Pulse's Per Rotation

[Jfreeman1412]

I am in the Process of converting my Analog guages to Digital and need to know how to find the pulses per rotation to be able to calibrate the converter box. Does anyone know a good way of find this out or maybe you know from experience. Motors are 1980 Mercruiser t-228's. Thanks.

[prowlersfish]

4 That was a easy question ( I assume you getting the pulses from the coil or dist. )

[Jfreeman1412]

I read that's what it could be, but though that was far too easy. But after some confirmation from the forum! Four it is! Thanks!

[BarryL]

Makes sense. 4 stroke engine... I wouldn't have guessed that without a little extra thought but sometimes the simple answer is right in front of us.

[Jfreeman1412]

Actually.....that wasn't it. Or at least not for my digital gauges. With 4ppr programmed it said I was idling at 6k Rpms.

[prowlersfish]

It has to to be 4 pulses . 8 cylinders that each fires every other revolution 8 cylinders divided by 2 = 4 . your picking up the signal at the - side of the coil right ? I set up tachs on both gas and diesels and have not have issues ( diesels can drive off alt pulses , fly wheel teeth or a sender )

I would contact the tach company for support . what brand of tach ?

[Jfreeman1412]

I am pulling right off the sender off the back of the tach to a Noland RS11 analog to digital converter. Then it displays on a lowrance head unit. The ppr is a number I program into the RS11.

[mikeandanne]

I am pulling right off the sender off the back of the tach to a Noland RS11 analog to digital converter. Then it displays on a lowrance head unit. The ppr is a number I program into the RS11.

Try running a temporary wire from coil like prowlerfish mentioned instead of from your tach ,sometimes these tachs have their own converters for certain ignitions--- then you can compare---- was reading the manual for this unit,would be nice to see how they arrive at that pulse per rev numbers that show in the specs ,really cool stuff,I would like to convert all my old gauges to this so this will be very interesting to follow-- hope this helps--Mike

[larryeddington]

Generally there is one pule per revolution for each cylinder ergo 8. Spark plugs in most engines fire TDC of exhaust stroke as well as compression stroke, as there is no explosive charge in cylinder nothing happens at the extra pulse.

[Big D]

On an 8 cylinder engine, all cylinders fire during one complete rotation of the cam, so 8 pulses per one cam rotation. For every rotation of the crank, there are 4 pulses since the crank rotates twice for every complete rotation of the cam.

Waste spark is used in batch fire EFI systems but not in systems with distributors; you can't fire two cylinders at once or at two different stroke positions of the same cylinder with a distributor.

[prowlersfish]

Larry , that is incorrect . At least with a distributor ignition system like we are dealing with in this case . Now with a DIS type system you would be firing every rotation .

[prowlersfish]

Big D looks like we where posting about the same time . With about the same info

[Big D]

++1

[larryeddington]

did not mean the cylinder fires a fuel air mixture once each revolution. But on retrospect given a distributor that is turning a half engine revs it would be 4 pulses per crank turn. There are many engines that fire the sparkplug each time the cylinder comes up, but you all are correct and I stand corrected on this one.

[LandVF36]

I will check this weekend what I used on my RS11 config. I do not believe i used either 4 or 8! If I remember right, I had to use 16. I am certain that the reason is that the RS11 is getting the pulse it is suppose to, but it is also getting a "reflection" pulse that is transmitted on the wire. Perhaps I have a bad ground somewhere. Anyway, I'll check when I'm on board Friday night and try to remember to send another post.