what exactly happens when we re-jet and change over to a K&N filter do to our bikes? better performance right?

define "better performance".

...and why do you ask?

i see that this is mod that almost all do to there bikes so does it make the bike better?

by rejetting, we're bringing the air/fuel mixture to the stoichiometric air-fuel ratio (14.7:1) that is "ideal".

By using pod filters (no airbox) and a low restriction exhaust, we are allowing more air to be passed through the engine and theoretically allowing it to produce more power.

By rejetting to match the increased air flow capabilities (again to closely match the "ideal" fuel mixture) one should see more peak HP.

The Area P dyno's show about a 10% increase with their systems. That's typical of all full systems that I've seen.

What I 'm very pleased about in the rejetting and removal of the airbox is how linear the power has become. Instead of the peaky, no power at the low end and no top end to pass cars as the stock bikes come, my bike, with all the mods I've done, has become a bike that is very responsive from low revs to redline. Once it starts pulling at about 4k, it continues to do so smoothly all the way to redline. That makes the bike extremely easy to ride and there is always at least "some" power not matter where in the rev range you are.

If that is defined as better performance, then yes, rejetting and the K&N filter are parts of the puzzle of building a complete "system".

Read Purspeeds thread on what Area P has done to his bike. His latest post about results from the work they have done almost had me thinking he was riding my bike with the words he used to describe the powerband he now has. Opening up the breathing on engines and proper rejetting has long been the secret to maximizing what power the engine design can safely deliver.

thank you kkim now a full system is exhaust that come with tubes and muffler together as oppose to stock tubes and slip on muffler

yes, full system as in header and muffler as opposed to just a slip on for he main reason that the stock exhaust system has 2 cats. One in the header and one in the muffler. If you only get a slip on, you are only getting rid of 1/2 of the restrictions of the stock setup.

by going with a full aftermarket system you are eliminating them both.

thank you again kkim