Adding ”SYSTEM” to the word EXHAUST makes it sound a lot more complex/scientific, doesn’t it ? In fact it is a complex enough issue, system or no system word added to it. I have elaborated a bit in the January 1 log book chapter on the job the exhaust system has to do and what some of the parameters are that determine whether it will be able to perform this job well or not. The job target is simple: help the engine produce as much maximum power as possible and that over as much of a power band as possible. In my case, we are aiming at maximum power at 8000 rpm and to have a useful power band between 4 or 5000 up to the red line at 8500 rpm. With the former owner of NRE, Dave Nourish, being out of the game and the new owner, Chris Bushell, still going through Dave’s notes without having found anything on the exhaust system dimensions, I have taken on trying to come up with a solution on my own. Coming up with does not mean that I am going to design this on my own. It means having somebody do it for me that is able to convince me that he knows how to do it.
I was on the NYC Norton website one day and saw that they had built a bike for a guy called John Magyar, a Seeley MkII with a NRE engine – exactly what I am doing. There are pictures and text about the guy building the exhaust and I thought – this is the guy to contact. He must have at least the dimensions noted for John Magyar’s exhaust and if I am lucky that engine is of the same bore and stroke as mine – meaning I could get a copy done for me. Seth Rosko is his name and he seems to know his stuff. Not only how to bend thin walled tubes with them getting wrinkles on the inside bends but he also seems to have the theoretical background, or at least the ability to apply hard data like exhaust valve diameter, exhaust tract length, exhaust valve opening point and the desired max power and red line limits into a formula or two that should produce what we are looking for: exhaust pipe diameter and length and the same data for the following megaphone. Seth mentioned last night in an email that he had run the numbers in the Jennings* formula and was going to do the same with the Blair formula. This brought back memories to me.
Jennings must be the late Gordon Jennings, a motor journalist whose articles in the CYCLE magazine about especially two stroke tuning I was completely impressed by. Gordon Jennings was able to, through very thorough investigative journalism, obtain theories and formulas that was only available to very few two stroke engine designers in Japan, and maybe the best of them all – doctor Walter Kaden of the East German MZ factory, and make them understandable and useable for the everyday engine tuner. I don’t know if Gordon Jennings also presented theories for four stroke engine exhaust design but maybe Seth is using the two stroke theories and applies them to four stroke problems. At the end of the day the task is the same: extract as much burnt gases as possible from the cylinder, including a certain amount of fresh air/fuel mixture and push the fresh mixture back into the cylinder before the exhaust valve closes.
Blair must refer to the late Gordon Blair, Professor at Queens University in Belfast. He made his name ( in my world ) in the design of racing motorcycle engines, part of which was of course exhaust systems. I first got to hear about him when he designed a 2 in 1 in 2 system for Paul Dunstall’s Norton Atlas/Commando based race and road bikes. I assume that Gordon Blair has left theories behind that can be used for the calculations that Seth Rosko is now doing for my bike.
Anyway, the Seth dropped these two names convinced me that he will be able to come up with a design that will be close to the ideal system, bearing in mind that we will not, at least not within the foreseeable future, be able to make any experiments with alternate designs. What will be built now is what is going to be used, without questions being asked afterwards.
The picture shows the Dunstall 2 into 1 into 2 system.