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Ironhead Sportster Motorcycle Talk (1957-1985) For all those that wanna talk about Ironhead Sportster Motorcycles

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  #21  
Old 3 Weeks Ago
Scott Scott is offline
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Quote:
Originally Posted by ryder rick View Post
Dual plug it and the motor won't want stupid high advance numbers.
High spark advance needs are a sign of a poor combustion chamber design.

Bonus - the motor will now make more power and run more efficiently.
Rick - this is probably an ignorant question, so please bear with me. Doesn't dual plugging help reduce detonation? Wouldn't that allow high advance numbers? Aside from the obvious risk of detonation, what is the disadvantage of 45 degrees on a 1000 ironhead.

Last edited by Scott; 3 Weeks Ago at 16:50.. Reason: typos
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  #22  
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Dual plugging reduces a motors tendency to detonate by getting MORE of the mix lit, BEFORE the cylinder pressure rises to the point of detonation.

By getting more of the mix lit sooner it does not need so much advance.

Advancing a dual plug motor to 45° would be a mistake as it would detonate bad as peak cylinder pressure would occur too early.

High advance numbers are a band aid for a poorly designed combustion chamber.

Spark timing is a balancing act while juggling grenades. More advance does not always make more power.
Motors with crappy combustion chamber design require more advance, BUT they make less power and are more susceptible to detonation.
Making the chamber more efficient (by dual plugging) reduces the spark advance needed which also reduces the tendency to detonate while at the same time making more power.

I am convinced that the MoCo did not dual plug the Ironhead as it would have spanked their premier product.
Can't have the premier product (a Big Twin) getting beat by a girls bike....
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