Looking for some help. Does anyone know if the 3xxx motors (GenIII) exhaust manifolds will fit on the 3.1 GenII heads. The GenIII mani's look like they would flow much better than the stock GenII manifolds. I think the GenIII heads have "D" shaped exhaust ports, vice the "O" round ones on the GenII, but again, from pictures they look like they will fit.

Thanks in advance.
edit - Well they fit. I got a pair of front manifolds off a Malibu 3500 motor and they fit on the 3.1L heads just fine. Yes, they are "D" shaped and the heads are "O" shaped, but there is enough material on the heads for a slight porting on the edge. They didn't require any slotting, or grinding, they just bolted on.

[This message has been edited by qwikgta (edited 09-15-2020).]

The "D" shape was a common trick on some engines. (Some of the higher performance/later Duke, and maybe SD4 engines used it.)
I think it was used to prevent gases from reversing flow. If the manifolds will bolt up, I wouldn't hesitate to use them. (With this caveat... I have never owned one of those engines. Just iron head 2.8s and 3.4s. The 3500 in my G6 has never been "into", but is completely different anyway. So take this for whatever it's worth.)
Perhaps some others will jump in.

I plan to hit the j-yard tomorrow to find out.

yep, they fit. edit on first post.

Rob