Knob on 440 Right Exhaust Manifold Collector

[vacuumbed]

There is what looks like a steel knob by the collector on the RH exhaust manifold. What is this for and does anyone try to remove it?

[RV Mech Tech]

That's probably a mounting flange known as a 'bung' for an oxygen sensor or an exhaust temperature gauge sending unit the racers use -if its threaded  check what kind of threads are in it and plug it- many e.manifold and header manufactures now  include these  in there products so that if you have to install an O2 sensor then the hole is already there.

[vacuumbed]

This thing actually spins. If I turn it with a screwdriver (so I don't burn my finger) with the engine running the exhaust sound changes. I think there is a plate inside the manifold attached to it. The manifold is stock.

[RV Mech Tech]

vacumbed-  my mistake -that's a heat riser valve- when they started putting emission devices  on  older vehicles one of the things the manufactures did was to heat the intake manifold when the engine was cold around the bottom of the base of the carb so that the cold fuel would not 'puddle' and it would flow more evenly when cold - that knob attaches to a plate that closes by a thermostatic spring (or a rod that's attached to it and is controlled by a vacum diaphragm mounted higher on the engine) that's mounted on the outside and the plate inside the exhaust manifold blocks off the exhaust in that manifold and temporarily redirects the hot gases  up and through  a heat riser passage in the cylinder head through to a matching passage in the intake manifold then through to the other exhaust manifold to warm the cold fuel- once the engine was warmed up that plate was then opened and the exhaust gases would flow in the normal path - this was called the 'Early Fuel Evaporation system' (EVAP) and was an early  band aid method of emission control - you may see this on earlier vehicles before the 70's as well- it will also muffle the sound of the exhaust when closed so make sure it stays  in the open position - it sounds like the thermostatic spring is missing -they usually rust and fall off- if it stays closed and you drive the RV in extremely hot weather then the exhaust gases can overheat the area around the passages and crack the cylinder heads and intake manifold - a common practice on race cars and vehicles driven only in the warm weather is to install  block-off plates where the intake manifold gaskets are between the intake and cylinder heads to block off those passages under the intake manifold  as the warming of the fuel is not required in those vehicles but you can get by with just making sure it stays open - usually I use a piece of wire or fabricate a bracket to do this. 

[RV Mech Tech]

vacumbed- one other thing- some of those heat riser valves only had a counterweight and the thermostatic spring was usually beside it on the outside -when the engine was cold the spring would wind up and close the plate inside the e.manifold - once the spring warmed up and expanded the counterweight would overcome the spring tension and open the plate up - it sounds from your description that the knob is  a counterweight and the spring is missing.

[DaveVA78Chieftain]

In the picture below , if you are talking about the portion at the bottom where the exhaust pipe connects to the manifold, then that is a heat riser.  The heat riser is almost fully closed when the engine is cold.  It restricts exhaust gas flow when the engine is warming up.  It opens as the engine heats up.  If it fails in the closed postion it can cause an engine to overheat and lose power.

Dave

[vacuumbed]

Hi again everyone, I would like to thank everyone that posted here with all of the great information you provided me. The heat riser description was very accurate. Mine is there with the spring intact.

FYI: I bought the Mr. Gasket copper exhaust manifold gaskets MRG-7167 and they do not fit well, I will be returning them. There is a casting ridge below the valve cover on the heads that prevents the gasket from seating flush with the head. I am just going to try the Fel-Pro MS90029. My exhaust manifold was baldy warped. I took it to the machine shop and they milled it for $25.