i'm currently running the following

36mm front
24mm rear
12 bolt rear
SFC
lower control arm relocation brackets
tokico illumina adjustable shocks and struts

with the struts and shocks set at 4 this car is down right scary to drive.. i set them all to 1 and it's a lot better but the rear still feels a little loose

i want to put a smaller bar in the rear i was thinking a 21mm bar question is, is there a big difference say going from a 24mm to a 23mm ?

 

I ran into the same problem with my 12 bolt.

Problem is that the rear is some 40lbs heavier than either the 10 or 9 bolts and changes the handling of the car - understeer is much more pronounced. I remedied the problem by moving from a 23mm rear bar to an 18mm one, making it much more predictable and controllable. Springs have an effect too, similar to the rear bars...

Obviously you need to have the best combination of rear bar and shock setting to make the most of what you have.

 

Seems rather backwards... one typically ADDS rear bar to GET RID OF understeer.



And, a heavier rear axle, usually pushes downward more equally on the 2 tires (since it's unsprung dead weight), which would be expected to tighten a car up (remove OVERsteer and/or increase UNDERsteer by increasing the load on the inside tire).

That aside, the rate of a sway bar changes in proportion to the CUBE of the change in diameter; so, a little change goes A LONG way.