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]

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?

 

[Bilgediver]

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?

This is a two stroke diesel so the crankshaft and camshaft run at the same speed so I would expect 8000 pulses per min. I doubt the
sensor would be driven by an odd sized idler gear.

If it was my engine I would just turn the engine by hand through 1 revolution and see how many turns the sensor drive turns. I am sure
there will be a way to turn it . Alf up in Peterhead (Divemaster) knows these engines and might be along soon.. We had them on our rigs but I was shore based.

 

[penfold]

I had this problem with a Caterpillar tacho; I just hooked it up with the engine running and clicked the selector switch through the options until the readout made sense, i.e. 600rpm rather than 60.

 

[Divemaster1]

.......
If it was my engine I would just turn the engine by hand through 1 revolution and see how many turns the sensor drive turns. I am sure
there will be a way to turn it . Alf up in Peterhead (Divemaster) knows these engines and might be along soon.. We had them on our rigs but I was shore based.

...apart from not being located up there any more (boat in Inverness now) ... ... yup I know the iron lumps pretty well... Have not got the manual in front of me (just back from Rotterdam and Den Haag, so hope to be forgiven)... but seem to recall that these come straight off the cam, so believe them to be 1:1 ratio... pretty accurate when I checked with our two engines and actually use the mechanical gauges for setting idle, instead of using helm ones... then check the helm ones agains the mechanical ones at various RPM's underway, just to double check that I run the engines with equal load... Will check the manual when I get home tonight and correct if I am wrong in respect to the ratio...

 

[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?

 

[Divemaster1]

Can not tell you why your instruments don't pick this up, unless there is a lower minimum for the sende..., but those lumps when in-tune should idle nice and smoothly from about 450 RPM upwards (think your governor idle for the In Line 6/70 will be rated 450 - 900 RPM) ... best one for pick-up of load in between gear changes, and keeping speed down is around 550 RPM. We had ours down at 450, but dropped down to 400 when getting in gear.... goveror did pick up to 450, but basically felt too low... IMHO of course..

 

[Divemaster1]

Looked at manual, and now-where in the 800 page document does it state it is not 1:1 .... camshaft of blower shaft connection...

 

[nnyerges]

Looked at manual, and now-where in the 800 page document does it state it is not 1:1 .... camshaft of blower shaft connection...

So its not 1:1? Its?...

I guess the pickup is a coil with a rotating magnet. Not only is the frequency a function of the rotational speed, the amplitude also increases with rpm. Below 1000 rpm the trigger level isn't reached, so there is no indication. Could it be the tachometers and pickups do not belong to the same "family"? Right. The amplitud also increases with rpm, and my actual sender is from Teleflex/Signaflex and the tachometers are Faria. Faria have they own mechanical takeoff senders.

They are physically identical but may have different responses. Does any body else can confirm this?

If these tachometers dependent voltage emitted by the sender, then I'm screwed. I'll have to change the senders. If the RPM reading is done by number of pulses, then no problem.

 

[Divemaster1]

OK, gone back to square 1... could be a blower take up and the blower have gear ratios...

6/71T engines Blower gear ratio is 1.65:1 or 6/71TT 1.51:1 etc .... actually... here's the table from the book....

 

[nnyerges]

Thank you Divemaster1 for the table.
My engine is a 71N. What is the difference between CURRENT and FORMER?
There are two different ratios for the 71N (2:1 & 1.65:1)....

 

[Divemaster1]

Believe the former is pre 1979 and refer to the 2 valve head engines, but not entirely sure...

 

[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.