BT-H Magnetos Ltd.

TERRY'S IGNITION TWEAKS BY TERRY WRIGHT

This article was originally written as a submission to the CAMS Eligibility Committee and it resulted in approval being given for a self-contained electronic substitute for magnetos on air-cooled cars. Here Terry Wright looks at the history of ignition systems and some of the problems they pose for the air-cooled car.

Historic race engines are inherently unreliable when key components have long exceeded their intended service life and/or conditions.
This can be especially so in the case of ignition instruments whose repair may be beyond all but the most skilled technician. Magnetos need to be spot on mechanically, as well as electrically, and finding someone who can reliably deal with the mechanical side of things, as well as the electrical, is not easy.
This has long been the case, as the following in a 1948 book (1) on ignition testifies:
"The rotary armature magneto … can only be really successful when constructed to precision limits. The initial assembly and also the servicing of this type of instrument is a highly skilled job"
If, like me, you find yourself with seemingly unreliable magnetos, what are your options? We can first learn a bit about ignition systems from history. 
Right back to the internal-combustion engine first able to perform continuously under industrial conditions (Lenoir 1860/1) there is a common thread - a rapid drop in voltage in a primary coil induces a high voltage in a secondary coil which produces a high-tension spark to ignite the compressed fuel.
The coils may stand-alone (as in points-triggered coil-ignition) where the current is usually supplied by a battery, or be part of a self-contained unit (magneto-ignition) where the current is generated internally.
The collapse of the primary voltage was almost universally triggered by 'make-and-break' or 'contact breaker' points driven by the engine. An orthodox contact breaker is not an accurate timing device, with errors of 1½ degrees of crankshaft angle being reported (2). 
So along came various 'electronic’ aids. Transistor-assisted ignition was an early one where contact-breakers could control higher primary currents (thus producing fatter sparks) without overloading the points and causing them to erode.  Eventually points were superseded altogether by magnetic or optical switches and transistorised circuits. Without points the currents in the coils and the energy of the sparks can be much higher because the deterioration of the points is eliminated and much greater precision and control of the spark is possible.

Compared with CD systems, modern inductive ignition systems (3) are said to have "high energy and long, programmable arc duration [which] are an advantage since they provide better lighting of lean or non-homogeneous air/fuel mixtures". 
One authority says that while a points-triggered system can only produce enough sparks for 9,000 r.p.m. in a four cylinder engine a modern high energy system is good for 15,000 r.p.m (4).
Apart from having fewer cylinders to worry about, none of this matters much to Loose Fillings readers, as conversion of distributors, and magnetos, to electronic or 'breakerless' operation is widely prohibited in historic competition. e.g. FIA, CAMS (Australia), VSCC, 500OA, HFJRA (UK).
However conversion from magneto ignition to points-triggered coil-ignition is often permitted. It requires the installation of a battery and one or more coils as well as modification or replacement of the magnetos themselves.
In the case of air-cooled (that is mainly motorcycle-type engined cars) the ubiquitous rotating-armature magneto is not easily converted to points-triggered coil ignition. This is because of the difficulty of finding small enough contact breakers to fit within typical housing dimensions with the contact breakers rotating inside the cam-ring.
One solution is to modify a rotating-magnet type magneto which has a fixed contact breaker operated by a rotating cam similar to a conventional distributor. It is relatively simple to use just the contact breaker in conjunction with an external coil and battery in a way that just isn't possible with the traditional type.
This is the path I went down with the Walton-Cooper, using, as other have, the relatively common Lucas SR1 which was first introduced in the late 'forties or early 'fifties and, in a racing version, was fitted to the Manx Norton amongst others.
However a problem of an elusive top-end misfire held me back until late 2006. when I sorted it with the addition of a Dyna ‘ignition booster ‘. Sounding a bit too good to be true, this little box of tricks cleans up and reinforces the primary voltage from the contact breakers to the coils.(5). I used dual-fire Harley-Davidson after-market coils with one for a pair of plugs on each cylinder. Those that I bought in my innocence (Andrews street coils) just weren't up to the job of trying to fire two plugs under compression at the same time in conjunction with supercharging. I probably should have used the highly regarded coils made by Dyna which have proved themselves in a multitude of conversions in the UK.
Although I think I have solved my early problems, my enthusiasm for coil-ignition is now much diminished and I am making a new effort to service my aging BT-H dirt track magnetos and give them another chance. I also hope to be testing a UK developed self-contained unit that looks roughly like a 1950's magneto and which comprises a generator with electronic ignition components internally i.e. no points.
These are being made for twins and singles and are being supplied under the BT-H trademark; for further particulars see http://www.bt-h.biz
In a recent review of its policy on ignitions systems, the CAMS Historic Eligibility Committee decided not to allow any further penetration of electronic components except as follows (from the minutes):
"The Committee further discussed the replacement of magnetos with distributors which has been approved in the past. The Committee agreed that this practice will be permitted to continue.
The Committee then discussed permitting the use of an electronic magneto substitution. It was the decision of the Committee to permit the use of an electronic magneto substitution for single and twin cylinder air-cooled motors only. These units must be self-contained and require no external battery, or external electronic controls, mount in the same position as the original magneto and have an appearance similar to a period magneto."
A conversion to points-triggered coil -ignition will cost you anything up to $500 per cylinder for coil, instrument purchase and conversion and rewiring. Buying and rebuilding a period magneto will cost you somewhat more. A new BT-H unit will come to about $1100, so it’s not cheap.
But the premium for a new box of tricks, which automatically retards to aid starting, may be well worthwhile in performance and reliability. We’ll let you know what we think when we have tried one out.

REFERENCES
1.'Ignition Equipment' by Edgar T Westbury, ;first published 1948, reprinted Tee Publishing 1993.
2.See the now available reprint (Ricardo plc) of the 5th edition (1968) of the classic High Speed Internal Combustion Engine by Sir Harry Ricardo and J G Henson.
3.When the energy developed in the coil during the dwell period (when the ignition trigger is closed) is stored within the coil or coils the system is described as "inductive". (See 'Inductive ignitions yield reliable firing', by Don Weaver and Kathy Boutin at http://www.dynalco.com). See also http://www.chevyhiperformance.com/techarticles/45618_inductive_cd_ignitions_basics/ ). 
4.A.Graham Bell's "Four stroke Performance Tuning" published by Haynes has a very good chapter on ignition systems.
5.The unit is simply inserted in the circuit between the contact breaker and the coil.

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Copyright © 2008 E. Grew