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t dub has a wasted spark but the exhaust/wasted spark occurs on a whole new revolution, so on a tachometer install (which I'm not even doing, this question is just driving me crazy) I want it to count every spark even though one of them is a "wasted" spark, right? because no matter what it's one spark per revolution---



sincerely,



--aspiring to wrench my own tw
 

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t dub has a wasted spark but the exhaust/wasted spark occurs on a whole new revolution, so on a tachometer install (which I'm not even doing, this question is just driving me crazy) I want it to count every spark even though one of them is a "wasted" spark, right? because no matter what it's one spark per revolution---



sincerely,



--aspiring to wrench my own tw


I don't think so. Cut the revolution down into quarters:



  1. Intake
  2. Compression
  3. Detonation
  4. Exhaust
A spark happens very near the end of 2 and very near the end of 4, thus occuring within the same revolution. Only if the spark was more than 3 steps away from the next would it be in a different revolution, and thus wouldn't really be a wasted spark system for a 4 stroke engine. I could see how counting this out in your head might make you think it's part of a different revolution, but it's never been a full revolution until you're through all 4 quarters.



EDIT: A much simpler way is to realize you're not considering crankshaft RPM, but camshaft RPM. I was definitely reading the question weird. One spark per revolution, two sparks per complete cycle.



You still cut the number in half on those devices because it's a chicken or the egg thing. If you were wanting to know how often it sparks by reading a tach, you'd keep your thinking, but to install one, you want to know the rpm by reading a spark, so you have to divide to eliminate the wasted one.
 

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One spark per revolution.



Every other spark sets gasoline on fire. The one in between does nothing hence it is called wasted.



If you buy the tach get the one that says it is for the kind of engine that has one spark per revolution.
 

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Right, the TW's ignition system looks only at the crankshaft, it doesn't have a clue as to what the camshaft is doing so it blindly sparks with every turn of the crank.
 
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