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Why have a heater pipe that runs through the manifold


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Early cars do not have water passing through the inlet manifold. It was done to reduce warm up time and to keep the manifold at a temperature more constant than without the water flowing through.

In extreme conditions, a hot inlet manifold can transfer so much heat to the carbs that fuel can evaporate. Hence some engines have an insulating barrier between carbs and manifold. On cross flow engines, the inlet manifold can cause freezing in really cold weather. (My throttle slide once froze open on my Triumph motorcycle in freezing fog!)

Heralds and Vitesses do not have cross flow heads, so the inlet can become warmer during hot weather if the water is not present in the inlet manifold.

You can choose not to have the water connected if the pipework is dodgy. It will not affect normal use.


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In the later installations and with the correct return pipe fitted with a restrictor, there is a low flow of hot water through the manifold to slightly heat up the interior surfaces.  This is to prevent petrol condensing on the inside to the manifold which would lead to poor mixture distribution to the cylinders.  If the weather is cold and you turn on the heater, more hot water flows through the manifold and supplies more heat to counter the increased tendency for condensation to occur because of the cold ambient.  Some engines, such as on the Mini, had the inlet manifold in direct contact with the exhaust manifold but this can supply excessive heat.

If you want to go racing and need as much power as possible and are not concerned about smooth running at slow speeds, then yes there would be a theoretical advantage in not heating the manifold.

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8 hours ago, Andrew said:

Are you saying it is there to warm up the inlet manifold so the air entering the engine is pre warmed. Sorry to be dense. 

Once the engine is up to temperature the flow through the manifold does very little but on a cold start, as said, it reduces the time and amount of choke needed so fuel consumption and emissions are minimised😁 

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