ehall

Out of curiosity, does the size of the filter matter, as long as it flows/passes air as well as a different sized filter; either smaller or larger? Discuss.

Dougs951S

volumetric flow is volumetric flow, I would think. so long as it can keep up with the engine's air demands i dont think size or shape matters.

ehall

That's what I was thinking, but some stuff on other boards, appears to say that it matters. So, I still don't know. If your engine is getting the air it needs, the only difference, to me, would be cleaning intervals. But, I don't know from a science stand point.

ttAmerica RoadsterAWD

Yes, make sure it's clean. Not only will it work well, but you will avoid premature wear of your turbo!

I recently looked into the Lindsey Racing MAF2 setup and I can clearly see the blades of the turbine marred-like it had spun up against metal. I'm sure it's the micro silica and other debris sucked in by a dirty filter.

If you have a similar setup, next time you take off the filter to clean, take a peek inside. You'll have to get on your knees and look up into the setup and you can see the turbine blades.

Interestingly, though mine look chomped up it still spools up well and get good pressures under boost.

Paulyy

id say yes, it can matter. You can measure if it's to big or to small, by putting a very accurate vacuum gauge just after the filter and seeing what vacuum it pulls under WOT. more vacuum the more resistance there is trying to get air into the system.

That's really if you're trying to get every bit of HP

ehall

All good points, but I guess the real question I'm asking, is will, for example, a 5 inch long x 6 inch wide K&N allow a better flow rate, to the engine, than a 4 inch long x 4 inch wide K&N filter? If so, why? I guess if not, then it doesn't matter. So all things being the same, except for size, is there any important difference, in performance?

refresh951

It is about sizing the filter properly. Putting a giant filter on a stock car will not make a meaningful difference, put a stock 2.5 filter on a 3.0L and it makes a difference.

http://www.knfilters.com/filter_facts.htm

TurboTommy

Well, the larger the filter, the more air it's gonna flow, of course.
But, like somebody already said, you only have to provide a filter size big enough for the engine demands. Go smaller, and the turbo will have to start making boost from less air, costing power. And a bigger filter than needed won't give you any more performance.

For 350 crank horsepower the engine needs roughly 500 cfm of air at sea level (higher elevation needs more cfm for the same power).

Somewhere on the K&N site it gives a simple formula to calculate the cfm that various sizes of their filters will flow.
I believe it is: surface area x 6 = max cfm before pressure drop will occur.
(You'll have to use some simple math, including pi and whatnot, to get the surface area)

So, in the examples above, a 4 inch by 4 inch will flow 300 cfm; a 5 inch by 6 inch wide will flow 565 cfm. (They will flow more, but at the cost of pressure drop.)

So, an obviously too small filter will cost power.

Tom M'Guinn

Use the biggest filter that will fit The filter can't flow too much, but it can flow too little. It's not something you'll notice, but horsepower can add up if you find one here and one there...

Willard Bridgham 3

Of course it matters. Larger filter, lower velocity, better filtration.

A better question is what is the optimum size.

ehall

So folks will find this in a search, and to add the equations to this thread, here is what K&N's site says.

"Filter Selection

If maximum horsepower is the objective, the size and shape of the air filter element is paramount.

Let's first consider shape. When fitting a conventional round filter on top of the engine, such as a carburetor, central fuel injection or throttle body fuel injection, we have found a large diameter, short filter will flow more air than a small diameter, tall filter. For example, a 10-inch diameter filter 2-inches tall will flow more air than a 5-inch diameter filter that is 4-inches tall. Where space permits, the height of the filter should be between 1/5 and 1/4 of its diameter.

The shape of the filter is less important if the application calls for a remote mounted filter, which includes many late model fuel injected models. Typically these vehicles will use a flat panel filter or a conical or cylindrical shaped filter with a rubber mounting flange designed to be mounted on the end of the inlet hose.

That brings us to size.

Use the formula below to compute the minimum size filter required for your particular application. The usable portion of the filter is called the EFFECTIVE FILTERING AREA which is determined by multiplying the diameter of the filter times Pi (3.1416) times the height of the air filter in inches, then subtracting .75-inch. We subtract .75-inch to compensate for the rubber seals on each end of the element and the filter material near them since very little air flows through this area.


A=(CID X RPM) / 20,839

A = effective filtering area
CID = cubic inch displacement
RPM = revolutions per minute at maximum power


Example: A 350 CID Chevy engine with a horsepower peak at 5,500 rpm.

A=(350 X 5500) / 20,839 = 92.4 square inches


If you are sizing a panel filter, multiply the width of the filter area (not the rubber seal) times its length. If you are sizing a round filter, use the following formula to determine the height of the filter.


H=(A/D*3.14)+0.75

A = effective filtering area
H = height
D = outside diameter of the filter
3.14 = pi
0.75 = the rubber end caps


Example:

H=(92.4 / 12 * 3.14)+0.75 = 3.20 inches


Referencing the K&N catalog shows the proper filter for this application would be an E-1500 which is 3.5 inches tall. Keep in mind, this is the minimum size requirement. To extend the service interval and to provide an even greater volume of air to the engine, install the largest filter that will fit in the space allotted. If the space above the engine is restrictive, perhaps a remote filter arrangement could be used to gain space."

ehall

Indeed, that is the better question. That's kind of what I'm after.

Dougs951S

Those formulas dont make any sense, they imply that filter size is purely a function of engine rpm and displacement. We know that isnt true, power/air flow requirements are more important (a filter media of X surface area can only flow Y amount of air without a pressure drop). Without taking into account the motor's volumetric airlflow requirements, I'm calling BS on K&N's calculations and will go so far as to state that without taking flow into account, there IS NO WAY to know how to properly size a filter for a given engine.

Or to state it another way, those formulas imply that a 997 GT2RS (612 hp @ 6500 rpm, 3.6 liters) would be ok with the same filter as a 997 carrera (340 hp @6500 rpm, 3.6 liters). We all know that isnt true, because the GT2 needs almost twice the volumetric flow as the carrera and would be absolutely strangled by a filter properly sized for the N/A car.

Dash01

As a filter filters, it gets progressively less able to take in the flow. The progressively smaller passages then mean increasing aerodynamic drag, which increases exponentially. So, if smaller passages wind up with 2 times the air velocity to let the engine breathe at a given rpm, there will be 8 times the drag.

Better to use a larger filter, which will last longer before it becomes too restrictive.

Also, our engine intake plumbing has WAY too many bends and restrictions, is WAY too long, and absorbs WAY too much engine bay heat enroute to the cylinders.

Better to have shorter, straighter, and cooler intake plumbing. I suspect that an otherwise stock 2.5 liter engine and turbo would perform as well or better than a stock 3.0 liter engine and turbo, if the intake plumbing of the former were modified to be as short, straight, and cool as practical.

ttAmerica RoadsterAWD

IMO, simply put, assuming same material used, a filter the size of a soft ball will work just fine as long as you clean it or replace it every hour........ conversely, you may get a larger filter and clean it or replace it at longer time intervals. The question of big or small is really a question of capacitance.

This is equivalent to the small battery discussion. You can get a very small battery for your Porsche, one that weight very little. It will get the job done but it's function is very limited to just turning over your car in warm weather and hopefully in within seconds. However, a larger battery has higher reserve, higher capacitance, so you can crank your car in the cold, run your stereo while the car is off, etc.,

capacitance is what you get with bigger filters