Hi all, I have heard that magneto's for engines put out a much higher voltage than a coil, is this true ? and what sort of output could I expect to get from the mag ? Can you have to much voltage for a spark plug or are the plugs matched to the coil / magneto voltage output ? What is the difference between spark plugs, when they all look the same I know about temp ranges but how is this achieved? can some one explain in detail if possible. Thanks Richard
-- To reply remove " spam "
Back in the 1960s, I lived next door to a young lad who was studying electronics. He built himself an electronic ignition system, which he fitted to his HA Viva. After that, it would never start on foggy mornings. I was asked to take a look (he wasn't by any stretch of the imagination a mechanic!). This was a winter evening when I came home from work.
Looking at under the bonnet when he tried to start it, I saw the whole HT system light up with St Elmo's fire. I told him that the reason his car wouldn't start was because all his sparks were running around his engine bay instead of driving the plugs. I asked him what the design output was, and he thought for a bit and then told me it was just over 600,000V. To cut a long story short, he ended up encasing all his HT leads in thick wall plastic tubing with all the ends sealed with shrink wrap. His car started OK after that.
So yes, you can have too much voltage, but I wouldn't expect you to get enough out of a magneto to really worry about.
Jim
Magnetos are very reliable and give a very good spark at low speed. Modern uses are usually small aero engines.
Plug temperature ratings are varied by the amount of porcelain around the internal electrode (broadly speaking) A hot running plug will have little porcelain, while a cold, lots.
If the problem is wet fouling of plugs then use a hotter plug, if the plugs burn out then use a colder plug.
mrcheerful
The downside of magnetos is that it's not as easy to implement ignition-timing variation (vacuum and/or centrifugal) as with a coil. To do it with a magneto means you have to either move the coils or the rotor - it's hard to relatively move the rotor because it's transmitting significant torque (and torque which is reversing with every passage of the pole-pieces) and the coils are heavy too so aren't really amenable to sensitive control by weights or vacuum.
It's also hard to implement any real spark- energy-control on magnetos (there's no equivalent of variable-dwell systems or ballast-controlled energisation).
Most British bikes had a bob weight advance retard in the drive gear, I assume cars did too. A lot bigger than the little things you find in a points and coil distributor. The Merlin and other R-R aero engines had a bevel gear differential in the mag drive so the timing could be moved by a lever.
-- 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!
MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.