ive heard some people leave/take this spring out... whats the effect of this?

Cheers

It stiffens the front differential for better cornering. If you use 10k silicon oil at the rear and 10k silicon oil at the front + the spring then your actual front diff stifness will be 40-50k.

what spring is it? i never know what it is.

That's the spring budy

You definably want to leave the spring in there, I would recommend picking up this spring set HPI makes just for the differentials. It comes with springs of different stiffnesses so you can try to see what one you like best. You could use Silicone oil but because the RS4 3 does not use sealed gear differentials the oil will leak out over time and make a real mess, not fun to clean up.

http://www3.towerhobbies.com/cgi-bin...?&I=LXZT47&P=7

oh, so it pushes out the two little gears so it pushes it harder?

Yes, it acts like oil. I run the Gold spring in the front and the black in the rear. My rs4 3 is setup for high traction surfaces and I am not sure how it would handle in lets say a parking lot.

oh, is the spring in the original RS4 3 model too?

yes