There are three reasons for the exhaust passages through the intake manifold.
1. When a heat riser is installed on the right side exhaust manifold outlet, it remains open when the engine is warm, causing no effect. When the engine is cold it shuts and directs a lot of exhaust across the intake to the left manifold. This caused the intake to heat up and causes better fuel atomization (sp?) This allows the car to operate better and cleaner during cold warm up, with better throttle response. When it opens with more equal pressure on both sides of the manifold, flow stops and the manifold stops heating. This also allows for faster engine warm up.
If you have removed the heatriser, as I have due to my headers, the engine will have an increased warm up time, and on very short repeated trips, could foul spark plugs.
2. It is a passage that supplies exhaust for the EGR valve. If you don't have and EGR, not worries.
3. It is a heat chamber for the choke heater tube. If it is blocked off the choke will open more slowly. This may or may not be a good thing, but can be adjusted and compensated for to a point.
If you have an electric choke, it does not matter.
With my headers and no EGR, my choke works fine, but I installed an electric conversion.
kstyer2010-01-01 12:16:33