Hey guys, the pre-cat on my stock exhaust manifold broke in half, and instead of buying either a cheapy manifold or a super expensive one, i plan to weld in a chunk of stainless pipe in place of the pre-cat.

That being said, does anyone have a stock manifold laying around that could measure what size the pipe is on the inlet and outlet of the pre-cat, and about how long the pre-cat is?

 

I can measure the OD's for you when I get home.

That being said,

I DO NOT! advise using stainless.

while stainless will weld to mild steel,

Welding different materials to one another will still result in a dieletric point due to the differing metals.

This dielectric difference will cause advanced levels and rates of corrosion. Especially at the weld joint and surrounding areas.

AKA: you will eat through the stock manifold and the welds will eventually fail.

Granted this is an extreme case, but none the less. I do not advise it.

Also, different materials expand at different rates when heated.

Stainless moves much less as well as it radiates much less heat than mild steel.

Because of this, it can also cause welds to fail. Especially when located in the hottest part of your exhaust. The mild headers will expand at a faster rate than the stainless and weaken the joint.

Even if it is not immediately, heat cycling and expansion differences will cause fatigue, and I would not be surprised if the weld fails within a few years of normal use.

Just my .02.

You can weld somewhat different grades of stainless together, or different grades of mild together, but mixing the two is generally advised against.

same with filler metal. Don't use mild filler on stainless, or stainless on mild.

 

Yeah I don't know why I keep thinking the exhaust on my car is stainless.

 

Just a note: I think what you are calling a Pre-Cat is not a catalytic at all or is the only cat.

I do not think there is two on the 1st generations cars.

 

I can look up inside the broken pipe, and I see a honey comb catalyst bed. It's not the large cat either. It's too small to be the only catalyst.

In this pic, it's the lump that is just after the upstream O2 sensor. Mine is broken off at the bottom.



Now that I look at the photo, it doesn't seem like the inlet and outlet of that precat are a uniform size or shape. I'm just going to toss an OBX header on this beast and be done with it.