Changing the filter
Owners agree it is a more awkward job than on most cars. There are two ways to reach it.
From the wheel arch (works on both NA and Turbo)
- Jack up the nearside (left) side and take the rear wheel off. Jack at the side jacking point and lift the car so that both wheels on that side come up together. Support it on stands.
- Take the wheel arch liner out, or fold it back. One owner says just remove the front liner screws and fold it back. Another says the liner is the worst part of the job. Screws can spin in the clam, and one owner cracked a liner that way, so take care and have spare clips.
- The airbox is then in front of you. Undo the spring clips (four, from one owner's memory) or screws, lift the lid and swap the element.
From the engine bay
Owners say the NA filter can be changed from above with no wheel or liner removal. It is less easy on the Turbo. One owner who tried it from above found opening the clips manageable but closing them, especially the far one, difficult, and said he cut his hands. Others said they would rather take the wheel off.
How often
- One owner thought it was a 4th service or 40,000 mile item and said the service book will confirm. He changed his every year. See the service schedule.
- Another owner says changing the NA filter at 5k rather than 10k keeps the car happier. He added that the original filter size isn't important, it's how quickly it fills up.
- One owner says many cars have filthy filters and dirt in the box because the job is so awkward that owners put it off. It is a cheap part.
Which filter?
Standard paper
- The most common advice is to stay with a standard paper element. Owners say it gives the best filtration, and is cheap.
- One owner priced the paper NA element at about £4 (2016). A 2016 post pointed to GM part 90512447 for it, but that is a single post, so check it against your car before ordering.
- For the Turbo, one owner said the filter is the same as the Astra with the Z20LET. That is one post too.
Modified standard airbox with a paper element
- Owners recommend this for the best mix of filtration and flow. The usual modification is to open up the box and remove the pipes inside. Many also drill holes in the bottom of the box. Owners say it is fine on a Turbo.
- One owner reported that on a dyno, a standard filter in a modified airbox gave no difference from having no filter at all, even over 400 bhp. It was on a car that wasn't supercharged, and it is a single report.
- One owner warned that removing the ram pipe inside the box could slow the airflow and cost power. This is his view, and others advise the opposite.
- Another owner built a panel filter from two standard elements joined together and says cutting the pressure loss across the filter gave him a large power gain on a high-power car, along with larger intake pipes. His figure was his own guess.
Oiled foam or cotton (K&N, Pipercross, ITG and similar)
- For: they sound good, especially with an airbox mod, and some owners have run them for years with no trouble. A tuner is said to have told one owner a panel filter is fine.
- Against: several owners say they filter worse than paper and so wear the engine faster. Others point out you need to clean and re-oil them regularly, and that it is easy to get the oil level wrong.
- The air mass sensor. Owners disagree on whether oiled filters kill it. One owner says he saw a few sensor failures with one brand, but none with another. Another had used a big oiled cone for years with the original sensor still working. A common view is that the damage comes from over-oiling, or oiling a panel filter from the wrong side, so the oil coats the sensor. Follow the maker's instructions. If you have a Turbo and the sensor has failed, see MAF sensor faults.
- An owner who thought his air cone had killed his sensor was advised to choose a dry filter or the standard box before fitting a new one.
- The ITG panel filter was claimed (2004, one post) to give about 2 bhp, though others said a filter change gives no extra power.
Dry filters
An Apexi dry filter was recommended by several owners as having no oil to harm the sensor and a good sound, with some of them running it for five years or more. Buy a genuine one, as owners warn of fakes on eBay. One owner said it is probably not the best for flow. One owner of a supercharged car also used one.
Supercharged cars
Owners debated what to fit at 250 to 350 bhp and above. Suggestions included the large Pipercross cone, a Pipercross enclosed unit (one owner runs one with a 100 mm inlet and 80 mm outlet, said to be good for 400 bhp, and says it is quieter than a big cone), the Apexi, and the standard NA airbox modified. Power ratings quoted for each are the makers' or owners' claims, not tested figures. One owner claims the standard paper filter is good for over 500 bhp and filters better than aftermarket ones. That is one post.