Wanted to buy used R.A.R.E cast iron round port exhaust manifolds

[hectore3]

Specifically looking for part number "RM-4 OSA". To fit an Edelbrock round port head in a 1967 Lemans/GTO. Thank you.

[tflyboy77]

How does the 77 Trans Am RM-4 differ from the RM-4 OSA? I have considered selling my set in favor of Tri-Y headers.

[hectore3]

Your collector size is smaller. Which equals less potential exhaust flow and power.

[tflyboy77]

Ahh I remember now. I have the RM-4OS (OS=oversized). I knew I had the larger collectors but I didn't remember the p/n change.