General Spark plug question - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 20 (permalink) Old 10-03-2011, 10:37 AM Thread Starter
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General Spark plug question

This may seem like a silly question, but what is the purpose of 2 spark plugs per cylinder. From the diagrams, as far as I can tell they both fire at the same time. Is it to cut down on pollution, increase power, or what ?
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post #2 of 20 (permalink) Old 10-03-2011, 10:48 AM
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I asked the same thing when I first got this bike. I was told it is for a more complete burn of the fuel. I guess that works for better fuel efficiency/emissions?
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post #3 of 20 (permalink) Old 10-03-2011, 11:47 AM
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Some aircraft engines use two plugs per cylinder, and Kawi is following their lead by using this same approach.

Yes, better power, better fuel burn, cleaner emissions....it's a win-win.

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post #4 of 20 (permalink) Old 10-03-2011, 03:58 PM
Crap, I WAS in 5th gear.
 
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The are two main reasons there are two spark plugs: one is to better control the flame-front for even burning, which allows a sightly higer compression ratio and/or leaner mixtures without detonation. Reason two is that on aircraft, as on our bikes, it is nice to have redundant systems in case a coil, plug or wire fails.

Pull one plug wire off of each cylinder, and you will see how much the second plug helps!
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post #5 of 20 (permalink) Old 10-03-2011, 04:43 PM
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If you really want to get into the nitty gritty if it, it has been discussed in the past that the second set of plugs is definitly part of the emissions system for a cleaner burn. Mainly to burn up the excess fuel on the exit side of things. I'm not sure how one would verify this but I'm sure the answer is in the manual somewhere. I know there's only two pick ups for the coils so it's probably a timing thing. That's about all I know about it.

Kinda like a 4cyl ranger I had with 8 plugs. One set burns on the way in & one set burns on the way out.
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post #6 of 20 (permalink) Old 10-03-2011, 06:39 PM
If only it had 6th gear..
 
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I think it's mostly for performance as Kelly2001 explained since they fire in unison off the same coil. Higher compression Dual overhead cam, 8 valves, and dual carbs all help to make this little v twin perform beyond it's size. If it had one more gear I really think it would top out beyond 125 wich is prettu darn good for a v. I'm not discounting the emissions though as I'm told that the plugs fire on the exhaust stroke as well as the power stroke but you're only burning remnant fuel at that stage. I've played with a few hyped up engines and my reasoning leans to the performance side.

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post #7 of 20 (permalink) Old 10-03-2011, 09:38 PM
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Performance.....Emissions....? Tastes great, less filling. Your all kinda right here.... I don't think the plugs fire on the exhaust stroke... Would be kinda dangerous don't you think? ( and noisey )

The two plugs are to insure complete combustion....this does of course reduce emissions ( and of course helps with performance.)

But I would wager the driving reason Kawasaki used the two plugs per cylinder on the Vulcan was to pass US emissions.....not to just increase power, (even if it was another benefit of doing so)

It was really just something that cost little to do and was efficient...

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post #8 of 20 (permalink) Old 10-03-2011, 10:00 PM
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Ford, Nissan, Dodge, Toyota all run 2 plugs per cylinder on the 4 cylinder engines to help with emissions.
80's Yamaha 650's ran 1 coil to fire both cylinders with one pickup coil. As one fired on compression, the other would fire on exhaust. No extra noise, and it was primarily to help with fuel burn.
One would think as it fired on the exhaust, it would pop out that side, but mine never did that. Ran really good, especially after I uncapped the mixture screws and set them equally for both cylinders. I was getting in the high 50 MPG with mine. The gearing was pretty good which helped. The final ratio was 2:1, 17 tooth primary, 34 tooth final.
She would get up to 60 in second gear pretty easily.
I hated to sell her, but I'm looking to get another one, but when I do, it's going to be an earlier model with points and dual coils. Easier to work on, and can get better performance out of them.
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post #9 of 20 (permalink) Old 10-03-2011, 10:25 PM
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All sounds good, but,, thank goodness we dont have two (2) magnetos to time up like the receps do on aircraft. Been there done that!


Vulcan #1 1993 - 28K
Vulcan #2 1999 - 26k
It All Comes Down To 3 Things:
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#2 Spark
#3 Mechanical
Any One Fails - Then It Dont Run

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post #10 of 20 (permalink) Old 10-03-2011, 11:00 PM
If only it had 6th gear..
 
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I'm definitely learning here (as usual on this site) and another thought comes to mind when thinking about even combustion. Given that there's no head room above the engine and the dohc set up, placement of a single plug may not have been feasible.

Dave

Mods:
Jetted, shaved & coastered
Home made under-tank single filter
Converted ACCT's to manual (thanks wib714)
Dual vac lines on petcock (my cure for ticking)
LED tail/plate lights
Relocated helmet lock to license plate

Added stuff:
Customized hard bags w/ lights
Chrome levers
Hellcat windscreen
LED running lights
Home made 'Vita-Volt' meter
Foot rests on engine guard
Tunes

Hmm....she was on the back when we left.... ??
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