Dual points for R69S (or any twin with magneto ignition)

[schrader7032]

Tommy -

I thought we were talking about the stock Pentacomm plate...one set of points.

[jwonder]

I figured I would send this out to be clear what dual points do and what their function are. Generally they are to increase the dwell allowing for a hotter spark. One set of points makes the connection the other one breaks. I have yet to see a dual point system with two (2) coils, but not saying they do or don't exist.

Dual points are very positive for 8 cylinder motors to increase the dwell time. New engines (and Cosworth race engines, etc) with a 180 degree crank typically fire two plugs at a time like ours do. Engineers found that firing the plug on the exhaust stroke promotes better evacuation of the combustion chamber allowing for a better intake charge.

The dwell time of our opposed twins (180 degree crank) is huge, actually overkill. If you are having problems with not having a hot spark I would look at magnetizing the magneto, a new coil and all the other things Vech mentioned.

For a great way to figure out dwell time check out this article from Denso: https://www.denso-am.eu/news/deneur21_0 ... -charge-up

I LOVE the pentacomm plate as you can really adjust it perfectly, and easily; BUT, it does its work by varying the points gap, hence affecting the dwell. Again, the dwell in our opposed twins are so much overkill I am not worried by the small amount you may lose with the Pentacomm plate.

[johnlacko]

James, thanks for that explanation. I’m not familiar with two sets of points operating in tandem like that, but it makes perfect sense, especially in 8+ cylinder engines, or when revs start approaching 5 digit territory. I agree with other comments that without multiple coils and some type of a distributor cap to ‘steer’ the spark & split up the workload, dual points do not seem to be a huge benefit to our relatively low-reving bikes. The points design/mounting/adjustment on the BMW magneto twins is probably the most un-German idea I think I’ve ever seen. To my mind the Pentacom points plate is a far superior design, and probably gives you more real-world usable gains than these other more exotic options.

[vechorik1373]

Thanks Vech for detailed explanation. Can this be done with a Pentacomm plate?

Yes, same procedure. The only thing a pentacomm plate does, is it forces you to change the point gap, via use of an allen wrench, to adjust the timing. Unlike the original set up, here is no movable breaker plate that the points are mounted on.
With the Penticom plate, whatever point gap you end up with, be it too wide or too narrow (and there for changing the dwell out of the original specficiations) (the amount of time the points remain open, expressed in degrees) .
Which is precisely why I don't care for the Penticom plate.

[niall4473]

Personally, were I that bothered, I would adjust the mounting holes in the points and keep the gaps even.
Given that the inlet tract lengths and exhaust lengths are not equal ( though the exhaust length difference must be at least partially mitigated by the balance pipe), some timing differential may actually be desirable to get both pots running exactly right....

[schrader7032]

I'm not sure how the intake/exhaust tracks play in this. It comes down to those explosions in each cylinder and the timing of those power strokes. If they are too early or too late, the power delivery will be uneven to the crankshaft.

[FL54]

Thanks Vech for detailed explanation. Can this be done with a Pentacomm plate?

Yes, same procedure. The only thing a pentacomm plate does, is it forces you to change the point gap, via use of an allen wrench, to adjust the timing. Unlike the original set up, here is no movable breaker plate that the points are mounted on.
With the Penticom plate, whatever point gap you end up with, be it too wide or too narrow (and there for changing the dwell out of the original specficiations) (the amount of time the points remain open, expressed in degrees) .
Which is precisely why I don't care for the Penticom plate.

Update. Followed Vech’s instructions. Differential timing was off 3/16 to 1/4” so borderline according to BMW. One tap halved the distance between marks. Second tap went too far. Slight tap in opposite direction and now spot on. Bike started on first kick (although that’s pretty normal.) Waiting for some decent weather to road test. Thanks Vech. You’re the man.

[niall4473]

Schrader,
because the length of the inlet tract affects the filling pressure of the cylinder, and the exhaust length affects the back-pressure which in turn affects the exhausting of the spent charge, which in turn affects the induction of the fresh charge, each engine has an optimum length for both.
The resulting density of the charge in the cylinder has an effect on the speed of the flame-front post ignition, which dictates the amount of time required for complete combustion. The point at which the spark is delivered must correspond to the time required for the flame front to traverse the whole combustionchamber.
I am talking about very small differences here but, nonetheless they exist. The point that I was trying to make is that getting the timing exactly the same on both cylinders, to a figure which is probably a bit of a compromise should not be the prime objective, rather getting the bike to run as well as is possible.

[Seek]

Question then: How would you adjust the well running behaviour of the engine with the ignition timing difference? Do you have a procedure for that?

Running well is a very subjective meassure, depending on many variables and adjusting the timing difference is not a very exact procedure.

Getting a boxer engine well balanced is a good idea anyway. Starting with the crankshaft offset at as close to 180 degrees as possible. Equal combustion chambers. Equal valve spring heights. Balanced carburettors. And ignition timing difference as close as possible.

[schrader7032]

Niall, that's a good point. There are other features of a running engine for which we have no/little control over. What about the amount of carbon build up inside the combustion chamber? I think we're focused on those things that we do have the ability to measure and change and that will make a difference in performance.