Topic: 350 Intake Manifold
in Forum: C3 Engines
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I have a 71 350 with a GM intake manifold with a stock carb. There is excessive heat, over 300+ degrees right under the carb on both sides of the intake. I assume this is where the exhaust valves are on the heads. It is discoloring the orange paint on the intake at these two points. Should I be concerned?
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No. Actually, what you are seeing is the result of the Exhaust heat riser system. What that is, is there is a valve on the exhaust manifold on the pass side. When cold, this valve is closed, and it forces exhaust gas/heat up thru the bottom of the intake, crossing from the pass side to the driver side. The idea is to warm the engine up quicker, mainly for emissions reasons.
This is what is burning the paint on the manifold. Some paints hold up to the heat better than others, but eventually, the paint WILL discolor/burn...not much you can do, other than have the manifold powder coated, maybe.
This is what is burning the paint on the manifold. Some paints hold up to the heat better than others, but eventually, the paint WILL discolor/burn...not much you can do, other than have the manifold powder coated, maybe.
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If you don't drive the car when it is cold, and don't care about quick warm-up, you can remove the intake, and block those passages in the new gaskets. That will stop most of the heat.
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It won't discolor if you put on an aluminium intake..... 
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Check to see that the heat riser valve is free. Joel told you its location (right exhaust manifold) for I have seen a few seize in the closed position which would always divert hot gases back under the manifold. When engine is cold, just move it to the open position and it should close on its own power. Being a 71, your car is prone for it to seize due to its age. You may not notice a decrease in performance due to your driving habits but it does effect perfomance if not working properly. Hope that helps.
There are three reasons for the exhaust passages through the intake manifold.
kstyer 2010-01-01 12:16:33
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.
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