Email came in from Denny Barber, the frame guy, advising that the frame is ready except for the rear wheel adjusters so I guess I will have to arrange for shipping it over here in the next week or two. Denny has also made the engine plates in 8 mm Dural and he is offering to supply the oil tank as well. His price is good so I have to make up my mind but since I have already decided to try to make the oil tank a part of the seat I will have to decline his offer.

I am back to the exhaust system. I am being told that Dave Nourish, the former owner of NRE ( Nourish Racing Engines ) was very specific about the dimensions of the exhaust system and that tells me that in order to attain a reasonably wide power band and optimum maximum power, I need to stick to Dave’s specifications. The problem is that I cannot find them. Chris Bushell who now owns NRE does not seem to have the information, or he is quite understandably too busy building engines for digging in Dave’s old files, so he is not of any help here. Kenny Cummings has built a similar bike to mine for a chap called John Magyar but I have a feeling that Kenny does not want to help me. He is of course in the business of building racing bikes for other people and not to give free advise to guys like me that he makes no money on. However, a guy called Seth Rosco was the man that built the exhaust for Johns bike and I am now pinning my on that Seth remembers the dimensions he used AND that John’s motor has the same bore/stroke dimensions as mine. Only if these two question marks are straightened out do I have a solution.

I should maybe explain why the exhaust system is so important. An a racing engine, the exhaust system has two or maybe three functions to fill. The first is the simplest, it shall lead the exhaust gases away from the rider and the motorcycle. The second is the tough part. The exhaust system is used to scavenge the burnt air/fuel mixture from the cylinder in order to make room for the fresh air/fuel mixture that is at the same time being sucked into the cylinder by the piston moving downwards from the top dead centre. When the exhaust valve opens, the overpressure in the cylinder sends a slug of burnt gases down the exhaust pipe, and with it also a load of fresh gases and of course a sound wave ( noise ) that travels down the exhaust tube at a negative pressure. As this slug and sound wave travels through the straight exhaust tube and megaphone and finally exits into the free air, the sound wave is reflected back as a positive pressure wave and is now pushing the fresh air that has followed the slug, back into the cylinder and so aiding is filling it up with as much fresh air/fuel as possible. More air/fuel = more power. Simple as that. BUT, the trick is to get all these dimensions right: Exhaust pipe inner diameter and length, megaphone taper angle and length and in some cases also the dimensions of the reverse cone at the end of the megaphone. There are theories and formulas for calculating approximate dimensions but the end results is only possible to attain in practice: putting the engine on a dynamometer (power absorbing measuring unit) and try out various combinations of these parameters. This is what Dave Nourish has done over the years and therefore I need his figures.

The third job of an exhaust system can be to reduce the exhaust noise to acceptable levels. In racing that is something that we these days also have to observe so it is another parameter. How to reduce the noise without impairing the power enhancing qualities of the system?

I have spent the rest of the day figuring out how to fix the oil system. I think I have the component side sorted out and need to get down to the drawing board once the frame is here. Tomorrow I will go and purchase a PROXXON hand grinder to be able to do minor adjustments here and there. The first is to grind off a piece of a Helicoil that sticks out in the upper fork yoke in a way that it should not.