Thermo quad 6138s ?

I'm not sure if it makes a difference or not.But I am not planning on using a choke.. .. Also had a different question, i read that one of the difference being the 6138s is for a 4 speed 340 carb that it has a is richend idle circuit witch allows the carburetor to come to an idle slower for a four speed.. Can someone explain, is it that noticeable when you are at an idle and reving it up and letting it come to an idle