This is kind of a tough one cause it all depends on how you like your car to feel. I generally prefer to run stock rear inserts cut down to 75% of foams left and stuff them in there. They also tend to me at least to stand up to tracks with big sweepers a lot better then closed cell foams. At least in my experience. Both have their good and bad qualities (cannot get stock foams wet, or mush time quickly, Closed cells take time to break in before you have optimal traction). If you use sauce on your tires I would say to use closed cells as they stand up to getting wet a bit better then stock foams. I clean my tires with a wet rag on the tread pattern instead of dunking them in a sink full of simple green. But it all comes back to what feels better for you on the track.