Even the best aftermarket, free-flow, mega-power-gain filter isn't much good when it gets dirty. Why not? Cos a really dirty filter doesn't flow nearly as well as a clean one. One way around this is to replace (or clean) the airfilter every couple of weeks - but that's not much fun. If you're using factory elements it can also get pretty expensive - some factory filters are up around $80 each! So what you need to do is build this warning system that shows you when the filter's starting to lose its free flow status.
It'll take you only a few hours and cost less than $20 all-up.
How it Works
The Dirty Filter Alarm uses a pressure switch to detect when there is an excessive pressure drop across the filter element. When the switch closes, a warning light illuminates inside the cabin, indicating that the filter is restricting airflow.
But how does it work? If the filter is flowing without any restriction at all, there will be no pressure drop across it. But as the filter becomes more and more blocked, the pressure difference across the filter will increase - especially at higher loads. So if a pressure switch of the right sensitivity is plumbed so that it can compare the pressures in front and behind the filter, it'll trigger when the accurately measured filter blockage becomes too high.
The Bits
To make the system really useful, you need to use a pressure switch that clicks over even at low vacuums - in other words, one that is extremely sensitive.
And, up until now, that's been a real problem. Why? Because the only switch sensitive enough to work with high performance automotive filters cost over $65!
(Sure there are plenty of other vacuum switches designed to work as filter blocked alarms. But they're meant to be used on industrial machines and usually trip at about 0.9 psi vacuum - that's about 25 inches of water! And that's a helluva lot too much if you want a high performance free-flowing intake....)
However, AutoSpeed has managed to source a limited number of high quality vacuum switches that are available at an excellent price - just $14.95. Made of glass-filled polyester, the switch triggers at just a few inches of water vacuum - less than one-tenth of one psi! On one side of the switch is a nipple to take 5mm ID hosing, while the other sensing port takes the form of a small hole in the rear surface of the switch.
Mounting the Switch
Making it all happen is pretty easy. In addition to the vacuum switch you'll also need:
- A dashboard warning light
- A short piece of vacuum hose
- A rubber grommet that the hose will j-u-s-t push through
- Some high quality double-sided tape.
- Some hook-up wire
Your local auto parts place should be able to get you these bits without a problem. Add an electric drill, soldering iron, solder and some electrical tape or heatshrink to make things easier and/or neater.
The first step is to remove the section of airbox that leads to the engine. If the car runs airflow metering, it will be the part of the box with the meter attached.
Drill a hole in a wall of the airbox just big enough that you can push the rubber grommet into.
Insert the grommet and then push the hose through the grommet so it is flush with the inside edge of the grommet. If you want, you can apply a little glue or sealant to keep it in place, but if you've sized your components right, none should be needed.
The body of the switch is mounted on the other half of the airbox. On this part of the airbox find a flat, cool and accessible area that's large enough for the 25mm square switch to sit snugly against the surface. The spot should also be protected from water.
Drill a small hole in the airbox plastic, and then align the flat side of the switch so that the small pressure-sensing hole in the rear of the switch is directly over the top of the newly-drilled airbox hole. That's where the switch will sit. Clean the airbox surface around the hole with methylated spirits, then clean the back of the pressure switch in the same way.
Cut out a section of double-sided tape the same size as the switch, and stick it to the airbox over the hole that you have made. Carefully make a hole through the tape in the middle of the square.
Then carefully stick the switch to the airbox...
...again lining up the holes so that the switch can sense airbox pressure on this side of the filter through its rear port.
Re-assemble the airbox, cut the hose to length and connect it to the pressure-sensing nipple of the switch. Whenever the filter needs to be cleaned or changed, simply pull the hose off the switch (or out through the grommet) to allow the box halves to easily separate.
If you're fitting the warning switch to an aftermarket ram pod or the like, first take off the whole rampod. Drill a hole in the plumbing after the filter - eg on the rampod trumpet or in the adaptor tube on which the filter is mounted. Place the rubber grommet in the hole and then push the sensing tube through it. In this case, the rear sensing port of the switch can be left open to atmosphere - so simply mount the switch in a cool dry location nearby, making sure that its rear port isn't blocked.
Wiring
Wire up the warning system like this. Simple, huh? Mount the warning light somewhere on the dash where it's a bit out of sight - in this application you don't want it staring you straight in the eyeball.
Testing
With everything hooked up, go for a drive.
In gentle driving the warning light will stay off, but when you go for full throttle at high revs, expect the light to come on - even when you have a clean filter in place. That's because at full throttle and peak revs, even the best filter normally has a high enough pressure drop to trigger the light.
So how can you use the warning to tell you when to change the filter, then?
Easy.
As the filter gets dirtier and dirtier, the light will come on at lower and lower rpm.
For example, a clean filter may cause the warning lamp to illuminate at full throttle, 4500 rpm. But as the filter gets blocked, that will gradually drop. So you might decide that when the warning light is on at full throttle at all revs over 2000 rpm, it's time to change the filter. Or, if you have an exceptionally good filter - where the starting point with a clean filter might have been 6000 rpm - you could make your filter change point when the light starts constantly glowing over 4000 rpm.
Being able to see the full-load revs at which the filter pressure drop exceeds the switch-on point has other benefits, too. Say you're driving in the country and pass through a dust storm. Your big cold air intake picks up all of the dust and channels it to the filter. Suddenly, in a matter of hours, your filter warning light is coming on 1000 rpm earlier..... ooops that was a heap of dirt that you picked up - time to clean or change the element!
Real Results
One of the beauties of the system is that it gives a real-life guide to what's actually going on in the filter element. To show this, we did some testing on Dep Ed Michael Knowling's Commodore VL Turbo.
The starting point was an old genuine filter, one that had about 15,000km on it. Testing with the filter block alarm in place showed that the switch triggered at 2700 rpm, full throttle. (On my Audi, the switch triggers about 1000 rpm higher.)
Then we placed a brand new OE filter in the box. Amazingly, this filter was actually worse, consistently triggering the alarm at 2400 rpm! What was going on? Well, the new filter had black glue that had crept in around the edges of the filter during its construction, reducing its exposed area. And, of course, the newie also hadn't had several hundred thousand cubic metres of air pass through it, too. Perhaps the opening up of the tiny pores in the filter more than makes up for those blocked by the dust of this slightly dirty filter? We always knew that paper filters are exceptionally good at posing little restriction (even when dirty), but we hadn't seen the new versus old filter test done so accurately.
(Incidentally, manometer testing also supported these results. And yes - when we took the filter out of the box, the alarm wouldn't sound at all.... Zero restriction you see!)
Blocking off the better flowing (old!) filter gave these results:
- One quarter of the element covered over - light triggered at 2700 rpm
- One third of the element covered over - light triggered at 2500 rpm
- One half of the element covered over - light triggered at 2100 rpm
As you can see, as the restriction gets higher, the full-throttle rpm at which the light turns on gets lower and lower.
Conclusion
So there you go - a very cheap system that will always be monitoring your filter for even the tiniest restriction. Now you don't need to guess when the filter is restrictive. Instead, you'll actually know it!
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Comparisons
Another use of the filter warning light is for comparing different filters. For example, it lets you very easily see if an aftermarket filter element is less restrictive than the standard element that you normally use. How do you do that? Well, let's say that with a brand-new factory element the light comes on at 4500 rpm, full throttle. Pop in the aftermarket element and you find that the light doesn't glow until a full-noise 5000 rpm. That means it flows better....
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