Engine takeoff ratio and tachometer calibration

[nnyerges]

Gentlemen,
My boat engines DD 671N/J&T (1984) use mechanical takeoff for the tachometer sender. Questions:
1. The take-off is in the back side of the engine (see image). I need to know what is "Engine takeoff ratio" (1/2:1, 1:1, 1-1/2:1 or 2:1)? to set my new tachometers.
2. I will like to calibrate my tachometers, using my osciloscope. The sender is a SIGNAFLEX (8 pulse per turn).
Case study:
With the sender outside the engine, if I manually turn the needle at 1000 RPM, the sender will give me 1000x8 = 8000 pulses in one minute, equivalent 8000/60 = 133.3 pulses per second = 133.3Hhz.
With the sender installed in the engines takeoff port, do I have to consider the "engine take-off radio" to calculate the real engine RPM?. In other words, does the "engine take-off radio" affects the sender reading or just is to setup the tachometer?
Example:
Suppose that the engine take-off ratio is 2:1 and I measure 133.3Hhz in my osciloscope, does the final RPM is 1000/2=500RPM, 1000*2=2000RPM or 1000RPM?
Please Help
Nicolas

 

[nnyerges]

I'll make the question simpler: If the DD 6-71 engine runs at real 1000RPM, how many pulses per minute is going to give me the SIGNAFLEX sender output?

 

[nnyerges]

Some confutions...

The take-off is mounted high on the engine, so it uses the camshaft; you get 4000 pulses/min at 1000 rpm

This is a two stroke diesel so the crankshaft and camshaft run at the same speed so I would expect 8000 pulses per min

Some discrepancies, since one takeoff ratio is 1/2:1 and the other is 1:1
Bilgediver give me a nice an perfect idea (how not occurred to me before) !Thank you!:

...turn the engine by hand through 1 revolution and see how many turns the sensor drive turns....

I will give it a try this week.
Now, aside from all that, no matter what position I place the takeoff ratio switch selector on my two Faria tachometers, there is no reading for both engines at IDLE or LOW RPM. They start to “steadily move up” at +/-1000rpm (in both engines).
Any idea why?

 

[jgmo]

I admit that most of your post is confusing to me including the last question in red but I can provide this:

I haven't seen an inline 71 in forever but I do know that on the V-71 and 92 engines the tachometer drive is driven at BLOWER speed, not crank OR camshaft speed. That is why you see all the differing ratios listed. The superchargers on those engines are selectively driven at varying ratios depending on application and accessories. There is one turbocharged version with a drive ratio of 1.95: 1, for example, which is not on your list.

But, if yours IS camshaft driven it will, MOST CERTAINLY, be 1:1.

Bilgediver's method for verifying the ratio is a valid method for confirmation.

Therefore, if you hook this up and see your 133.3Hz on the scope, that, at least in my mind, would indicate you are measuring 1,000 rpm if what you wrote above is accurate.

I don't know but is it possible that you have a compatibility issue between the Faria meters and the Signaflex drive? I see that a lot in the marine tachometer world but I am not familiar with most of it.

Good luck.

 

[nnyerges]

SOLVED:
I have measure the ratio by rotating the crank wheel and counting the number of pulses of the sender. It’s 1:1. Tachometers are now calibrated... Thanks to all.