Today I started taking some of the parts off a motor we have on a stand in the shop. I took off the front and rear exhaust manifolds to prep them to swap into my car over the summer. This motor had 90k original miles on it and showed some of the things we were expecting to see.
First off, to my surprise, the entire front manifold and crossover pipe (all one piece) was lighter than the little rear manifold. The steel is much thicker on the rear.
I had read about problems with cracking manifolds on the series I and expected the rear manifolds to be more prone since they are really stuffed in there with not much room for the heat to escape but they are solid and I think it would take ALOT of heat to hurt them.
The fronts were a different story. There were four cracks around 2-4" long.
None of the cracks were visible with it still on the motor and 2 couldnt be seen until removing the heat shied around the crossover.
The next thing I noticed after was the welding on the inlets of the manifolds. Each is welded from the inside creating a lip of about half a dime size in spots all the way around. This would most certainly limit airflow at high RPMs I think the rear manifolds were worse but it could vary from car to car. The welds had created a buildup of hardened black junk from the exhaust that made probably cut between 1/4 and 1/2" off of the diameter of the inlets. After cleaning the junk off the welds were still more than I would like to have interfering with the exhaust....
I plan on removing the rust and corrosion and welding the cracks first, then welding inlets from the outside so I can trim away the welded lip from the inside. Then they'll probably be painted to keep them from rusting too quickly, most importantly the welds. Then we're going to wrap both sets and replace. I am not going to have these in anytime soon but hopefully mid-summer or earlier. I am not yet sure of any benefits but I will be sure to update on anything I learn or anything that can be gained.
If theres anything I might have missed or any ideas anyone has had for a job like this feel free to chime in!
Also, not sure if there is anything issues with the Series II but having headers available, its probably best to go that route anyway!