cam vs crank rotations

[perkinja]

Anyone off the top of their head know how many cam rotations there are in order to get 1 complete crank rotation? The engine in question is rephased, with a pamco ignition, i'm trying to make a digital speedometer using the pamcos 12 to 6v signal when it sparks. I've got the micro controller to read this and count it as an rpm but i think its off.

 

[schlakeandbake308]

Four stroke motor. So can rotations to one crank rotation

 

[schlakeandbake308]

2 cam rotations... Sorry auto correct

 

[5twins]

1/2 cam rotation to each 1 full crank rotation. Cam chain sprockets are 17T on the crank and 34T on the cam for the 256 engines, 18T on the crank and 36T on the cam for the 447 engines. Turning the crank one full revolution on either turns the cam 1/2 revolution.

 

[schlakeandbake308]

Pamco is a wasted spark system, it sparks on both compression and exhaust

 

[perkinja]

So for every spark the crank has rotated twice?

edit.

 

[5twins]

No, once. The Pamco fires at every TDC. One is at TDC (actually a few degrees before) on the compression stroke, where you need and want it, the other is at the TDC when the cylinder is just finishing it's exhaust stroke. That's the wasted spark.

Did you mean to say you're trying to make a digital tach?

 

[grizld1]

Right! Cylinders fire alternately, and one (but not both) fires with every revolution of the crank. Any given cylinder fires every other revolution. Think about it: valves must be closed on compression and power strokes; that's a full rotation of the crank. What does that tell you about camshaft rotation?

 

[perkinja]

Did you mean to say you're trying to make a digital tach?

trying to yup.

 

[XSLeo]

Do you still have the stock tach? And can you hook it to the engine? If so, do so, compare the readings. If you have the electronic tach set right it should match the stock tach. If you have it wrong it will read twice as fast or half as fast.
Leo

 

[perkinja]

Thats what ill be doing once i get everything soldered to my pcb board, hooked up to the micro controller, coded, and then put on the bike. I just needed to know what the ratio was so i can start the programming

 

[TwoManyXS1Bs]

... i'm trying to make a digital speedometer using the pamcos 12 to 6v signal when it sparks. I've got the micro controller to read this ...

Perkinja, what kind of signal are you getting?
Got a scope trace?

 

[gggGary]

 

[fredintoon]

Anyone off the top of their head know how many cam rotations there are in order to get 1 complete crank rotation? The engine in question is rephased, with a pamco ignition, i'm trying to make a digital speedometer using the pamcos 12 to 6v signal when it sparks. I've got the micro controller to read this and count it as an rpm but i think its off.

Hi perkinja,
hopefully you are building a tach, not a speedo?
Camshaft runs at 1/2 engine speed but pamco is wasted spark so it sparks every engine rev.
Does the re-phase use 2 pamcos or only one?
If it's only one the pulses will be 2 per rev although they won't be spaced evenly on a re-phased engine, perhaps that's what's confusing things?

 

[650Skull]

Correct me if i'm wrong here.........but............a re-phased 277/270 crank with a re-phased Pamco requires 2 coils so there will be no wasted spark. Wouldn't the spedo work if it is connected to one of the coils and timed for 2 electrical charges as 1 revolution

 

[grizld1]

I'm not sure what kind of firing pulses that 277 igniter box produces, but you'd be working with either 1 pulse per rev or 1 pulse every other rev, never 2 per rev with a 4-stroke twin.

 

[650Skull]

True, i still got mixed up in the wasted spark idea when there isn't one with the re-phase and 2 coils. I meant timing the spedo off one coil