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[Pan1]

Why does the M8 have two plugs per cylinder? Do they both fire at the same time?

[GregKhougaz]

Theoretically, this creates faster combustion over a shorter period of time leading to a cooler motor.

Good article on the Milwaukee 8:  https://www.cycleworld.com/harley-davidson-motorcycles-new-milwaukee-eight-big-twin-engine#page-3

[Fired00d]

Also makes it a PITA to change two of those four plugs... they require removal of the gas tank to get too... which (more than likely) will be more billable hours.. IE: costing owner's (that don't do their own services) more $$$.

 
Ride Safe,
Fired00d
 

[Pan1]

I would think of both plugs ignite fuel at the same time that when the two burning fronts meet , itcwould csuse a setinationvknock.

[NH Bulldog]

Two spark plugs per cylinder provides for more efficient and complete ignition of the fuel in the cylinder, lowering emissions and squeezing more power out of the motor.  My Ram truck with the HEMI V8 also has two plugs per cylinder.

[Phreakyz]

Theoretically, this creates faster combustion over a shorter period of time leading to a cooler motor.

Good article on the Milwaukee 8:  https://www.cycleworld.com/harley-davidson-motorcycles-new-milwaukee-eight-big-twin-engine#page-3

Great Article!  Thanks for sharing!

[hrdtail78]

Pretty small combustion chamber and flat top for it to be a huge advantage.   I’d bet it has more to do with lower engine speeds and EPA.

[Ironhorse]

I had a '91 Evo Heritage I modded out. One of the things I did was dual plug the heads and go for single fire ignition. It helped smooth the engine out by burning cleaner with no detonation.

[MCE]

It makes for faster flame propagation, (which requires less spark advance) The combustion chamber is rather large
too (compared to the TC), so it would take longer for a single flame front to propagate the entire chamber, which would
waste energy and likely result in higher emissions.

I'm sure allot of research went into them. They are quite a bit better in several ways and they're not going away.

I'm wondering if this ancient pushrod style motor will be able to take full advantage of the breathing capacity. I'm not
convinced, but it's still early in the game.

 

[hdaliaconis]

Nothing new.  Been done before!  Just ride it!

[smkymtnboy]

it takes two to burn the ethanol!!

[Threephase]

I dont believe they fire simultaneously. If they are like my Hemi which has 2 plugs per cylinder, the 2nd plug fires on the exhaust stroke to burn any unspent fuel. It is an emission thing. The secondary plugs look almost new even after 30k miles.