I'm looking to split my exhaust manifold on my Pontiac straight 8 and was wondering about the heat riser. My thoughts were to lop off the heat riser, then run two pipes out of the dump that's right there from both halves of the manifold. My other thought was to cut the manifold at the back between the group of 3 runners and the center (there's about 3 inches there) and make a 3-5 manifold. Any preference? My questions are really on the heat riser. If I remove it, am I going to have problems? I don't drive the car in the winter. Also, just on the heat riser in general. My flapper is still working properly, but man...it looks like it would be restrictive as all hell. Has anyone just cut the flapper out and welded up the pivot holes? I'm curious what people have done.
When I split the manifold on my flathead Ford 6, I cut out the flapper and welded up the holes. I have not had any issues. Of course, I'm in north Texas, not quite so cold.
I don't have a heat riser on my Poncho 6. I thought about cutting it off and running pipes the way you describe. I decided not to go that route and built a manifold and ran one pipe per exhaust port. Runs great. I run two 97's on an Edmunds Custom intake. The intake has provisions to run your coolant through which would make up for the heat riser. However, I am not running the coolant through it. I do live in the south though so cold is not an issue.
The only other thing I'm wondering is that the heat riser kinda holds the whole unit together when you hang it on the studs, since there isn't any actual "holes" in the flanges. When you cut the riser off, you essentially have two floating parts then. I'm thinking keeping them together while tightening the fasteners would be a bit of a pain, but I might be over-thinking that. I'm not too worried about the cold. I do live in Indy but I'm not driving it in the winter anyways unless its over 40 out. I'm just thinking the 5-3 might be better because it leaves the heat riser in place. I can just go in and cut out the flapper and I'd think there will still be enough general heat there to keep the carb from frosting up in sub 50 degree weather. It looks like there's enough room to make a 90 degree turn-down off of the runner inbetween where exhaust port 3 and 4 start towards the back. Plus a 5-3 would sound pretty cool!
Boy does this post bring back the memories. in 1956 i had a 47 pontiac conv that i split the manifold, had to go 5-3 it was sweet. everyone said i had put in a olds v8. cant remember about the the heat riser. i remember i went 5-3. Go for it. 8-Ball
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