Ok i need a 100% for sure answer, since every link i find there is not right answer, and all pics are dead. I just need to know what size piping to use to extend my top hats?

Also i was thinking 1.5'' in front and 1'' in back? 91 ef si chassis. Any one can tell me what they are using would be great.

 

I ended up using some 2" OD tube that I turned down to ~1.9" to fit in the hole in the shock towers. It was a long time ago so I don't remember the exact dia needed. Its somewhere in the 1.8-1.9" range. Not 100% answer but best I can do.

Also 1.5" maybe too much in the front. Depending on the shocks you're using, the shaft may hit the hood before then. With my Illuminas I could only get about 1.25" before contact. I settled for 1" to give myself some clearance.

 

I think i used sch 40 pipe