Geoff Battick's Notes on 2 Stroke Engine Modification

Suggested Reading

  I have all of these in my collection, which I have gathered for over twenty years. I also have many old magazines and copies of articles from magazines that have items of interest. The books in the first section all deal with motorcycles only. The second section includes information from any field that I have found relevant over the years.

    Back when the GP's were still a two stroke show, there were many informative articles written on the technology of the time. Old editions of Motocourse are also great sources of photos and tech pieces along with often stunning reports on the individual GP races of the day. The English bike magazine industry is also a good source, as Yamaha and all the other manufacturers continued to sell further evolved road going bikes long after the RZ350 met its demise over here.

Some of the books I list are out of print may through the miracle of the internet you may be able to find them with some luck. I bet the Jennings book commands a few bucks nowadays if you can find one. I will say that while that is the book that first brought a studied scientific approach to the tuning community, now thirty plus years (1973 pub date) later it is a bit dated in some areas.

It is a great read and a worthy addition to any library, but do not stay up nights wishing you had a copy. I would suggest the modern would-be tuner find Bell or Robinson first. They have the advantage of being written more recently (1983 and 1986 respectively) and therefore provide better information on reed valve intake systems and other areas as well as using RD series engines as examples on occasion in their texts.

It is very interesting to read all of the " big three ", Jennings, Bell, and Robinson, and compare their various styles and techniques. Jennings was the first to really stress the use of mathematical formulae in separating fact from fiction and defining just what you could realistically expect to achieve with your engine.

Jennings was very good at pointing out the flaws in the " black magic " approach to tuning so common then. The biggest hurdle you had to face when trying to learn from him was the level of math skills needed to use what he taught. For a person like myself for whom math was the least favourite school subject, it was either learn it or forget about it.

If you took the time to acquire the skills needed, Jennings could show you how to design pipes and port designs that would really work and would yield predictable results. His book was written in the pre-computer age, so you did all the work with pencil and paper and a scientific calculator. This was the first place I ever heard of things like Specific Time Area and Angle Area design, now the accepted method of design and tuning easily found by anyone with a modest home computer.

Bell published his work ten years later, and took a much simpler approach to the work. Through his many years of experience he came to see that the vast majority of engine designs used similar designs by type, meaning an enduro motor would usually fall within a narrow set of specifications, as would a roadracer or motocrosser no matter who made them. He could also from these observations see how an unsuccessful design could be improved by modifying it to mirror a successful design if physically possible.

He placed his emphasis on a less math intensive approach and simply stated that for an engine to make peak power at (X) rpm it would need (Y) amount of port open duration for each port type. So you would then get out your degree wheel, find your engines current timings and then modify all the ports in the cylinder to the duration's called out in his book. Port width was approached as a secondary consideration with the actually very good advice of start with the stock width and widen slowly until the best results were obtained or you broke something, usually the piston.

So Bells' simpler emphasis on timing is very easy to understand and it does have merit. It is more of a cut and try approach than Jennings had put forth ten years earlier but might be easier understood and experimented with by beginners. Like Jennings, Bell also goes into great detail regarding exhaust pipe design and does offer a math intensive design formula you can use to design your own system. Again, this was before you could simply fire up the PC and punch numbers into a TSR program and get a design in three minutes. With Bell and Jennings you have to get to know cotangents, square roots and the like before you can get results. One of Bell's strengths is the sheer amount of diagrams, charts, tuning mods and illustrations in his book, many of which are actual specific examples. You'll find actual physical dimensions for TZ250D/E exhaust pipes, port diagrams for an MT125R Honda, and much more. He also has a much greater understanding of reed valves and the porting designs that came with their development, something Jennings book is just too early to help with.

John Robinson's book, even though just three years newer than Bell, shows the tremendous leap in technology that was happening then. He fully embraces the full scientific approach of Jennings, where you can design a cylinder or motor mathematically by using time area and angle figures, or examine what you have now and see just what its potential is, mainly because he benefits from the advent of the home computer, and provides programs written out in BBC (acorn) BASIC for the real ambitious tuner to use.

WEBMASTERS Note:- Geo of the K.G.B of T&T collaborated with John in re-writing 2 versions of the BBC Basic program (with corrections to the original ) in Pascal & C to work in DOS for the IBM PC. Sadly John became very ill & died before the long promised version 2 of his books were ever published.
Geo has decided to make a downloadable ISO image of these programs for use on this site

Again, the progress of time has made this even simpler now, but he was the first person I am aware of in this field to espouse the use of a computer to the average man. Robinson also provides much practical advice, touching on things like port to cylinder entry angles and their effect on power spread or how to determine points of major restriction in airflow and what limits they may be setting on power production.

He makes good use of charts and tests to back up his findings, like a test of twelve different reed types to show just what that type of change actually does yield. His book is perhaps the most balanced of the three, offering a simpler practical approach like Bell for the most part but blended with a Jennings-style belief in math and engineering to explain why things work or do not.

As a plus, Robinson was very involved in Yamaha's LC racing series and his book shows that, which can be of great interest to those of us tuning the aircooled versions since those two engines are virtually the same aside from different their cooling methods.

None of these three books will disappoint someone who wants a greater understanding of how this specific type of engine works and how to improve them in a studied proven way. None of them merely repeat old lore and legends or are one model specific only or give you no genuine tools to use. The ideas and methods that all three espouse are applicable to any motor no matter what who the manufacturer, because they rise above being simply "tuning handbook for the XYZ-123". Once understood by the reader, they allow him to apply this knowledge to any two stroke engine you might come across and modify that motor to be optimised for the type of use intended.

They were all written by diligent, thoughtful and intelligent men and it shows in their writing and the method of thinking they impart to the reader. Sadly both Gordon Jennings and John Robinson have since left us in recent years, both still relatively young men in the scheme of things. It leaves me to wonder what more they might have had to add to to our knowledge if they still had wrench and word processor in hand.

Both were prodigious writers for various magazines during their times, so there is much more to be learned from them if you take the time to find their old works.

©  Geo of K.G.B. of T & T.