Originally Posted by 89cobranotch
I have the stock swaybars on now. I would like to get rid of some understeer. I already have eiback springs and tokico shocks/struts. I plan on autocrossing the car and some roadracing but the car is mostly street driven.
FYI springs are generally rated in #/". What are your Eibachs? I suspect the fronts, e.g., are less than 800#/" which would be in the area of a dual purpose front spring. Rears need to be balanced to the front and that depends on rear suspension changes among other things.
As a bandaid for understeer, a bigger diameter rear bar (or a smaller front one) will help balance the car, as will stronger rear springs; so will running wider wheels and tires in the front for that matter.
Balance is a good thing.
But, there are many considerations, especially with Mustangs, that need to be addressed to actually achieve good handling. For example, rear suspension bind, roll centers (especially if lowered), camber and camber curve (see lowered), body roll, bump steer, Ackerman, and more.
Find a copy of Mathis' Mustang Performance Book 2
and read it a few times. It's a little out of date, but will help you understand a lot of what you will need to know in order to make informed decisions.
Edit to add: If the understeer is on corner entry, more camber (up to 3 1/2*), and some toe out (up to 1/4") will help. It won't help tire wear though, especially the toe out.
Of course, there's always driver error - entering corners too hot = understeer. But I'm sure you wouldn't do that.