So about 5k miles since i got my 86 Gt back on the street, and i have had both (at different times) manifolds crack at the flange closest to the y-pipe collector.
This is not the common cast flange to stainless pipe weld separation failure, but rather some sort of stress fracture. Take a look at the pic, and I tell you what i think is happening.



So my car has been improperly lowered, it's a bumpy ride. (I plan to fix this when i have the $$$$) I'm thinking that the tension of the y-pipe and the lower exhaust system is bumping pulling and causing the manifolds to crack. Someone once told me that the lower system, and the y-pipe are designed to be flexible with motion...meaning you're not to tighten the hell out of the collector flange at the lower end, nor the manifold end. Well i thought having flexible collector flanges woudl create a leak, and also cause flex tention on the EGR pipe, but now i'm wondering.

Anyone run there 2.8's with a lose collectors? Just tight enough to seal and not leak, but lose enough that the y-pipe actually moves when you hit a bump, absorbing the impact instead of passing the stress on to a sold bolted manifold? I'm considering this option.

I should also add I'm running with polly engine mounts, and aluminum cradle mounts, so this exhaust system is really getting the full beating of the road. I did replace all springs, but I just don't think that is enough to absorb impact. I'm also wondering about having the lower exhaust system directly welded to the cradle with flat bar. That would be sure the lower exhaust system wouldn't pull on the y-pype, thus pulling on the manifolds.

i donno... suggestions...

Your theory is one idea.

The other idea is that the log expands as it heats up and becomes longer. This pushes toward the crossover side which would cause the stress on the joint where it cracked. Also the crossover itself expands pushing the logs apart from each other.
Also you don't want to weld the lower part solid since the motor still moves around some on accel an decel. Maybe stiff rubber mounts like used on other cars would be better.

I have Rodney's polly mounts, whit the polly dog bone. That engine hardly moves at all. Acceleration and deacceleration snaps my head. lol.
I see your point though. There is some engine movement, so solid strapping of the exhaust wouldn't be a good idea.

So in newer cars i notice they have a wire mesh just under the y-pipe..any in mod this in to ours?

A flex pipe may help to solve that. Be careful though, there are some very pathetic flex pipes (aka cheap chinese parts) floating around.

All good points. The exhaust system needs to be able to flex under engine movement, rough road conditions etc. Too stiff and it puts stress on the ends of the logs. Too loose or "hanging" on the logs is bad too. I think Dodge has it right though regarding Fiero logs. I can not tell you how many I have seen that are broken just like yours. I think it's more atributed to poor manifold design and heating/contraction. More so on the rear - firewall side due to water/moisture running off the decklid and hitting the manifold. Does your decklid have the "weatherstrip"?

quote Originally posted by Seanpaul: Anyone run there 2.8's with a lose collectors? Just tight enough to seal and not leak, but lose enough that the y-pipe actually moves when you hit a bump, absorbing the impact instead of passing the stress on to a sold bolted manifold? I'm considering this option.

That's how I reassembled the exhaust, no manifold cracks in 8 years.

I believe the manifold-to-Y pipe connection IS supposed to be loose enough to rotate, to avoid exactly what you are describing.

Also, the Y pipe-to-cat connection is definitely supposed to able to rotate because the bolts holding the joint together have springs under them to give an exact preload on the joint.