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Aftermarket Intake Users, Stock Airbox Flow

5310 Views 10 Replies 5 Participants Last post by  Merc6
Let me start by saying I completely understand that an aftermarket intake will flow better than the OEM unit will, especially when fitted with a less restrictive filter.

I get that.

I was curious to see how well the stock intake flowed, but how to measure this? Well, it is quite common in vehicle fleets to see filter monitors installed. These are devices that measure how much vacuum is present in the vehicle's intake system after the filter. The more vacuum measured after the filter, the more restrictive (plugged) the filter has become.

As time goes by, air filters get plugged up with dirt and debris. The more dirt and debris, the less air it will pass for a given amount of vacuum from the engine. Less air to the engine, less power.

Most commercial air filter monitors consider a vacuum level of 25" H2O (water) to be the point where the filter should be changed. To visualize this, consider a hose connected to the air filter box behind the filter and the other end left to hang down into a bucket of water. The engine, operating at maximum power, would be drawing a vacuum in the intake sufficient enough to lift water 25" up the hose above the water level.

Here's an example of a popular 25" H2O filter monitor, sold by Wix: Wix 24800 Air Filter Monitor Kit: Automotive

The higher the vacuum level the farther towards the red area that yellow plunger is pulled. The plunger stays put at its "peak" vacuum reading and can be released by pressing the button on the bottom of the unit.

I decided to take a chance. I wanted to know how restrictive the Cruze intake was with a relatively clean air filter.

K+N, makers of high flow air filters (as most of you know) sells a filter monitor that recommends a filter change (or cleaning) when restriction gets to 10" H2O, much less than the 25" H2O commercial units. This makes sense since aftermarket performance intakes SHOULD flow better, and should be maintained that way.

Here's the 10" H2O monitor sold by K+N:

And here it is installed on my air filter housing:

For the record, my intake is 100% completely stock. No tube delete, resonator still in place, everything exactly like it was when the car left the assembly plant... including the almost three year old air filter. Stock tune.

I drove around for a couple of days, making sure to do a few 2nd gear pulls to redline. I popped the hood and saw this:

Yep, a 1.4T with a stock intake and a three year old air filter is only showing 5" H2O vacuum, about HALF (50% on the monitor) of what K+N recommends as maximum restriction for a high flow after market intake system.

Like I said, I'm not debating whether an aftermarket intake flows more air, it absolutely does. I'm just pointing out that the stock intake flows much better than most people will give it credit for.

If I decide to modify my intake in the future I will provide updates as to whether anything changed for the better. Also, the fact that it's winter time may have an effect also. We'll see if warmer temperatures change things.
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Looks like what the old factory silverado airbox has on it. Have reset more then I can think of lol. Nice writeup tho. And so is this on the factory paper filter?
Yeah, these are very common on trucks since they often operate in dirty environments and end up with plugged filters much sooner than the average car.

Yes, this is with the stock filter, untouched. It still has the foam "pre-filter" pad attached and has been filtering my engine's air since the spring of 2012. Mind you, I don't typically drive in dirty environments and my filter still looks pretty clean. It's nice to have this minder on there to know for sure it doesn't need changing.

Which brings up another good point. Even though this filter minder costs about $30, if it saves you the cost of changing filters prematurely it will likely pay for itself. I was planning to put a new filter in this spring but I'll hold off now... maybe I'll get 4-5 years out of this air filter?
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It'd be a little interesting on how different filters would read on that thing. Prob wouldn't make much of a difference if any at all and be a waste of $ just kinda wonder 1 paper really is better then another? Lol I don't know just rambling...
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The air filter in this car is very over sized for an engine this small, think its double the size of the one in my 3800 v6. I swapped out my air filter at 35K and 60K at the same time as my cabin filter, but it was not necessary. figured I had the air box apart to check it might as well put in the new one.

GM recommends 45K air filter maintenance, which may even be low for some considering what the filter looked like at 30-35K in my car.
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Ah yes, I should add my mileage... The car has just over 30k miles (50k kms) on it, so it's not exactly a high mileage car.
It'd be a little interesting on how different filters would read on that thing. Prob wouldn't make much of a difference if any at all and be a waste of $ just kinda wonder 1 paper really is better then another? Lol I don't know just rambling...
I'm kinda interested in finding that out myself too
Only problem with testing different filters with this is it would not really tell you anything useful. What I mean is we know a stock air filters filtration is far superior to a K&N, which means the K&N would always show less vacuum since its always less restrictive. K&N even claimed in the past their filters flowed more air dirty than a stock paper filter did brand new, can assume this claim could easily be seen with this installed.

Just remember this is only going to measure how restrictive the filter is, not how well it filters the air because of it.

Blue Angel, before you swap out to a new filter I would love for you to test removing the foam pre-filter from the stock filter. I did this last summer on my previous air filter and it really seemed to gain allot of power for such a simple change. Guessing it would lower the vacuum to some measurable degree.
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Good points spacedout.

There are TONS of things I would like to experiment with now that I have that gauge installed (I think I'll just refer to it as a vacuum gauge from now on). Bypassing the intake resonator for example... how effective is that really? Different filters? Pulling the foam prefilter? Maybe there's enough restriction in the resonator and snorkel that the filter choice doesn't make a difference unless they're bypassed?

I doubt I'll be investing in an aftermarket filter unless I see some reason to do so, but at least I'll have some sort of educated guess going into it now.
Update: I was under the hood the other day and noticed the vacuum gauge reading had climbed to between the 50% and 75% lines. Is this due to warmer weather? A rare moment of higher than normal airflow? I don't know yet, but I'll keep updating with any relevant findings.
I replaced mine with STP one after my old ole was visibly coated in pollen and other things. I been running no foam pre filter since a month of ownership.
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