Dual Plugging Part 1 – Timing
There are two main camps for the spark timing of a dual plugged airhead engine. Both seem in rough agreement that timing should be retarded to about 28 degrees of maximum advance compared to the stock BMW timing specification of 31 or 34 degrees depending on model (see note 1). This retardation of the spark (compared to the stock setting) is necessary because the two flame fronts from two spark plugs result in a shorter burn duration which necessitates a delay in combustion in order that the peak cylinder pressure remains at the optimum crank angle.
Note 1: Earlier airheads up to and including /6 series had a static timing of 9 degrees Before Top Dead Centre (BTDC) – the ‘S’ mark on the flywheel. And a maximum timing advance of 34 degrees BTDC – the ‘F’ mark on the flywheel.
From 1977 with the introduction of the /7 series, the static timing and ‘S’ mark was changed to 6 degrees BTDC and a maximun timing, ‘Z’ mark of 31 degrees.
Note that in both cases, the range of the advance mechanism has remained 25 degrees.
So, to achieve a maximum advance of 28 degrees for dual sparked machines, earlier models need to have their maximum advance reduced by around 6 degrees (34-6=28) and later models by 3 degrees (31-3=28).
The two camps may be roughly summarized as follows.
Camp 1 – Retard Static Timing
For those who don’t want to fiddle with the innards of the mysterious timing canister or ‘bean can’* or install an aftermarket electronic ignition that caters for dual plugging, there is little choice but to retard the timing at idle (static timing) from the BMW spec by said 6 or 3 degrees depending on model. This does not change the shape of the timing curve that the stock bean can delivers, but shifts it in its entirety by the same fixed amount across the rev range and causes the maximum advanced to be reduced to around 28 degrees.
The ‘OT’ mark on the flywheel represents TDC (zero degrees). So, to retard the stock timing by 6 degrees for earlier models and 3 degrees for later models, people are timing their machines at idle so that the OT mark is at, or around the center of the timing viewing port.
The stock maximum advance of 34 or 31 degrees (depending on model) is designated by the ‘F’ or ‘Z’ mark on the flywheel and occurs beyond 3000 RPM with the stock bean can. The retardation that is introduced at idle also shifts the point of maximum advance by 6 or 3 degrees to the target 28 degrees. In which case, you and your strobe may just see the ‘F’ or ‘Z’ mark hanging around the very bottom of the viewing port at engine speeds of 3000 RPM and above.
A handy number to keep in mind is that one degree of crank angle corresponds to a tad under 2mm along the periphery of the flywheel i.e. where the various timing marks are stamped. So for /6 machines and earlier, the required 6 degree shift for both ‘S’ and ‘F’ marks should see them move around 12mm down the viewing window. Later machines that require a 3 degree reduction in maximum timing should be set with a 6mm shift of the ‘S’ and ‘Z’ marks down the window.
* In this article ‘bean can’ refers to the timing canister found in >1981 airhead motors. i.e. when electronic ignition was introduced. Not to be confused with the bean can found in models immediately proceeding 1981 which contained points style ignition.
Camp 2 – Modify Advance Mechanism
For those who are brave enough to mess about with the fiddly bits in the bean can, springs can be played with, bobwieght masses changed, and travels altered in order to change the shape of the original timing curve. An infinite array of curves can be achieved in this way.
There is rough consensus within this camp that in addition to retarding the maximum advance by 6 or 3 degrees depending on model, dual plugged engines enjoy curves that are stretched compared to the stock curve so that maximum advance occures at higher engine speeds than stock. Some say 3500 RPM is good. This is likely due to the fact that people often dual plug in order to increase compression ratio.
For this scenario, the timing at idle is generally set at around stock (the ‘S’ mark) and the range of advance that the bean can delivers is compressed by our 3 or 6 degrees (depending on model) in order that the timing at maximum advance is retarded by that same amount.
In other words, for later machines, the range of the auto advance mechanism is reduced from the stock 25 degrees to 22 degrees in order to obtain the target maximum advance of 28 degrees i.e. 6 + 22 = 28. The 6 degrees being the stock timing at idle.
For /6 and earlier machines with stock timing at idle of 9 degrees and max timing of 34 degrees, one could obtain the required 6 degree shift to 28 degree maximum timing by reducing the static timing to the later spec of 6 degrees and using the modified 22 degree advance mechanism. i.e. 9 – 3 + 22 = 28.
Details of how to modify the auto advance range of the beancan can be found here
So is it worth it to modify the bean can? Only one way to find out…
I went about the business of modifying a bean can according to the vagaries of the 2nd camp as described above. Installed it in my bike and set the timing at idle to around stock. In this case, just a tad retarded on the stock figure at 5 degrees BTDC. With degree wheel and strobe, the curve of the modified can was mapped and graphed.
The two camps are represented below. The black curve is the stock spark timing. The red curve is camp 1. and the blue is camp 2 – my modified bean can.
dyno comparison of the two camps
Off to the dyno man I went to compare both camps. The dyno chart below is pretty ugly due to some tuning issues that were fixed up later, but what it clearly shows is that modifying the bean can – camp 2- improves performance almost over the entire rev range compared to using an un-modified with retarded advance – camp 1.
So, yes, it is definitely worth modifying the bean can or installing an aftermarket electronic ignition that provides an ignition curve that is designed for dual plugged airheads – Silent Hektik, Dignition, and Omega are some of the better known brands.