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MG MGB Technical - Air Rail Manifold

If someone could spare a little time and explain for me the purpose of the air rail manifold? I have a 68 B that has what muat be an abandoned air rail. My Haynes manual simply does not discuss it. I see in a parts cataloge the need for a check valve, and maybe a gulp valve, but its getting a bit hazy at this point. I also have a Weber DGV carburator, and don't know if the air rails are 1) even part of the fuel system, 2) are compatible with the Weber carb.

Thanks for your thoughts.
Rick

Rick. The air rail hooks into the cylinder head and forces air into the exhaust system--when the air rail is hooked up to an air pump through a one way check valve. It was part of the early emissions control systems, introduced in the 1968 model year and used through about the 1995 model year. Modern emissions control systems do not use air pumps and the value of an air pump is a matter of some debate.

The air pump also provided air into the intake manifold, through a gulp valve, which opened up when you closed the throttle. Under such conditions, small droplets of fuel, which had condensed out of the incoming mixture, would be drawn into the cylinder head, causing a short period of excessively rich mixture and leading to a higher level of unburned hydrocarbons. Various measures were used to reduce the excessively rich mixture, under these limited conditions, including modifications to the carb throttle plates and the use of a gulp valve to allow extra air to be introduced into the intake system.

There is no advantage to having just an air rail hooked up to the cylinder head. There is considerable doubt about the value of an air rail hooked up to an air pump through a check valve. The gulp valve system is of some value, especially when used with a carb not having the poppet valve system found on the HIF spec North American cars. The use of the gulp valve will reduce/eliminate the backfiring, due to an excessively rich mixture, when shifting or decelerating.

The holes in the cylinder head, which receive the branches of the air rail, are 7/16" UNF thread. The air rail can be removed and the holes plugged with either set screws or hex headed machine bolts.

Les
Les Bengtson

"the value of an air pump is a matter of some debate"

The debate seems to be whether it just dilutes the emissions i.e. there is the same volume of emissions but since there is a greater volume of air there is a lower *percentage* of emissions i.e. it is a con, or whether injecting air promotes combustion of unburnt fuel in the exhaust and so genuinely does reduce that type of emission. As the injection cause backfiring in the exhaust, which required the addition of a gulp valve to control, personally I suspect the latter.
Paul Hunt

Hi Guys
Please correct me if I'm wrong but I always thought that the air pump and gulp valve were a part of early emissions systems to protect the cat. converter from any rich mixture situations that would cause it to melt. Also any richer than 14.7-1 is when the nasties start coming out the exhaust. Gary
Gary

Gary,

I think you are not exactly right because the cars had the air pump / gulp valve prior to the addition of the cat.

However, I have always heard that cats don't deal well with excessively rich exhaust so, after the cat went on, I think you may be right.

I would like to know myself because the 77 that I bought a few weeks ago has a downdraft Weber and an aftermarket cat at the end of the header, and no air injection/gulp valve system. I am wondering if it is going to cook the cat.

Charley
C R Huff

It was progressive. First came air injection and gulp valve in 68. Then evaporative loss collection in 70. Then reduction in compression ratio and HIFs in 72. Then Zenith and cat in 75/76 i.e. seven/eight years later. All the while the factory were having to fiddle with ignition as well, and all these changes were to keep within the ever-tightening emissions limits. Cats are sensitive to all sorts of things, my 2004 ZS handbook warns against simply running out fuel, for example.

Cooking shouldn't be an issue, at one time there were spark plugs fitted to them to ignite unburnt fuel in the exhaust which gives a much quicker warm-up for the cat, causing it to glow, as it is only when they are hot that they start converting the nasties anyway. Even in normal use they often glow red-hot. It is physical shock and extremes of gasses that damage them.
Paul Hunt

This thread was discussed between 18/08/2008 and 19/08/2008

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