why so many spark plugs? - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 14 (permalink) Old 05-23-2007, 12:03 AM Thread Starter
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why so many spark plugs?

i have a 86' vulcan vn750. My manual says I have twin cylinders. So why do I have 4 spark plugs?
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post #2 of 14 (permalink) Old 05-23-2007, 04:43 AM
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Most people seem to agree that it's to get a better combustion of the gas.
If I remember correctly, for each cylinder, one plug fires very quickly after the other. Not both at once.


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post #3 of 14 (permalink) Old 05-23-2007, 10:27 AM
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I'm not a engineer but I've thought the same thing.

My opinion
In the combustion chamber, the location, number of and size of valves is less than ideal for a single plug, so they tend to compensate a incomplete burn by adding a plug. I believe that in all U.S. cars since the late 70's or so, there is a EGR valve to burn, unburned gases instead of two plugs but for pollution reasons.

There is a tendency in the past 30 plus years for more and larger valves, for better air flow which means more power and or efficient.
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post #4 of 14 (permalink) Old 05-23-2007, 10:29 AM
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some people also ask "why DOHC?" or "Why does it rev so high?"

The answer is that this bike was introduced with such a high tech V-Twin package at the time that it lasted another 20 years in the market virtually unchanged and could still smoke all the other stock V-twin 750's released!
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post #5 of 14 (permalink) Old 05-23-2007, 11:18 AM
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I agree with Hyper on this one. The bike has very good acceleration for a 750cc and it looks like the two plugs per cylinder are the reason.

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post #6 of 14 (permalink) Old 05-23-2007, 12:51 PM
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With two plugs there is better combustion in the cylinders. Better burning of the gas vapers. The way the old piston planes worked they both fired at the same time, this causes the gas to burn evenly in the cylinder and more complete. Old Air Force Dude.

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post #7 of 14 (permalink) Old 05-23-2007, 02:07 PM
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Had a Nissan pickup with a Nap-Z engine that had two plugs per cylinder and one was for combustion and the other plug was for pre exhaust, but I think
on our bike both plugs are for combustion.

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post #8 of 14 (permalink) Old 05-23-2007, 09:28 PM
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I to thought the 2nd set of plugs were for emissions mostly. Either way it does smoke all of the true V-twins out there. Even outran a 1100 Vstar... twice.

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post #9 of 14 (permalink) Old 05-24-2007, 01:31 AM
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Quote:
Originally Posted by bradweist View Post
i have a 86' vulcan vn750. My manual says I have twin cylinders. So why do I have 4 spark plugs?
There's no room on top of the cylinder for a spark plug so a spark plug is put on the side. That gives a funky asymmetrical burn so a second plug is put on the other side to give two flame fronts that meet in the center. A good thing for an engine that has to cruise at over 4500 rpm on the freeway.

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post #10 of 14 (permalink) Old 05-24-2007, 11:22 AM
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The two plug design has to do also with the speed the flame front spreads inside the combustion chamber. All things been the same the leaner the mixture the slower the flame front spreads so having 2 instead of just one is very good for high speed burning of the fuel.

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