Boost gauge = air filter health-o-meter?

[tonydca]

So I'm downstairs blowing plenty of dust and crap out of my air filter with my compressor (Blow it from the inside out; backwards through the filter element. Works pretty well, BTW) and I'm wondering to myself "Self, did I really need to do this today?" Lots of forum comments about sluggish performance possibly caused by clogged air filters. How can I check if this is really the case with me at the moment?

And I got to thinking, I installed a $25 Princess Auto boost gauge, measuring the intake pressure downstream of the turbo and air filter (obviously).

When I step on the gas pedal, the gauge swings up fairly promptly to its normal 11 psi (or so) where it sits obediently while the waste gate dumps off any excess.

I'm wondering this - if a dirty air filter is starving the engine of air, wouldn't I be showing less than 11 psi? Or have a hard time reaching that point?

It seems to me that as long as you are getting full boost from your turbo, then the air filter must be pushing through enough air.

So if the inside surface of the filter is reasonably clean and you are getting your normal boost reading, no need to $$pring for a replacement air filter just yet.

Yes?
No?
Thoughts?

[rezdiver]

blowing compressed air through air filter is usually not good on a paper filter, you can easily damage the paper element. usually a good tapping and shaking should take care of it.

the turbo will give you boost pressure irrelevant of the filter quality as that is a function of the fuel mixture and exhaust speed and temperatures, unless your filter is totally plugged. you may have the turbo working hard to provide pressure which you can read on the gauge, but this does not indicate if the flow is getting restricted on the low side.

its just like your fuel filter. the injection pump is still going to push the required high pressure fuel to your injectors but that doesnt mean your dirty fuel filter is not causing a resitriction making the suction side work harder reducing the flow.

pressure does not equal flow.

[tonydca]

the turbo will give you boost pressure irrelevant of the filter quality as that is a function of the fuel mixture and exhaust speed and temperatures, unless your filter is totally plugged...

pressure does not equal flow.

Not sure I agree with your logic there.

True, pressure is not necessarily flow, but at the point where the pressure is being measured, there is no restriction between there and the engine. The air filter/turbo is upstream of that point, so it's not part of the equation any more. The turbo might have to spin faster as the filter starts to clog up, but that's not the point I'm trying to make -

I'm still getting a consistent 11 psi when I'm on throttle, and that's all the rest of the engine cares about. So if the performance is sluggish, it's not because it is not getting enough air, so changing the air filter won't make it run better. Might just let the turbo spin a bit slower to push through the same amount of air.

I'm not saying that changing filters is *bad*, just not a performance cure if the boost gauge already shows full operating pressure.

[rezdiver]

edit:
you are correct that a low boost indication can be caused by a dirty air filter. but it has to be a very very dirty filter. i think you will lose milage and performance from a dirty air filter before this will show up on your boost gauge.by this time your engine is running harder, hotter, with greater fuel consumption to give you the same boost.

[tonydca]

Why? The only thing working harder might be the turbocharger, and I can't think that'd have such a huge impact on mileage since it's driven from the exhaust gasses that're on the way out anyways. Maybe causing slightly higher back pressure in the exhaust manifold, but not much I'd imagine...

The rest of the engine will work the same if it's getting its downstream 11 psi through an upstream clean filter or a dirty filter or a bottle of compressed air through a regulator.

[jessef]

I will get full boost with clean paper filter and an insanely muddy, dirty filter. Same boost 14psi with horrible lugging on the throttle (dirty mucked up filter).

This is at the gauge. With the turbo suffocating, I'm not so sure the 'actual' boost is accurate between them.

[psilosin]

I'm wondering this - if a dirty air filter is starving the engine of air, wouldn't I be showing less than 11 psi? Or have a hard time reaching that point?

A harder time time anyway. On both the L300 and L400 I believe base fuel metering is based simply on throttle position (not including boost comp). Since your air filter is clogged you will end up applying more go pedal to get the engine spinning at the same rpms to make the boost. Since it is being starved for air at the given throttle position compared to normal there is excess fuel from optimum being delivered and this results in poorer ecconomy (and increased smokey exhaust) and higher temps.

This might not be noticable via observing the boost gauge as factory boost setting are reached pretty easily and nearly instantly at a fraction of full throttle.

[92pajero]

If every one is worried install a "filter vacuum" gauge on the filter housing (after the filter and before the turbo). You see them on big rigs and forklifts. This lil gauge will let you know when to clean your filter. It has a viewing window and a colored slider that locks into place to register the worst situation. If you see yellow its dirty if its red clean it.

[Raule Duke]

Pretty sure you can buy these at lordco. They come factory on the ford power strokes

[EnviroImports.com]

if you are really wanting to know how much boost you are getting from your turbo, or more importantly, to your motor, you need to take off the egr valve, put on a blanking plate with a welded bung/fitting into the plate that goes into your intake manifold put the boost/pressure gauge there, its right in front when you look at your motor, and very easy to do. measureing off a accessory tube that may or may not be quality or 15+ year old tubing is not a great place for getting a REAL reading, the only real reading and ideal place to check this is direct from your intake manifold, after all , if the turbo puts out 15 psi, and your manifold only gets 8psi you wont know it if your reading from a crappy old rubber tube.
air filters should be checked and kept as clean as possible, but no matter if you have a 20 boost gauge or a 200 boost gauge , if your base line for the reading is faulty then the rest of the info does not matter and you can rely on it to make changes or know whats really going on .
$.02

[EnviroImports.com]

If you want to measure your air flow from your air filter, put in an air filter meter, you can get them on most heavy duty trucks, easy to plumb in and easy to read.

[madmazda]

There are two ways to see how pluged your air filter is first.. the easiest way and cheapest is to open your hoot take filter out look at filter to see if there is a ton of crap on it... second some how hook up to vacume guages before filter and after look to see if there is any differantial in vacume will only be a bit of in-hg ( or inches of mucury because unlike pressure which is measured in psi-psig, suction is measured in in-hg) I like the first one my self