Silly short question- How many sparks?

[rooooney]

I have a 1999 9.9 4 stroke merc that I'm setting a tach for. It needs to know how many times the spark plug fires per revolution. Once, twice or every other. I'm guessing once?

 

[Charlie61]

Re: Silly short question- How many sparks?

it would be the number of spark plug you have = if you have 4 plug it would fire every four revol

 

[Chris1956]

Re: Silly short question- How many sparks?

each cylinder in a Four cycle motors will spark once in 2 full revolutions. So a 2 cylinder 4 cycle motor will spark once per revolution.

 

[schematic]

Re: Silly short question- How many sparks?

What kind of tach is it? If the engine has only one ignition coil, it will fire once every revolution on a 4-stroke 2 cyl.

 

[Chris1956]

Re: Silly short question- How many sparks?

Normally Outboard tachometers run off stator pulses, and number of cylinders is not important.

 

[Silvertip]

Re: Silly short question- How many sparks?

I suspect he is buying a Tiny Tach that runs off a wire twisted around one of the spark plug wires. Even so, it doesn't matter how many cylinders the engine has when ordering a Tiny Tach. Why the "How Many Sparks/Rev" question is asked is some engines incorporate what's called "wasted spark". For example, if the engine has two coils but is a four cylinder, there is a pretty good chance the engine is a wasted spark design where each plug fires every revolution (wasted spark design) rather than once every other rev as it does in a standard 4-stroke. The non-power stroke firing is the "wasted spark". So the Tiny Tach people need to know which design the engine has. The number of cylinders is of no importance. Two strokes fire every revolution regardless.

 

[Texasmark]

Re: Silly short question- How many sparks?

I suspect he is buying a Tiny Tach that runs off a wire twisted around one of the spark plug wires. Even so, it doesn't matter how many cylinders the engine has when ordering a Tiny Tach. Why the "How Many Sparks/Rev" question is asked is some engines incorporate what's called "wasted spark". For example, if the engine has two coils but is a four cylinder, there is a pretty good chance the engine is a wasted spark design where each plug fires every revolution (wasted spark design) rather than once every other rev as it does in a standard 4-stroke. The non-power stroke firing is the "wasted spark". So the Tiny Tach people need to know which design the engine has. The number of cylinders is of no importance. Two strokes fire every revolution regardless.

The reason for the "wasted spark" is simple. You put a trigger coil under the flywheel. You mount a magnet on the flywheel. When the magnet passes the coil it produces a pulse of energy (magneto). The magnet is mounted inside the flywheel so every time the crankshaft revolves it passes the coil, hence one spark per rev. whether you are using it or not (wasting it on a 4 cycle OB). In autos, using 4 cycle combustion technology, distributors were used and were turned by camshafts which were operating at 1/2 the speed of the crankshaft turning them. Hence the distributor only passed each cylinder every other time which is all that is required for a 4 cycle engine.

Mark