Just out of curiosity really, I was wondering what advantages there are in having two sets of contact breaker points in your distributor. I know that the early Triumph Stags had one set, but were changed to twin points during the production run. I've seen race spec distributors advertised as having two sets as well, but can't get my head round why it would be an advantage - the Stag's are a pain to set up!
If anybody could point me in the direction of, or even write an explanation here, that would be great.
Quite simple really, the cam will have less lobes on it, thus giving a greater point movement between each cylinder. A cam with 8 lobes on it will only have a small amount of movement on the points, the point gap is a lot more critical. With two sets of points, the cam only has 4 lobes.
I believe on some of the jag v12 engines they went even further and had two distributors.
I've seen several different setups. Two sets of points merely in parallel. Suppose it's a way of keeping the inertia down while retaining the correct current switching capability.
A lobe with half the number of cylinders and two sets of points and coils. This makes sense as you can get a better dwell characteristic at high revs, so therefore a more reliable spark. They do need careful setting up as of course each bank of cylinders can end up with different timing.
Not UK spec Rover V-8s I had one of the very early ones which had a single point distributor. What it did have that was useful was external adjustment of the points, so you simply connected a dwell meter and turned a bolt until the meter read correctly - a two minute job. Which was just as well as it needed doing rather more regularly than points setting on a four cylinder.
This was the set up I thought was used on Rover V8 (Mallory distributer??) the points were in parallel and one set broke the circuit the other made it, it was to force a certain amount of dwell between sparks so the current in the primary circuit of the coil had enough energy.
I was a bit disappointed to find that the new distributor for my 1966 car doesn't even have a Vernier to set the timing - you have to slacken the pinch bolt and turn the whole thing by hand. Not exactly a guarantee of accuracy. Wonder when Verniers disappeared - I don't buy distributors very often!
...until the plastic heel of the points starts to wear; then it's very useful. A click or two towards 'advance' now and then as required keeps my Morris 1000 lively. Tufnol-insulated points were far better in this respect.
I'm currently looking around shedspace trying to locate the home-made capacitor discharge ignition unit which I took out of the Imp van I scrapped twenty-mumble years ago; that device to lower the points current to 50mA or so, plus a pair of hard-wearing NOS Tufnol points, should virtually eliminate timing drift.
Years ago I was told that motorcycles didn't have distributors and had a separate set of points and a coil for each cylinder or pair of cylinders (wasted spark on 180 deg twins and 4's) due to the number of sparks required per sec causing points contact bounce. Around 400 sparks/sec - 12,000 rpm for a 4, 8.000 on a 6, 6000 rpm for a 8. This was a limiting rate and pushing it was a crude rev limiter, the ignition would become so erratic that there was a rapid drop in power, anyone that wanted good reliable ignition didn't push much above 200 sparks/sec. It also allowed the precise timing of each cylinder to match any slight misalignments due to angular tolerance between cylinders or in the case of a 'V' between banks.
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Well, as I rather expected you've had lots of answers and every single one has missed the point (excuse the pun). One even suggested you'd use fewer lobes - obviously he's never seen a Stag distributor.
The way points work is that they close (to build up current in the coil) then open (to cause a spark) as the cam lifts them. Now for obvious mechanical reasons there is a limit to how steep the cam can be. There is a limit to how fast the points can move. There is a limit to how non-linear the lift profile is. And there is a limit to how quickly the points will close.
The next thing to consider is the electrical side of things. When the current in the coil is interrupted by the points opening, there is a large backlash EMF (voltage) generated. On the HT side this is what makes the spark, but it is also inevitably present (in reduced form) on the low tension primary. In order to avoid pitting of the points, you need to prevent this from causing (too much of) a spark there. The condenser helps, mostly by slowing the rise of the voltage. However, the really important thing is to make sure the points gap is big enough soon enough.
Now given the mechanical limits, the speed at which the points reach a big enough gap depends on the full gap you can achieve (i.e. the measured points gap you have to set). Make this bigger and you get more safety margin. Unfortunately you also get less dwell. On a 4 cylinder engine that isn't a problem. On a moderately high revving V8 it is.
The neat thing about the twin contact distributors, is that the two sets of points are out of phase by about half a firing interval. In other words they follow the sequence : close 1, close 2, open 1, open 2. This means that if each set of contacts has a 50% dwell, then the overall system gets 75% dwell, and does so with the same opening rate and thus spark margin as a normal 50% dwell setup.
Aren't they just! My big bugbear was the set that is 90 degrees round from the vacuum advance lever. If you have any wear in the advance plate it will begin to shift sideways with the vacuum advance rather than rotate cleanly. That set of points consequently ends up with a gap that varies with engine load. Mine frequently got to a state where it would cut out completely on full load. If I adjusted them too far the other way it would cut out completely on lift-off.
That's why I fitted lumenition - first car I'd ever felt the need.
Many thanks for the info; I'll bear them in mind if the original one doesn't turn up. The Minor 1000 has been changed to negative earth, as with our other previously-positive-earth cars.
Loads of fun! The Bantam racing manual says stay well clear as you will chase elusive earth faults until you lose all your hair. You won't experience this as you have it under a distributer cap. A mate had one on a classic racing motorcycle, it would run going up the paddock but it missed and ran really badly coming back. When he got back everyone crowded round and it picked up just fine so off he went again, came back sad. It was a real mystery up, down, up etc. Walked up to where he was blipping the throttle, crowded round, it picked up and ran fine, he pulled foreward 6ft, dead flat. Then someone noticed that as they moved around to get a look it ran fine, duff, fine, duff. shade, sun, shade, sun. The pick up was getting confused by the IR from the sun, once we sprayed a plastic paint can lid black and tapped it over the pickup it ran just fine. The one I had on my Bantam ran just fine too as it was under an alloy sun shade.
-- Peter Hill Spamtrap reply domain as per NNTP-Posting-Host in header Can of worms - what every fisherman wants. Can of worms - what every PC owner gets!
Contemporary Rover V8s and Stag V8s used the same distributors - both engines were fitted with both single and twin breaker systems at various stages.
Also the Rover V8 at the time was a noticeably lower revving engine -
5200 RPM in the MGB IIRC, as against 6500 RPM for the Stag.
Certainly it is possible to get away with a single set of points. It does, however, need rather more precision in breaker settings than is entirely practical.
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