Replacing the radiator on my 85 GS. When I pulled out the old/original rad, it had a length of foam glued to the bottom.
What is this for?
Eliminate rattles between the rad and the underbelly cover?
Increase airflow through the rad (blocking airflow around the rad)?
Do I need to replace this foam?

 

That all depends what setup you're gonna run and/or personal preference imo

 

I would put something in there, it does exactly what you suspect. Those rattles will lead to wearing holes in things as well.

This is when I usually head for Homey's Race Supply(you may know the place as Home Depot) to find some good dense weatherstrip.

 

I didn't replace it and never had a problem.

I also don't run a front under tray as it doesn't fit well with my Mariah Mode 4 airdam, and at speed, the air traveling in the front of the car has nowhere else to go but straight through the radiator and FMOC below it. I also live in the AZ desert and street blacktop temps are way up there (~140F or more) on summer days. Never had an issue.

If you're going for that totally stock look, and like to see an underpan when you open the hood, have at it, otherwise, I'd probably ditch the undertray and foam, but that's just me.

Seems like every time I needed to work on something under the car, that tray got in the way.

 

it is a bad idea to remove the undertray under any circumstance.

 

Are you speaking from some catastrophic experience, or repeating what you've heard?

I ran an SA22C to 212k miles, about 1/2 of that without an undertray, and with the old SA oil cooler. My current 84SE is at 204k miles, with MOST of that without an undertray.

No problems, whatsoever.

Care to elaborate?

 

like I said before, it's mainly preference