Conventions and Terminology
The popularity of the motor had not gone unnoticed by the karting fraternity who used it in one of the many kart classes, and indeed still continue to do so today, it is from this source that some of the latter day expertise comes, as well as the supply of new parts. With the gaining popularity of pre-65 scrambles and trials events, and the renewed interest in road racing of anything that is old and British, the Villiers engine has once again found yet another niche in history. Today more riders are turning to the humble two-stroke to fulfil their ambitions of owning and riding a British machine, the bigger bikes becoming rarer and expensive as time goes by, and are turning to the Villiers product to satisfy their needs.
When first produced, the 9E churned out 8.4 bhp at 4000 rpm, but in the light of modern day technology, it can be made to produce some 30 bhp at 9000 rpm (197cc) and 40 bhp at 8500 rpm (250cc) without increasing the number of ports in the cylinder. This sort of increase is fine for road racing and scrambling, but not for trials riding. The trials rider and the road rider do not need all that extra power, but will benefit from the extra reliability and increased tractability that this type of engine preparation and enhancement would give them in the lower power role, so it can be seen that nothing is lost, and all this development can benefit every branch of motorcycle sport.
All riders of Villiers single cylinder powered machines, whatever the branch of motorcycling they are involved in, can profit from the information gathered together and documented by the authors and detailed in this manual. Many of the ideas and modifications listed, have been formulated, tried and tested by the authors, and are willingly passed on to other 9E users to help them understand, modify and maintain their motors.
Throughout this manual reference is made to the authors own engines, and in particular any theory and calculations are illustrated with real data taken from these engines. The engines used by the authors have received the use and abuse over many years that have taught many of the lessons repeated here. The engines used are:
The section on pistons, for example, is concerned with ring width as well as ring clearance. Ring width is of concern because of the phenomena of ring flutter, and the role that it plays in the onset of seizure. The sections on crankcase and carburation are both interested in the determination of maximum torque rpm, and the carburation discussion continues to determine the maximum and minimum gas speeds in order to calculate the optimum carburettor choke size. Where the discussion includes some mathematics, we have tried to provide a data table which is suitable for the majority of applications in an appendix. This reduces the complexity of the text, and allows the reader to apply the technique to a particular circumstance which is not covered by the data tables.
We believe that the readers of this book will either have access to a workshop, or be able to get minor engineering jobs done locally. The section on special tools gives the tools that are easily available and we can expect you to possess, it identifies others that are easily made, or may be found sometimes at autojumbles. It is almost always necessary to raise the barrel in order to change the standard power characteristics in any significant amount, and this will involve lathe work. If you are tuning a road barrel then you have to decide how much to lift the barrel with a spacer under the barrel to increase the exhaust and transfer port timings, and possibly how much to remove from the top of the exhaust port which increases the exhaust timing without affecting the transfer timing. Lifting the whole barrel will obviously affect the inlet port as well as exhaust and transfer ports. Raising the barrel has a less narrowing effect on the power band than raising the exhaust alone, the reasons for this are explained with the help of port time-area diagrams which is a major feature of the discussion on exhausts.
Keeping some harmony with the port shapes and sizes is important, and harmony is the prime aim of this manual. First, a word of caution when using theory: its only theory. Life is chaotic not simplistic, mathematical models and other devices help us cope with complexity, and we must recognise their purpose and limitations and not be lead astray by instabilities in our mathematics.
We have reviewed the original research papers published in the 1960's and 1970's which gave rise to the "rules of thumb" that have been passed down through time. This allows us now to replace the rules of thumb with "approximations by calculator" derived from the original research not based on hearsay. Many papers have been published, but those by Fujio Nagao of Kyoto University and Gordon Blair of Queens University Belfast have made a particular contribution. A number of references to their papers appear in the appendices. The researchers at QUB continue to publish papers relevant to 2-stroke performance, and are currently involved in rewriting the classical tuning theory based on acoustics, with a more precise prediction on behaviour and performance based on unsteady gas dynamics. In this environment, computers become mandatory. Research papers from the 1980's and early 1990's do not seem to have made it into such tuning literature that has been published recently. It is possible that the research material published over the last 15 years is being aired "publicly" for the first time outside of the laboratory or the works teams. The understanding of transfer port performance, scavenging and charge short circuit into the exhaust port is an example of the painstaking research accomplished by QUB. Its application demands new and higher standards of understanding and machining accuracy.
There are several conventions used in describing ports and compression ratios in two stroke engines, the conventions used in this book are as follows:
UnitsUnless specifically stated at each instance, the units used are
PortsA four port barrel has 1 inlet, 1 exhaust and 2 transfer ports.
A six port barrel has 1 inlet, 1 exhaust and 4 transfer ports.
Port timingPort timings for Inlet are degrees of opening before top dead-centre (BTDC).
A port is deemed closed when you cannot any longer get the tip of a 20 thou feeler gauge into the port while holding the gauge at 45o to the vertical.
Compression ratiosCompression ratios may be "real" or "geometric". Throughout this manual we deal with real compression ratios: primary compression starts when the inlet port closes, and the secondary compression starts when the exhaust port closes.