Which johnnyracecar intercooler do you guys recommend the 5 or 6 inch? Do they require modification to the bumper?

 

i have a 6 inch on an EJ8 ('96) coupe, with no modification at all

 

Yup, I too have the 6" and would recommend it. The 6" IC isn't that much more $ than the 5", so i would definately go for it. Just like Bamboo said, no modifications to the bumper at all.

 

how about on 4g's, modification of bumper or its support needed?

 

Yeah, I had to do a little cutting on my bumper on a 94 civic, but really it's pretty minor. No cutting of any supports or anything, just the side air ducts.
Look at this thread, they have some pics of an eg and how much the bumper had to be cut:
http://www.honda-tech.com/zerothread?id=684467

 

Which is easier to put on a 4th gen? How big is the max hp difference between them because I kinda like the look of the charge pipes on the 5 inch?

 

in a 4g they are both kind of a bitch to install. Both IC's are rated around 300whp.
If ya look in the FAQ I have a write up on a 6in in a CRX