shayne

The midrange spot that is hard to sort out is also about the same rpm spot that gets the gains from changing velocity stacks. The guy that designed them told me that his main aim was improving this area.

Personally I cannot see how an airfilter can have a flat spot in it - surely it must flow air at a linear rate. I suspect the problem is the design of the intake, not the filter itself. I did a lot of experimenting with this using another brand of filter (much bigger than a K&N) with the same results.

I accept of course, that if you have a standard intake set-up then the result will be the same. I just think it is not the filter that causes the problem, it just exacerbates it.

Tweety

Agreed it's not the filter at the root of the cause... But the K&N definetly brings out that hole very distinctively... That area is rough with an OEM filter, but usually there is a bulge upwards, just as the first run demonstrates... Add the K&N and 9 times out of 10 there is a dip... Just as the second run demonstrates...

To get any performance increase out of the K&N you need a fairly non-standard engine in most respects, airbox included... It did represent a gain when I tried it with the velocity stacks... But it also was a PITA to get consistancy out of even then, so I gave up...

The stacks did level out that area nicely for me... And a BMC street filter which defintly is more free flowing than an OEM filter and less then the K&N gave me the best results...

shayne

With my modified engine and intake (including billet velocity stacks) I was using a Unifilter. That sucker is huge compared to a K&N! I reckon it would be an easy 30% bigger in its surface area.

And I had a flat spot I copuld not get rid of. At WOT it was awesome, but not at part throttle, when riding easy. In the end a K&N and a re-tune was a temporary fix for me. It is more rideable on the street, but down on power compared to the more open set-up. I had 205 mains with the Unifilter and now I have 170's with the K&N.

So I had the same problem, but just up the tuning scale a bit more. I am hoping that my EFI conversion will be the answer to such problems when it is completed. It will be a way off yet though - new shed is not even built yet.

YOUNGJESTER

Hey guys. I noticed when I bought my hawk it pulled hard and smooth. I bought a K&N and installed it and now that I read this thread I wonder if its the K&N that caused my flat sucky spot in mid/low power? Its the only thing I added to the motor besides oil change... I'm going to get a stocker and see... thanks

Torque Junkie

The KN will add the flat spot, you can add a fin like the stock filter and get some of the power in the flat spot back. Airflow into an engine is all about pulses, sometimes the pulses of air bounce around in the air box and cancel each other out at certain rpms and create a flat spot. If they bounce just right they can create a resonance and increase the air flow. The fin seems to help some for mid range, but will take away some peak horsepower. The problem is our airbox and are too large 48mm carbs are sensitive to any changes in airflow. More power in the midrange will make your bike accelerate better on the street than just tuning for the peak hp. For acceleration you want as much area under your torque curve (looking at your dyno sheet) as possible and that means getting rid of the big dip at 4-6K rpm even if it means taking the peak down a few hp. For the race track the peak is more important and the KN works better.

Velocity stacks work by creating a resonance frequency at certain rpms which allow more air in for certain rpms, and unfortunately less air in at different rpms. You can tune for low rpms, mid, or high.

The KN sucks, I need to get mine out. The only advantage is it is reusable, and it helps keep my front wheel on the ground with the flat spot in power.

killer5280

I'm surprised it will run at all if the only thing you changed is the filter. Yes, the K&N is the source of your flat spot.

offroad_rider2008

ok so I just need to run oem filters? was thinking bout a k&n but after reading this thread, I think you guys talked me into saving some money are there any filters out there that are better than oem but not require new jets or other changes?

YOUNGJESTER

I always thought the K&N to be a wise upgrade. I should add that while it is the only thing "I" have added, I bought the bike because it had reworked suspension , scotts damper, and titanium remus cans. My airbox has been drilled with 25 or so .5" holes and small screens are inserted into those holes. It looks
more proffesional than hacked but I wonder if the PO went as far as the jetting ... the TPS was already adjusted and I noticed that the vent tube was out of stock position as well... I better take a look at the numbers in the jets...

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