrocarbon trap air filter has resulted in a flat-panel hydrocarbon trap filter relatively permanently secured to a downstream section of a conventional OEM closed air box, to inhibit tampering, with a conventional flat panel air filter releasably secured to an upstream section of the closed air box."

^ This. It's not replaceable because it's not supposed to be removed, ever. That said, removing any *ahem* restrictions between the intake mouth and cylinders will improve airflow to the engine.

[NJ Drive]

Originally Posted by WeeYari

"While normal air filters serve the purpose of removing particulate matter, such as dirt or debris, from the air entering the air intake manifold, the normal air filter provides little help in preventing evaporative emissions from the intake manifold from entering the atmosphere. Such emissions are a particular problem when the engine is turned off and the intake manifold is hot. This source of air pollution has received considerable attention from the federal and state governments with the goal of reducing such emissions.

To that end, the California Air Resources Board (CARB) which effectively sets the maximum air pollution exhaust standards for all motor vehicles sold in the United States, has passed a regulation which requires that a hydrocarbon trap filter be connected in series with the air intake of most motor vehicle internal combustion engines.

Designs of such a hydrocarbon trap air filter has resulted in a flat-panel hydrocarbon trap filter relatively permanently secured to a downstream section of a conventional OEM closed air box, to inhibit tampering, with a conventional flat panel air filter releasably secured to an upstream section of the closed air box."

This is EXACTLY the explanation I needed. Thank you!!