sparks per revolution: RPM mystery

[tz89]

I'm working on the software for the next version of my digital carb sync. My proto board will be here next week.

 

I'm trying to figure out a way to add RPM to the display. I remember reading somewhere that the TCI assumes 2 sparks per revolution. The Ignitech TCI software from Dingy says that, too.

 

But I didn't think our Ventures were double sparkers like some that will go nameless.

 

So my question is:

 

How many spark firings will I read off the plug per revolution of the crank?

 

Or if you prefer

 

How many revolutions of the crank per spark at the plug?

 

This probably seems obvious but it's got me perplexed. Too much screen time.

 

Thanks!

 

Tom

[MiCarl]

Each coil fires once per revolution.

[bongobobny]

Wrong, it's a 4 stroke, 1 spark per cylinder for every two revolutions. To refresh your memory, the 4 strokes are intake, compression, power and exhaust. With a 4 cylinder engine, for every revolution there are 2 cylinders sparking, but for just one cylinder it sparks once for every two revolutions...

[MiCarl]

Wrong, it's a 4 stroke, 1 spark per cylinder for every two revolutions. To refresh your memory, the 4 strokes are intake, compression, power and exhaust. With a 4 cylinder engine, for every revolution there are 2 cylinders sparking, but for just one cylinder it sparks once for every two revolutions...

 

Not in this case Bob. This is what is known as a "wasted spark" ignition. The plug fires at the top (more or less) of the compression stroke to fire the fuel charge. It also fires again at the end of the exhaust stroke where there is nothing to burn - thus it's wasted.

 

The system you describe has the trigger (points, pick up etc.) on the cam shaft which rotates at 1/2 engine speed - thus fires every other revolution. On most modern motorcycle engines the trigger is on the crank shaft, and fires once per revolution.

[bkuhr]

Wrong, it's a 4 stroke, 1 spark per cylinder for every two revolutions. To refresh your memory, the 4 strokes are intake, compression, power and exhaust. With a 4 cylinder engine, for every revolution there are 2 cylinders sparking, but for just one cylinder it sparks once for every two revolutions...

But the ignition pickups are crank driven, not cam driven to get the 1 spark per 2 rev.

we do have 1 spark per crank rev, or 2 sparks per 4 cycle,-cycle

[tz89]

Well I don't feel so bad now.

 

So, anyway, each time I read a spark at the plug how much revolution since the last spark at the plug?

 

Thanks!

[Flyinfool]

each coil will fire once every 360 degrees, or once per revolution.

[tz89]

Thanks!

 

btw are there identifiable groups of bikes that do something different?

 

I'm mostly interested in the Honda Valkyrie with 6 carbs. That's a GL1500 engine I believe.

 

What about the inline 4's from Yamaha or others?

 

Is it mostly just older v-twins?

 

I'm trying to make this thing as useful as I can.

[MiCarl]

Most are wasted spark ignitions like on the Venture. There are a few modern engines and the old OHC Honda singles and in line twins with breaker point ignitions that spark every other revolution.

 

If you want to make it truly flexible just put a multiplier in your algorithm that can be set to 1 or 2. Default it to 1 and it'll work most of the time.

[tz89]

If you want to make it truly flexible just put a multiplier in your algorithm that can be set to 1 or 2. Default it to 1 and it'll work most of the time.

 

Yep that's the plan. I'll store the value - warning geek alert - in eeprom.

 

Next I'm looking at various timing light inductance clips to see how best to get a clean pulse of 0-5v.

 

This is what I do while my mechanic has my bike held hostage.

 

Thanks.

[syscrusher]

 

Next I'm looking at various timing light inductance clips to see how best to get a clean pulse of 0-5v.

 

Thanks.

 

I don't know how sensitive the pulse detection amplifier would need to be but several digital tach versions from Trail Tech just require an insulated wire to be wrapped around an ignition wire 3-5 times.

[bongobobny]

MiCarl you're right, I forgot that fact about the exhaust blast! My apologies.