K&N air filters experience

How bad was your old paper one.... For a fair comparison, you must compare new for new, then old for old.

BTW, K&N burnt me years ago, don't use them now. BMC (best but $$) or Green Cotton (almost as good and about 60% $) for me.

 

crap product, sent wrong part first, then "correct" one didn't fit well and they didn't refund fully on wrong one. leaked at edges and generally bad quality. Had to mess a round with packing to make a seal to air box surfaces.

 

I really like their oil filters.

 

They work. Mike C and I proved it on the dyno last January in his 308.

http://ferrarichat.com/forum/showthread.php?t=88743&highlight=throttle+body

 

Yes. My K&N bad experience was years ago... maybe even the 80's but certainly early 90's. Don't care though. Happy elsewhere now. Bridge burnt.

Anyway, BMC are std fitment on Michelotto cars, Ferrari F1 team etc, so it's easy marketing.

Green Cotton are good though....

Curiously, BMC dont do one for TRossa (only TR and M)... I just tried. K&N had one, but I got Green Cotton instead.

 

Seems to me if the K&N makes any difference, your car was not set up right to begin with. Letting more air in should make the car run too lean and reduce horsepower. If your car runs better with more air, it was too rich to begin with.

Ken

 

*sigh*

just try one and see.

It's called volumetric efficiency. Google it.

 

Okay, I did. I looked at a few sites, some with nice equations to determine correct air flow. This quote stands out:

"If the cylinder is filled with fuel/air at atmospheric pressure, then the engine is said to have 100% volumetric efficiency."

On a normally aspirated engine, that's the best you can do. Now, I agree if your intake is not allowing the cylinder to fill to 100% of it's theoretical maximum of air/fuel at the correct ratio due to restricted airflow, then a K&N will help.

My point is a car such as a Ferrari is desinged by experts, and it *should* have the maximum airflow for the correct ratio of air/fuel incorporated into the design. If the K&N allows too much air in, especially on a carb car with no computers, you WILL lose horsepower. So anyone who sees any improvement with a K&N had a car that was not set up with maximum efficiency to begin with, or needed to change their old air filter, or has a poorly designed car.

If this is incorrect, I'm eager to learn.

Ken

 

Your faith is appealing, but misguided.

Production cars are a labyrinth of compromises.

"Suck it" and see, will ya?

 

Well, Ferrari et. al. spend a lot of money trying to squeeze out every horse they can. If a $30 air filter does the job, I would expect it would be original equipment. Porting costs a bit more than that.


Ken

 

Ken,
VE is a complex subject. It's rarely 100% across the operating range (we'll only think about WOT as that's where we want to improve power). The reality of a fixed camshaft profile means that you're compromised at most RPMs; valve overlap, charge motion, inertia of air column, and all that kind of thing. The tuning of the intake system has an impact on VE as well. A well designed system will exceed 100% VE at a particular 'tuned' RPM where the cylinders are supercharged by the mass of air resonating above the intake valve pushing its way in. (The mass of air in the column has a resonant frequency, like a tuning fork, so it vibrates on it's own. If the valve is opening in rythm with that vibrating column, the air will force itself into the cylinder producing >100% VE.) At other RPMs VE will be <100%.
Restriction in the intake tract (i.e. air filter restriction, loss through plumbing) means that you need to be "supercharging" in order to even get atmospheric pressure in the cylinder. Reducing these restrictions improves airflow. With proper fueling, improved airflow results in improved power.
Manufacturers have many tradeoffs to consider, and spending $30 for an air filter that may compromise durability and produce an overall increase in radiated noise is a no-brainer to leave off the car. Note that the mfr doesn't spend anything for "porting" as they cast the cylinder head as they wanted it to begin with!

 

Actually, that's not true. You can get greater than 100% volumetric efficiency. A properly designed intake and exhaust system will create beneficial pressure waves that force extra air in. Only works at a specific engine speed, unless you have variable intake length.... but outside of very well-developed race engines, 100+% VE is not the rule. Main culprit: pressure drops in the intake system, including the air filter.

I agree with all that: installing a K&N might reduce the pressure drop, but a well-designed car doesn't have excessive pressure drop across the air filter in the first place. That said, lots of cars don't qualify as "well designed." A prime exampe is my 1988 VW GTI 16V. A common mod is to gut the very restrictive airbox: in stock form the engine falls on its face above 6000 rpm. With a modified airbox it pulls hard all the way to the 7200 rpm redline. For those too timid to cut up their airbox, a K&N will have a beneficial effect. The reduced pressure drop across the K&N compensates for the excessive pressure drop in the airbox. My car had a K&N when I bought it.

I ripped it out and threw it away.

