Yes, and when I think about it, my Honda revs twice as fast as the Vin, so it can't be that.
H
Ignition timing for twin plug heads
- Thread starter vince998
- Start date
Yes, and when I think about it, my Honda revs twice as fast as the Vin, so it can't be that.
H
Here is my understanding of the situation with ignition coils. I may be wrong, in fact I am told I am usually wrong, but here goes.
A standard single output ignition coil, one end of the secondary coil connected to earth. When a high voltage is generated in the secondary the other end of the coil connected to the spark plug rises in voltage until the plug fires accross to earth (bike cylinder). That completes the electrical circuit and the secondary coil is discharged ready to start all over again.
In most dual output ignition coils both ends of the secondary coil are open ended and not connected to earth. The two HT leads out of a twin output coil are the two ends of the secondary. When a high voltage is generated in the secondary to complete the electrical circuit both HT outputs rise in voltage. One end will be a positive voltage and the other lead will be a negative voltage. Now for electricity to flow the circuit must be completed so both spark plugs will fire to earth and complete the circuit. Now the coils are designed to break down two plugs at a time, but at different voltages. The lead going to the plug in the cylinder which is on its exhaust stroke (wasted spark) will break down a a low voltage due to been at approx atmospheric pressure. This leaves a higher voltage at the opposite end of the secondary coil to break down the spark plug in the cylinder under compression as this spark plug will need a higher voltage to break down due to the higher pressure in the cylinder. Then as the cylinders swap strokes the breakdown voltages of the outputs swap.
The reason the coil will not break down two spark plugs under compression (ie both connected to one cylinder firing twin plugs) is the total breakdown voltage which is the voltage breakdown of both plugs added together required to fire both plugs in the high cylinder pressure is higher than the max voltage generated in the secondary. Its a trade off against size and the insulation breakdown of the coil.
Now I believe there are coils out there that can fire both plugs under compression, but they are large and expensive as the internals have to be able to withstand the higher voltages generated.
So on a twin cylinder bike running twin plugs on each cylinder you can use twin output coils, but each coil must have an ht lead going to each cylinder and the two coils firing at the same time. You could use 4 seperate coils if you have the room.
On a single cylinder bike running twin plugs you cannot use a twin output coil unless it is specifically designed to run like that. You will need two seperate coils firing at the same time.
The next confusion is the primary coil resistance. This all depends on the type of ignition you are using and the voltage you are running the bike on. With most points systems with no amplifier running on 12V you need to have a 12V coil with a primary resistance of 5 ohms approx. On a 6v points system you will need a 6V coil with a primary resistance of 2.5 ohms approx. This gives a current through the points of approx 2.5A so as not to burn the points out. Don't forget connecting two coils in series will double the total primary resistance in the system and so keep the current through the points down, but will half the voltage across each coil. So if you are on 12V points system running two coils in series you will need two 6V coils with a resistance of 2.5 ohms so to give a total primary resistance of 5 ohms as recommended for a 12V system. Running two coils in parallel on points will burn out the points a lot quicker as the current flowing through the points will be double.
If you have electronic ignition, you will have to refer to the manufacturers recommendation for coil primary resistance, voltage etc as they can be different for each design.
I feel I need a beer after writing that!!!
Cheers,
Simon.
A standard single output ignition coil, one end of the secondary coil connected to earth. When a high voltage is generated in the secondary the other end of the coil connected to the spark plug rises in voltage until the plug fires accross to earth (bike cylinder). That completes the electrical circuit and the secondary coil is discharged ready to start all over again.
In most dual output ignition coils both ends of the secondary coil are open ended and not connected to earth. The two HT leads out of a twin output coil are the two ends of the secondary. When a high voltage is generated in the secondary to complete the electrical circuit both HT outputs rise in voltage. One end will be a positive voltage and the other lead will be a negative voltage. Now for electricity to flow the circuit must be completed so both spark plugs will fire to earth and complete the circuit. Now the coils are designed to break down two plugs at a time, but at different voltages. The lead going to the plug in the cylinder which is on its exhaust stroke (wasted spark) will break down a a low voltage due to been at approx atmospheric pressure. This leaves a higher voltage at the opposite end of the secondary coil to break down the spark plug in the cylinder under compression as this spark plug will need a higher voltage to break down due to the higher pressure in the cylinder. Then as the cylinders swap strokes the breakdown voltages of the outputs swap.
The reason the coil will not break down two spark plugs under compression (ie both connected to one cylinder firing twin plugs) is the total breakdown voltage which is the voltage breakdown of both plugs added together required to fire both plugs in the high cylinder pressure is higher than the max voltage generated in the secondary. Its a trade off against size and the insulation breakdown of the coil.
Now I believe there are coils out there that can fire both plugs under compression, but they are large and expensive as the internals have to be able to withstand the higher voltages generated.
So on a twin cylinder bike running twin plugs on each cylinder you can use twin output coils, but each coil must have an ht lead going to each cylinder and the two coils firing at the same time. You could use 4 seperate coils if you have the room.
On a single cylinder bike running twin plugs you cannot use a twin output coil unless it is specifically designed to run like that. You will need two seperate coils firing at the same time.
The next confusion is the primary coil resistance. This all depends on the type of ignition you are using and the voltage you are running the bike on. With most points systems with no amplifier running on 12V you need to have a 12V coil with a primary resistance of 5 ohms approx. On a 6v points system you will need a 6V coil with a primary resistance of 2.5 ohms approx. This gives a current through the points of approx 2.5A so as not to burn the points out. Don't forget connecting two coils in series will double the total primary resistance in the system and so keep the current through the points down, but will half the voltage across each coil. So if you are on 12V points system running two coils in series you will need two 6V coils with a resistance of 2.5 ohms so to give a total primary resistance of 5 ohms as recommended for a 12V system. Running two coils in parallel on points will burn out the points a lot quicker as the current flowing through the points will be double.
If you have electronic ignition, you will have to refer to the manufacturers recommendation for coil primary resistance, voltage etc as they can be different for each design.
I feel I need a beer after writing that!!!
Cheers,
Simon.
Here is a gallery of the Talabach racing outfit. http://www.pbase.com/coreyl/red_molly
It has twin plugs. The owner is a Club member and probably could be contacted through the New England S.O. regarding ignition timing
I see his coils fire front and back cylinders. As another example, my '77 BMW twin plug coils fire the same cylinder.
It has twin plugs. The owner is a Club member and probably could be contacted through the New England S.O. regarding ignition timing
I see his coils fire front and back cylinders. As another example, my '77 BMW twin plug coils fire the same cylinder.
Many thanks for all of the advice here chaps.
The route i´ll take is:
1.) up size on main jets (already ordered)
2.) If this brings no results then i´ll try different coils.
3.) If this doesn´t work, then i´ll resort back to the original "D" distributor with single plug heads and see what that brings.
If all else fails, i´ll polish it so that it at least looks good (even if it don´t really go) and get someone else to have a look.
The route i´ll take is:
1.) up size on main jets (already ordered)
2.) If this brings no results then i´ll try different coils.
3.) If this doesn´t work, then i´ll resort back to the original "D" distributor with single plug heads and see what that brings.
If all else fails, i´ll polish it so that it at least looks good (even if it don´t really go) and get someone else to have a look.