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Classic Engines, Modern Fuel

Comments on Topic: Intake manifold designed in hotspots

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Submitted by Rog

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).

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Submitted by The Author

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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Submitted by Rick


Your book is a must read for anyone with a classic vehicle

My experience with type1 air cooled vw engines is without the heated manifold (an exhaust crossover under the inlet tract) the carburettor throttle butterfly ices up, sticks and causes erratic idle especially in humid weather. This happens when the crossover tube (thin pipe) burns through and the repairers (professional and diy ers) block of the ends. I've repaired or replaced a number of these (I'm a tech teacher, mechanic and own a private workshop).

This is not unique to vw's, fuel at idle enters the manifold right at the throttle plate which is where atomisation happens and vapourization starts. This change of state draws in heat which causes humidity to condense out of the air and ices around the throttle plate causing the idle issue.

The "Hot spot" or heated area is there to warm the material the manifold is made of (cast iron or aluminium) so that heat soak keeps the carburettor above a critical temperature. This gets worse where isolator blocks are used under the carburettor and heated manifolds are blocked up with carbon or blocked off.

Next time you run your engine to operating temperature grab the base of your carburettor while idling, you'll feel what I mean (Kool) and if you see condensation around the throttle valve area on the outside your close to having an icing problem.

This isn't a problem for modern engines as they have fuel injectors close to the inlet valve (some directly in the combustion chamber) and throttle bodies at the other end of the manifold. One advantage is a colder (read denser) charge which results in higher cylinders pressures and more power



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Submitted by The Author


This is a very interesting observation. And it is something that will be made worse by the ethanol in the petrol.

The problem is that when ethanol evaporates, it cools things almost twice as much as petrol and with a boiling point of about 78C this is something that can easily happen at under bonnet temperatures. The result is an increase in icing problems in carburettors. This problem mainly affects motor cycles or early cars with exposed carburettors. However, from your comments, it clearly can affect some cars as well.

All the best

Paul Ireland

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