Why? Because as a FILTER, a K&N is rubbish. When it's clean it's a lousy filter, and when it's dirty it has more pressure drop than a paper filter, due to a drastically reduced surface area. Count the pleats in a paper filter. Count the pleats in a K&N.

It's wortwhile to note that I live in a very dusty environment, and filtration efficiency is more important than in cleaner places. But regardless I don't like letting my engine eat the junk that a K&N lets through. YMMV.

 

http://www.mkiv.com/techarticles/filters_test/2/

.

 

Great post, thanks. One can see the complexity of maximum efficiency as it's not just at WOT that's important. I understand that everything is a compromis (as you point out): noise, desired torque curves, emissions, etc.

I am still skeptical that just adding a K&N filter to a correctly set up car should have a positive effect; in fact it can definitely make you run too lean. For those who can quantify an HP increase without richening up the mixture, I still think they were running too rich to begin with. How can less restrictve air flow not also require a mixture change, possible timing changes, even a cam change to compliment it IF the car was set up correctly to begin with?

Ferrari engines in particular are not a generic lump, mass produced for commuting. They are race engines for street use. They are already "ported" as you point out, balanced, pretty much blueprinted short of a full race engine that would not be streetable. The airflow system engineered for them is no accident. Perhaps you can kick up the HP with a K&N, but that begs the question of if this is a good thing for your engine in the long run.

Ken

 

So I suppose the real trick on a CIS 308 is to tune it slightly rich, then install a K&N?

 

ALL modern cars are too rich for best power at WOT. (I make a bold statement and recognize that fact.)

To keep catalysts (and possibly other components, such as exhaust valves/bridges) from overheating, even Ferrari has to add fuel beyond the Minimum for Best Torque. As a result, if you give the car extra air it will make more power. It's possible that they also apply the airflow learning that the engine does at part throttle to the WOT fueling as well.

 

Yes - is somewhat true for carburreted cars, as they respond to the overall negative pressure in the post filter system filling in a proportion of that with gas.
No - For an injected car (including K-jet) the amount of fuel is determined by the air mass sensor on the engine side of the air filter, automatically compensating for any air resistance at the filter.

My dynos have shown K&N to increase rwhp. I am sure they may filter less (duh), and those effects are a bit more hard to quantify on a seldom driven street car driven in good weather. A Baja racer would be another story.

best

 

Another good point! Like I said, even if it does increase HP a bit, is this a good thing? One reply was you get too much dirt, and you point out you'll run too hot for the cats. Of course, for a race car, neither consideration is important.

Ken

 

That's why I was confused. It seemed to me that the more air in, the further down the flap would move and allow more fuel to be metered in, etc., etc.

 

For a start, K(?)-Jetronic does not have an "Air Mass" sensor, it has an Air Flow Meter, a completetely different beast. These terms are interchanged but they mean different things. An AFM works by using a variable resistance potentiometer just like on a Throttle body, attached to a throttle plate (the flap). It does not measure the physical mass of air entering an engine dynamically, it works by already knowing the amount of air going into the system. The system was "sized" by the engineers and it only works over a fixed range. So even if you hooked a bloody jet engine to the front of it, the system would think it was receiving the same amount of air as usual.

Secondly, if you change the amount of air going into a K-Jet system you need to change the fuelling - within reason - depending on how much change you make to the engines VE. Doing this with K-Jet is quite difficult as unlike EFI the injectors are fuel pressure controlled and their opening times cannot be altered dynamically like they can be with modern EFI injectors and EFI controllers.

On a K-Jet system, the idle mixture is set at a baseline by altering the initial fuel supplied at idle - this is mechanical and it operates on the mechanical fuel metering head. It combines the value from the deflection of the AFM plate to provide information back to the computer.

Depending on the system and whether or not it uses Lambda modulation, the amount of fuel injected can be varied slightly by an electro-magnetic pressure regulator on the fuel metering head. So in the situation of sudden deceleration, sudden acceleration, partial load etc etc the fuel supplied can be altered - slightly.

K-Jet is a complex system and is not easy to tune. There are plenty of fudges out there to get one to deliver more fuel, but metering it is not very accurate science, hence the proliferation of EFI systems. They are light years ahead of K-Jet. K-Jet systems are great once set up and running but are not particularly adaptable. In limited applications they can be great for making power, but flexibility, driveability won't touch EFI.

Thats why even today on race cars Alpha-N EFI is the way to go, no measuring of airflow, it's calculated in advance because the engine normally only runs on a limited rev range and can be tuned to run best at WOT, forget idle etc etc etc.

 

i tried to match the little bit of air volume gained (and sound) with the K&Ns with tuned low-restriction exhaust pipes to see if it made a real difference...honestly, i can't tell but it felt good to lay into the gas peddle..which is why i can't rule out psychology from the whole experience - my right foot just felt heavier everytime i drove the car after the installation. maybe i picked the right week to try them.