Cruise at 35mph sound problem solved

The insulation is not necessarily needed.  It is often removed for reasons that others have already pointed out, but will result in some additional engine noise and an increase in cabin temperature, depending on your individual driving style and engine/exhaust type.

Products exist that can be used to combat the noise and temperature increase.  Dynamat or similar materials using a butyl rubber substrate can be used to combat noise increases, Dynamat Extreme or similar can effectively address the heat increase as well, as can any hybrid insulating material (cotton/wool strand fiber backed with aluminum) typically sourced through automotive upholstery suppliers.

I was thinking about this the other day. I removed my original Rover unit as it looked beat. All it was a thick piece of medium (if that) density foam with, what looked like, a bit of black paint on the outward facing surface. Maybe a thin undercoating type paint at best. Certainly nothing special. Only nice part was it was cut well for fitment. Was wondering if adding one back would do anything for me.

I also have a brand new under bonnet felt or whatever it is kit that I never installed. Been on the fence whether I want it or not for years now:-)

The problem could be the foam The original would be horse hair or jute (what ever the material is called). Current versions from Newton Commercial aren't that good either. Noise reduction depends on the density of the material so the pad will make no difference but the engine bay looks better with one.

Mid 80's cars had about 1/2" of foam plastic with a surface skin all across the engine side down as far as the firewall cross-member. Not only did it get tatty, but it held moisture against the back of the fusebox etc. My car also has a  plastic , concave panele to fill the central hole.

Well you don't really need a second kidney either....

Originally, BMC fitted a vinyl faced blanket acrosss upper firewall on the engine side.  This was often discarded as it became tatty.  A nice piece of felt carpet underlay faced with vinyl, cut to size and fitted across the upper firewall will significantly reduce noise (and smells).  

Later cars had a metal plate that was attached to cover the central hole  ..and, I think, even later they did away with the hole altogether....

So, no, you don't need the insulation but (IMHO) its useful.

Cheers, Ian 

I took off my engine bay firewall insulation, mainly because I needed access to the speedo connections, but then realized it was just messy rotten foam anyway, and actually I like the cleaner look without it.  I've noticed many pictures of other members' cars and most don't have the insulation, but they also don't have the a big hole in the center exposing a bunch of wires and contacts to the center speedo like mine.  I'm also afraid of dangerour fumes getting into the cabin (lthough I don't think the old foam was providing much protection against that).  Do other people put some kind of covering over this hole?  If I left it as is, could the speedo heat up too much or have other problems?  I have the speedo housing sealed pretty well to the passenger side of the firewall, but can imagine chances of leaks would be less with a layer of insulation on the engine side.