Quittintime: Do you want Extra Air with that?

Do you want Extra Air with that

The other day I asked Arthur my young (45 year old) motor mechanic what ever happened to "Extra Air". He scratched his head and said I don't know Max what is it?
I explained :-

Back in the nineteen twenties and thirties the carburetors of some cars, trucks and buses here in Sydney Australia were fitted with a small port located below the main jet. The port was similar to the ports on a Clarinet that open when a small lever key is pressed to change the note. I think the ports still exist on carbys today but are only used now for unscrewing a metal plug and temporarily fitting tune up equipment.

The idea was this, when the vehicle was traveling down a hill the driver would pull a knob marked Extra Air on the dashboard that linked to a lever on the port opening it up resulting in air entering the carburetor throat below the main jet.

The vacuum caused by the Venturi drawing the petrol through the main jet immediately ceased to be, so the vehicle continued traveling down the hill with the spark plugs still sparking but no fuel vapour was being made available to the cylinders for ignition. In doing this, a considerable fuel saving was possible.

It wasn't “coasting” in the true sense of the word of knocking the gear stick into neutral and rolling down the hill, which is positively dangerous and therefore illegal. The normal compression by the pistons in the cylinders was still available acting as a brake to give the driver full control of the vehicle.

Shortly before the next up hill section of the road was approached the knob on the dash board was pushed back in to it's normal position, the port closed, the vacuum returned and the fuel once again squirted through the main jet via the carby throat into the cylinders, the spark plugs ignited the petrol / air vapour and the vehicle powered on up the hill.

No great benefit was possible on the open plains due to them being mostly flat, but the Extra Air feature worked particularly well in hilly country.

I seem to recall Extra Air fell out of favour due to the cold unburnt air chilling the head sometimes causing it to warp or crack.

You'd think with the recent increase in the price of petroleum Extra Air would have made a swift comeback as electric heaters under control of thermostats being monitored and controlled by the on board computer could easily warm the air sufficiently to prevent warping or cracking of the head.

I'm not sure if this would work with vehicles fitted with automatic gear boxes but perhaps the next generation of bright young Engineers might like to re-examine this old idea and spruce it up to work properly, this time under electronic management and save us a fortune on gasoline consumption