All Norton Rotary engines (both the air cooled and the water cooled engines) use air-cooling to cool the centre of the rotors, the eccentric shaft and the main bearings. The centres of the rotors have air-cooling fins incorporated into the castings. Oil is ‘injected’ into the airflow by a metering pump that varies the flow according to engine revs (effectively a ‘dry sump’ system). On the air cooled engine, air is pulled in through a channel in the centre plate, oil mist is then injected at the centre plate and is drawn outwards to the left and right sides towards the end plates where it is then ‘sucked’ back out from the engine through two ports. For the race bikes the direction of the air flow was reversed with the oil being injected at the main bearings near both the end plates and being drawn in towards the centre plate and then extracted from the engine at one central point (by the ‘extractor’ or ‘ejector’ exhaust) where the air would normally enter the engine. This ‘reverse’ direction of flow was later used as standard for the water-cooled engines. For road use, both the air and the water-cooled engines use the suction of the combustion process, or the suction of the carbs if you prefer, to draw the cooling air through the centre of the engine.
All the (three) owners of air-cooled road bikes that now use an extractor/ejector exhaust have closely followed the example of the factory race team. The thing that has put me off connecting the ejector previously is the cost involved having the engine stripped, the route of the oil pipes modified (which involves drilling through castings etc), the cost of an expensive pair of amal carbs or similar and the relocation of the carbs. I was thinking about it and started to question why it has to be done the same way as the race team did it. Why must the ejector pull the air out from the centre plate? (it works well enough with the combustion process sucking the air out from the two \'ports\' at the sides)... and what’s wrong with keeping the SU carbs situated right where they are?
On the standard air cooled bike, the shiny alloy air transfer ports on top of both sides of the engine take the hot air out of the engine and feed it into the plenum chamber in the frame. This seems to me to be the ideal point at which to split the processes of the internal engine cooling and the carb intake. One small complication is that the air transfer ports also double as engine mountings, attaching the engine to the frame.
My idea is to replace the air transfer ports with basic stainless steel engine mountings complete with openings through to the engine and the plenum chamber. The openings from the engine will have twin flexible pipes connected going to the ejector exhaust venturi vacuum pipe to draw the hot air out from the centre of the engine. This cooling air will still enter the engine through the standard air filter and the oil-metering pump will still inject oil mist into it at the centre plate.
The openings into the plenum chamber in the frame will have air filters fitted (the K&Ns in the diagram) to filter the incoming air for the carbs and subsequently the combustion chambers. The plenum chamber will now act as an air box. 50:1 ‘Pre-mix’ lubrication for the rotor tips will now have to be added manually to the petrol tank (groan).
The carbs will have to be adjusted to flow more fuel (to cope with the denser cold air). I’m hoping this will be sufficient on its own without having to change to differently profiled needles.
So, there you have it! All I need to do is have the required bits made (I have no welding skills) in time for Rotorstock 3. With luck I will be able to chase my track rival down into the eleven-second bracket, hopefully without wrecking the rotor motor in the process.