I have heard that the outlets on '66 425 dual exhaust manifolds are different sizes. Can anybody tell me what they are w/o taking stuff apart? I am going from single to dual and have the Thornton repro (but not handy right now) and want to know what to tell my exhaust guy. Many thanks! Also - I just added a 650 AFB and a stock J-coded manifold. Didn't do anything for off the line but getting into the secondaries at freeway speeds - holy crap! Hoping the duals let me actually break the tires (well...tire on an open diff) loose from a light!

 

I'm not positive, but I believe J and G are the matched toronado pair.

Your 66 should have A and T If I remember correctly.

 

Oops - lt me clarify...the J-manifold referred to the 4bbl INTAKE manifold. Sorry for the confusion.

 

On the '66 425, the right side, common to both single and dual exhaust is cast with an "R", the left side for dual exhaust, original to the Starfire engine, is cast with an "S". Both outlets are the same size, using a 2 7/16" OD donut.