Performance

Lots of people have removed their heat riser valves and blocked off the cross-over passages on V8 engines. On inline engines, people sometimes wire their valves into the fully hot position or they remove them completely. If you have a V8, removing the valve would marginally reduce the flow restriction but the valve on an inline engine is a flow diverter not a flow blocker. I am not sure what people with inline engines are trying to gain by removing this valve completely but they aren't achieving better flow.

You can get by without this valve in warmer climates but why would you make your inline engine exhaust manifold flow more turbulently all the time? Beside the fact that engines work better with controlled intake manifold heat, the flapper helps to make the exhaust flow path smoother. The only time this valve blocks the flow is when the flapper is in the cold position to divert exhaust gas up to the intake for heating. As the engine warms up, the flapper rotates to cover the opening and makes a smooth exit for the exhaust gases. I call the fully hot position the closed position because the flapper closes off flow to the intake manifold.

If you have an inline engine like a slant six, see for yourself. Look in the exhaust manifold from the bottom where the exhaust pipe connects. With the valve closed (hot position), the valve blocks off flow to the intake manifold so the resulting flow path is smoother. Only when the engine is cold will you see the valve directing hot exhaust gases up to the intake manifold.

Whenever pressure drops are calculated in piping systems, the lowest restriction is found in a straight smooth section of pipe. Any change in flow area or direction causes a restriction (or pressure loss). When you remove the flapper, you create a permanent sudden increase and sudden decrease in the flow area of the exhaust manifold collector. Not a large restriction but measurable.

Factory engineers design engine components with sound engineering principles and research. Designs are always a compromise of a number of competing requirements. Before you re-engineer their designs by changing the operation or existence of this device, you should put a lot of thought into it before hand.