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That is the manifold and what you found is a heat shield. It has nothing to do with fuel efficiency. You should be able to bolt it right back on. Not so sure why it was off in the first place.

I wouldn't go spend money on an aftermarket manifold if you're not tuning it etc.. Most of them look like that anyway, and it won't extend the life of the engine. If it aint broke, don't change it.

Also, is that a picture of you're engine? Looks like the O2 sensor is unplugged.

 

with or without the heatshield, it wont change the efficiency of the car
as long as your exhaust manifold, aka header, is not cracked or damaged you should be fine

 

It's mainly there to keep you from scorching yourself on the hot exhaust header.
As long as you're careful, you really don't need it.
And if you wanna clean it up, either buff it sandpaper or paint it with high-temp paint (black is always clean-looking) and bolt it back on for something different
^^

 

Yes, that is a heat shield (Honda calls it an "exhaust manifold cover").

They are expensive new - I would look for one on ebay, then a local salvage yard. You don't really need it, but the rusty exhaust manifold is a bit unsightly and the shield covers it well.

Aftermarket headers aren't known for promoting better fuel economy.


EDIT: Oh yeah, 350k miles

 

You have 350k now?

Did you take off the shield? Why not just put it back on? You can paint with high temp paint if you want.

 

The two lower bolt holes are rusted out. It's a piece of scrap.

 

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You can go to your local junkyard to pick one up, it shouldnt be expensive

 

Throw it away...it will save you >0.5 Lb which is weight saving fuel economy!

 

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