SU theory on open air filters

There’s been a reasonable amount of conjecture here in recent times about performance air filters: whether they’re effective or not and what sort of filter is best.
These are my thoughts, the thoughts of a regular Saab owner with no technical expertise, but one who has had some seat-of-the-pants experience with both the stock air filter and an open-style performance air filter.
The Principle (according to me)
An engine is basically a big air pump that uses combustion as a means of turning the air into energy to drive the car. The more air you can push in there, the more energy (power) you can create at a given point in time and therefore, the faster you can go.
This principle is effected by the temperature of the air. Cold air is more dense than hot air so the more cold air you can get the better.
More important than air temperature is air filtration, as you don’t want to be pumping dirty air into your engine and causing damage to it.
So the ideal is to get more cold, clean air into your engine, thereby enabling it to make more power quickly.
Open air filters are attractive because they’re open. i.e. they’re not inside a box with a little pipe restricting the air feed. You have more ready access to a greater volume of air. BUT……. open filters are often compromised to some extent by the fact that they’re in the engine bay, where hot air resides. So whilst you’ve quicker access to more air, it’s often warm air and therefore not as dense (not as much pure volume) as cooler air.
Whether an open air filter is going to be suitable for you and your car might be effected by a number of things. The climate you live in, for example, could be an important factor. Someone living in a tropical region with higher temperatures and high humidity would probably do better with an enclosed air filter. In a colder, drier climate, your car could possibly cope a lot better with an open arrangement and the dynamic that it offers.
There are a number of thoughts about air filters, which I’ll deal with here. Again, this is just from a layman’s point of view, based on observed experiences both personal and 3rd party.
Theory #1 – The stock air filter is good for 300hp
I have no doubt whatsoever that this is an accurate statement. There are several things it doesn’t take into account, IMHO.
That statement, to me, says that the stock filter is inadequate for power ratings more than 300hp. It doesn’t tell me much about the performance characteristics of the stock filter below 300hp.
One of the things about making a mainstream production car is that it has to be durable and therefore, car companies go with ‘safe’ options. This is why your 9-3 makes 210hp when the engine’s quite capable of safely making 240hp (a-la the Hirsch remap output). When you boost the performance like that, you have to look after it a certain way. You have to use the right fuels, coolants, etc. Stuff the ordinary driver couldn’t be bothered with.
The fact that the stock air filter is good up to 300hp is reassuring, and safe, but it doesn’t tell me much about how it performs up to that level.
Theory #2 – open air filters don’t give any extra power
This may also be true. I’ve not dynoed my car in order to be able to tell. But I wouldn’t be troubled in the slightest if my open air filter didn’t boost my power output. That’s not why I bought it and it’s not why you should buy one, either.
The reason I was happy to get my open air filter was partly to do with……
Theory #3 – open air filters increase throttle response
I have no scientific evidence that this is the case.
But my own experience from owning a car that’s had both the stock air filter and an open air filter (fitted with a heat shield and run in a cool climate) is that yes, it does feel like the car responds quicker.
We’re talking about minute differences here. Spending a hundred-or-so dollars on an air filter is not going to take you from Driving Miss Daisy to The Rendezvous in one fell swoop. It’s an incremental thing.
But fitting the open air filter is something that I’m very pleased with for a number of reasons. The first of these is that I really do feel like it’s increased the throttle response in my car – however slightly. I’ll get to a few more reasons in a moment….
Theory #4 – Open vs enclosed
I find the performance-air-filters-are-bunk argument to be short-sighted and dismissive. If that’s the case, then a whole industry has millions of people, including some well-versed performance experts, completely duped. Maybe the more important question is “what type of performance air filter works best?”
I’ve been talking about the open type of air filter, like the one I’ve currently got fitted to my car.
There are others, though, one of which I’ve just put on order so that I can try it out. These are enclosed performance air filters that sit inside your factory airbox. They still have the cold air feed through a small pipe pointed away from the main area of the engine bay, but they have more free-flowing filtration properties that allow more air through the filter whilst retaining filtration quality.
This seems like the best of both worlds – free airflow of colder air whilst keeping the air clean. I hope that turns out to be the case.
Theory #5 – The intangibles
Those who are technically competent may giggle at this first intangible, but I can guarantee that this confession is not just my own. Other mechanically-challeneged people like me would feel the same.
It feels good to select, buy and then fit one of these performance air filters and then drive your car and feel a difference. It’s one thing that we dumb guys can do with minimal chance of stuffing something up. Add to that the fact that it seems to have a real-world effect, and it’s a win.
The second intangible is the fact that they look cool (and seem to have a real world effect – have I mentioned that enough yet?). If you’re into cars then it’s nice if you can have something that looks cool and does it’s job.
The third intangible is that the open air filter sounds absolutely awesome. Maybe this influences the perception that the car is responding quicker. I’m not sure, but it certainly adds a sense of theatre that I’ll miss if I don’t have access to it again in the future.
Put it this way – I live in a city full of hills, surrounding a big wide river. Those hills means that everyone gets a view of the water. Once you have access to that view on a regular basis, whether from your home or just driving around, it’s very hard to move to a city that doesn’t have the same pleasurable experiences associated with it.
Some have mentioned that using the stock airbox with a few holes drilled in the side will provide a similar aural sensation under boost. I’ll be trying a performance filter in the stock airbox soon and if it doesn’t deliver the noises I’ve grown to enjoy, then I might have to break out the Makita.
The bottom line
Do I have to say it again? These are just my own thoughts based on my own reading, conversations and personal experience. It’s not science.
But if you’ve got an open air filter and it’s performing well and you’re happy with it then you have my heartiest congratulations. Enjoy it. Love it, in fact. If it gives you more enjoyment from your car then that’s all that matters. Like me, you probably think that the car is responding better/quicker and there’s a very good chance that you’re right, especially if you’re in a cool climate.
If you haven’t got a performance filter and you’re happy, then congratulations to you, too.
This really is a horses-for-courses thing. The most important thing is that whichever way you choose to go, you should be happy with your car and you should modify it in a way that does no damage to the car.
Just look after the car and enjoy it.
My current air filter is an open BSR unit with heatshield, which I purchased from Elkparts.
Saab Air Filter fitting
I’m very happy with this setup except for a few vibration noises with the filter at certain airflow levels, which is a situation that isn’t common, but I believe is possibly to do with either my particular unit or my installation of that unit. I know of others with the same or similar units who are 100% happy.
The air filter I’ve just purchased and will try out soon is an enclosed performance filter from Maptun, that fits in the stock Saab airbox. I’ll let you know how that goes as soon as it arrives.

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