Detroit Diesel 671N/J&T "Engine takeoff ratio and tachometer calibration"

[nnyerges]

Detroit Diesel 671N/J&T "Engine takeoff ratio and tachometer calibration"

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]

Re: Detroit Diesel 671N/J&T "Engine takeoff ratio and tachometer calibration"

Re: Detroit Diesel 671N/J&T "Engine takeoff ratio and tachometer calibration"

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]

Re: Detroit Diesel 671N/J&T "Engine takeoff ratio and tachometer calibration"

Re: Detroit Diesel 671N/J&T "Engine takeoff ratio and tachometer calibration"

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?

 

[nnyerges]

Re: Detroit Diesel 671N/J&T "Engine takeoff ratio and tachometer calibration"

Re: Detroit Diesel 671N/J&T "Engine takeoff ratio and tachometer calibration"

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.