Anyone know abt these?
They came off a 1991 3rd gen.

 

The "High Flow" exhaust manifolds were available every year except for 82', so I wouldn't call them rare. They came on the 83'-86' L69 305HO, 85'-89' LB9 305TPI, 90'-92 LB9 305TPI with the G92 rear end option, and all the L98 350 TPI's. They are pretty much the same as the low output manifolds other than the outlets are 2 1/4" diameter where as the low outputs are just under 2". In order to use these, you will need the y-pipe off of any of the cars mentioned above, as a low-output y-pipe will not bolt up. As far as performance, they are OK for a stock motor, and if you can get the whole setup for almost nothting, then they may be worth using. For anything other than stock, they will quickly become a restriction. For the amount of work to remove and install, I would just do headers. I have changed from low output to high output manifolds on a 92' RS, and although the performance gain was noticeable, for the amount of work involved, i would do headers.