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Old 24-06-2013, 15:43   #1
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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
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Old 24-06-2013, 16:12   #2
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Re: Detroit Diesel 671N/J&T "Engine takeoff ratio and tachometer calibration"

The tach take off on my 8/71 turns at 1/2 the real rpm, they make an adaptor you can put on to get a real reading. I use a standard cable to the helm.
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Old 24-06-2013, 20:55   #3
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Re: Detroit Diesel 671N/J&T "Engine takeoff ratio and tachometer calibration"

Quote:
Originally Posted by lorenzo b View Post
The tach take off on my 8/71 turns at 1/2 the real rpm, they make an adaptor you can put on to get a real reading. I use a standard cable to the helm.
1. Thank you Lorenzo. I dont know if the 8-71 have the same take-off ratio of the 6-71. I dont need any adaptador, since the tachometer have a selector switch for the takeoff ratio, so i also use a standar cable to the helm.

2. What about question #2?

Regards,
Nicolas
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Old 24-06-2013, 21:15   #4
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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?
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Old 24-06-2013, 21:56   #5
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Re: Detroit Diesel 671N/J&T "Engine takeoff ratio and tachometer calibration"

I think all 71 series run off the star plates,usually trans.pump on other.marc
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Old 25-06-2013, 07:27   #6
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Re: Detroit Diesel 671N/J&T "Engine takeoff ratio and tachometer calibration"

Some confutions...

Quote:
Originally Posted by CDK on boatdesing.net
The take-off is mounted high on the engine, so it uses the camshaft; you get 4000 pulses/min at 1000 rpm
Quote:
Originally Posted by Bilgediver on ybw.com
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!:
Quote:
Originally Posted by Bilgediver on ybw.com
...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?
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Old 04-07-2013, 13:28   #7
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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.
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