To get cold air to the engine with less effort I removed the rubber coupling that connects the lower air filter case to the duct that runs along the drivers side fender and comes out behind the headlight.  I can feel, or at least percieve a little more power, espesially at high RPMs.

The next step will be to enlarge that opening to allow a higher volume of air to enter the engine.  Then top it off with a K&N Filter.

But before I go chopping up the air filter housing I just wanted to test the waters on this; Is there some kind of formula to determine how much air is just right?  Are there any downsides to this modification? 

Is there some kind of formula to determine how much air is just right?

How much air is just right?  No offense, but that's a rediculous question when it comes to naturally aspirated engines.  If your car were turbocharged or supercharged, it'd be relevant, as you can almost literally control the amount of air going into the engine and too much boost can be bad, but on a naturally aspirated engine, the engine will only draw in enough air to reach 0psi of manifold vacuum/pressure.

To make power, you want to get as much air (and fuel) into the engine as possible.  To do this, you want an intake system with the least restriction possible (and your cars fuel system will compensate with fuel depending on how much air the engine draws in).

So to make the most power, get the largest/highest flowing filter possible, within reason (of course, once you go too high flowing, you compromise filtering capability; once you get to a certain size, going larger won't make any real-world power differences because your intake piping is only so large and your engine can only draw in so much air).

As an example supporting this, in the 300ZX community there was a huge debate on which [cone-style] intakes were better for the 300ZX.  The two that were argued about the most were the Apex-i Power Charger and the Jim Wolf Technology Pop Charger.  Apex-i's filters flow higher per square inch than any other filter on the market (including JWT), but the JWT cones were significantly bigger.  As a result, in dyno comparisons, the JWT filters put down 5-8 rwhp more on several stock 300ZX Twin Turbos.

So, get as large of a filter as you can, within reason, to allow as much air into the engine as possible.

Is this an 8V or a 16V? If you are only modifying the intake to the air box then theres little point in making it any bigger than the smallest area along the whole intake. On an 8V its about 45mm (approx. 1 3/4") and IIRC thats pretty much the size of the opening. Before you start cutting you could try removing the airbox cover and the filter and then going for a quick drive to see if there are any gains to be had.

No offense, but you should never recommend driving a car without an air filter on.  The smallest pebble can be the demise of a healthy engine.

Also, to say you don't need a filter larger than the smallest part of the intake is also erroneous.  The larger the filter, the less air restriction. 

The best way to show this is to think of a huge filter and a small filter.  The engine is going to try to draw in X cubic feet of air per minute, no matter what (that's a function of displacement and compression ratios).  With the small filter, the engine is trying to pull in X cubic feet of air through a tiny opening; more air is going in through a smaller space (a single square inch of the filter is responsible for more air flow).  The more air you have going through a small space, the more restriction there is (basic physics).  With the larger air filter, the engine is still trying to draw in X cubic feet of air, but each square inch of the filter is responsible for less air (since there is more surface area, each square inch does less work).  This means that there is less air restriction on a larger filter.  Restriction kills horsepower. 

Like I said in my example, there was a 5-8rwhp difference in horsepower between two filters of different sizes, even though the smaller filter flowed more air per square inch.  Had these two filters been the same size, the Apex-i would've undoubtably performed better than the JWT.  But that wasn't the case... each square inch of the JWT had less work to do because it was a much larger filter - as a result, the JWT performed better.  The intake piping on a 300ZX is only 2 inches in diameter... the JWT cone filter has an ID of 6 inches and a height of about 8 inches.

I just thought of a good example.  Try to breathe through a piece of gauze that is the size of your mouth.  You should notice the restriction (if you don't, make it thicker until you do).  Now make a cone of gauze that is several times the size of your mouth and the same thickness as the smaller "filter," hold it up to your mouth and breathe now.  It's going to be easier to breathe with the larger "filter" because you're sucking in less air per square inch. 

In an engine, the easier it is to breathe, the more power the car makes.

Also, to say you don't need a filter larger than the smallest part of the intake is also erroneous.  The larger the filter, the less air restriction. 

I didn't say that, read it properly. What I was saying was that there isn't any point in cutting up the airbox if there is a further restriction along the intake. You have to consider the whole of the intake. I agree that there is a risk in removing the filter and I wouldn't run a vehicle off road without one - or on the road for any length of time. Its a pretty low risk on road but its for the owner to decide. I'm well aware of how an engine works and I have in fact done a fair bit of dyno testing with different sizes and makes of air filter.

Rhino: What I was saying was that there isn't any point in cutting up the airbox if there is a further restriction along the intake. You have to consider the whole of the intake.

so your telling me one 2" restriction point justifys two or more restriction points of 2"?  I'll save the physics lessons for ZLove4Life but it seems to me that just each time the flow is restricted it takes more effort from the engine.  I just want to point out my culmitive automotive engine experience is summed up to replacing a few alternators and on occasion a fan belt or two, I certainly expect to be wrong 50% of the time at least.

I took her out on the highway for the first time since removing the coupling.  It's deffinitly helping my mileage, before doing 65-70MPH over flat ground would put me on the far side of 4500RPM, now I never got past 4300, and at 75MPH it used to be close to 5000, I only got just over 4500.  Although I can hear a whistling sort of noise from my engine at these speeds, I think it may be the restriction at the opening.

I'm going to take alittle of both of you advice, I'll try removing the lower filter casing and duct tape the filter to the upper air box (sealed properly along the seams to prevent that nasty pebble...)  Then take it on the interstate for a quick run, if the whistling is gone and/or I get another spike in mileage I'll begin hacking away at the intake.

The whistle is probably because you removed the silencer that keeps the airflow quiet. 

If you switched over to a short ram intake instead of modifying your filter, you would get the increase in airflow and remove the whistling noise.  And get a little more power to boot.  There is a few threads comparing the two and which one is prefered.  If your going to cut up your airbox anyway, I would just look at replacing the entire thing with a cone filter.

You can get one of the new synthetic filters instead of a traditional re-usable filter.  They clean easier and have better airflow then even the paper ones. 

Just go buy a Cheap Open Elemet Filter and color you want for 29.99 and go to Home depot, and made an adapter outta PVC pipe.. pretty cheap and simple,


I have made 6 in the past, all for 4.0 liter v6's, back when i had a Lifted Explorer, and a lowered ranger...

Doesnt take a brain surgury to figure this one out guys come on..