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Classic Engines, Modern Fuel

Comments on Topic: Intake manifold designed in hotspots

Submitted by Rog
05-Jun-2020

I have a question relating to a classic engine (3.3L straight six, Vauxhall engine). Most of the straight six cylinder engines of the '60's used a hot spot on the intake manifold (typically from a connection to the exhaust, or maybe water heated). As I understand, this was for cold running, and also to help keep the fuel in vapour form, travelling along the length of the manifold. Of course the manifold does pick up a good amount of heat, just from being bolted to the cylinder head. With the volatility of modern fuels, do you think it is really necessary to keep the hot spot? It certainly doesn't help hot starts, or the volumetric efficiency.

Typically the manifold would have a centrally mounted carburettor, the longest runner length from the carb to the cylinder head is of the order 300mm, the shortest runner length about 80mm (these values need the intake port length adding to get to the back of the valve).

Submitted by The Author
05-Jun-2020

This is a very interesting question. Hot-spots or heated inlet manifolds are another example where an engine does not behave as one may expect.

Thinking about the 3.3L engine. During one full revolution 3 cylinders draw in mixture, i.e. 1.65L. This has to travel through the inlet manifold. Assuming the approximate diameter of the inlet manifold is 50mm (2”) then at 2,500 rpm, the mixture will take approximately 8ms (0.008s) to travel down the longest 300mm runing length. This is far too short a time for heat to be transferred from the hot-spot to the bulk of the mixture.

A heated manifold or hot-spot has virtually no effect on the temperature of the mixture in a running engine. So why are they there?

Engines can suffer from a problem called “pooling” where liquid petrol collects in the inlet manifold. Its flow into the engine is uncontrolled. When it does, it makes the mixture temporarily richer and causes uneven running. Pooling is worse when an engine has just been started or is running cold on the choke. To address this, the carburettors on the XPAG are semi-downdraft. The carburettors and inlet manifold slope downwards towards the engine to allow the petrol to drain into the cylinder. The hotspots or heated exhaust are an alternative way of addressing this problem. They vapourise any liquid petrol as it collects in the inlet manifold.

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