Interesting topic. I'm currently splitting restore time between a 49 3100 and a 72 Karmann Ghia and a common link between both is they both use a single 1-bbl carb and an intake manifold that has long runners. The Engr's of both power plants determined that the intakes be preheated via exhaust gases to get them warmed up to operating temps as quick as possible. Both for performance and economy sake....
The horizontally opposed air cooled VW's intake manifold has a cast-in heat chamber with 2 separate runners that connect to both exhaust pipes on either side of the opposed engine but it is somewhat unique it how they are plumbed. One side is plumbed directly into one sides exhaust right where it exits the head (can be cyl 2 or 4 depending upon orientation). The other side is plumbed into the muffler at a point where the exhaust exits the muffler. Basically it creates a low pressure area at this point which helps to ensure the hot exhaust gases will flow through the manifolds heat chamber. If both sides are plumbed directly at the same point on both exhaust pipes it would only produce a "pulsing" effect and not true flow, which would not provide the heat the proper way does.
Heres a pic of the
a stock muffler notice the small pipe running from the manifold connection straight into the muffler. On the other side the manifold connection is directly on the pipe to head connection.
Long story short, it appears that the method used to plumb hot exhaust gases into a dual carb stovebolt manifold isn't optimum for hot gas flow between both exhaust manifold. It "may be" if one header's exhaust is at a higher pressure than the other at any gven time but then again the pulse may not be long enough to get good hot gas flow. I dunno I could be wrong and that won't be the 1st time
Anyways I could be over thinking this but I thought I'd throw it out for discussion sake
Dave
