Help understanding advance curves

[sdyck]

I'm hoping someone can help me understand the avance cures in the FSM.  I have a D612-53 distributor on my 1972 Z, I've attached the the page from the FSM showing the mechanical advance curves for the D612-52 that I understand has the same curve.

The inital timing should be 5 degrees BTDC and the mechanical advance should max out at 24 degrees for a total maximum of 29 degrees at 3000 rpm.  Now the question I have is the advance curve in the manual shows two curves one at a max of 13 degrees and one at 10 degrees and I understand you add them together to get the maximum cetrifugal mechanical advance of 23 or 24 degrees.  But why two curves and why are they different? Is there one for each weight and is each spring slightly different?  Also for the Vacuum advance there are two curves, I understand the mechanism moves the breaker plate when there is differnet vacuum pressure, but why two curves? Is it the same reason as the mechanical advance?

I've read eveyrhgin I can get my hands on but haven't found an explanation for these questions.

Thanks for the help.

 

Steve

[Untitled].pdf

Edited October 17, 2016 by sdyck

[Zed Head]

I think that the two curves are for two different engines, and transmissions.  Page EM-34.  Match the spec. to the final advance on the curves.

Edit - added the comment about the transmissions.  Manual versus automatic.

Edit 2 - My image is from what's supposed to be the 1972 FSM.  The distributor numbers don't match yours though. I would not trust that " D612-52 that I understand has the same curve. " is true.  Overall though, I think that each one of those curves is meant to be used alone.  No adding together, unless you want to calculate centrifugal plus vacuum.

Edited October 17, 2016 by Zed Head

[Zed Head]

Actually, I see how confusing it can be.  My previous post may be worthless.  Don't forget to double the distributor numbers to get crankshaft numbers.  I would seek out the proper curves for -53 also.  

Edited October 17, 2016 by Zed Head

[Zed Head]

And the centrifugal advance mechanism should have a number stamped in to it that indicates maximum advance.  You have to remove the breaker plate to see it.  Won't tell you the rate but should show max.

In the big picture though, with the changes in fuel composition, the fine details of what Nissan did may not matter much.  As far as how to tune your engine.

[beermanpete]

I interpret the two lines in the charts as a tolerance window. If your distributor's performance is within the lines it is good. Be aware that the RPM and angles indicated are for the distributor (not the engine) and are intended for use with a distributor testing machine (distributor out of engine). For checking the timing curve in the car you need to double both the RPM and angle.

[Zed Head]

I think you're right.  The average of 375 and 525 is 450.  Matches the starting point of the specification (Post #2).  Maybe it's a standard method for showing distributor curves, in the 1960's/70's.  

 

 

[sdyck]

Excellent, that makes sense to me.  Thanks for the help!

Odd that so many distributors were used on the 1972 manual Z, I have found a number of discussions on another Z sight that discuss the D612-52 and D612-53 used on late Z's and the fact that they have similar curves. With the vacuum advance disconnected and initial advance set at 5 degrees it does seem to max out at a little over 30 degrees at a little over 3000-3100 rpm.   That would be consistent with the top graph of the curve in the book that shows to centrifugal advance at 13 degrees.  so 5 + (2X13) would put me at a max of 31 degrees.

Next I need to confirm my vacuum advance is working. I've tested it with a vacuum pump and it holds vacuum, the breaker plate moves but not much.  Is it a valid test to set the idle to 1000 and then use the vacuum pump to advance the vacuum mechanism to see how much advance I get on the timing? 

Thanks,

Steve

Edited October 18, 2016 by sdyck

[Zed Head]

You could set idle to 900 (the point that centrifugal starts to kick in - 2 x 450 [average]) or lower then connect the vacuum hose to a vacuum port.  This would give full vacuum advance.  Or use your vacuum pump.  You'll probably have to reset idle speed to below 900 since advancing igntion timing typically increases idle speed, which will cause the centrifugal advance to advance timing also.  Or you could use the curve from the FSM and subtract out the centrifugal part.

[sdyck]

 

Thanks Zed Head, one thought though.

As you accelerate vacuum pressure increases and then falls off so it isn't easy to see the impact of vacuum at increasing RPMs.  I did try something like what you suggested, I checked the advance at 500 RPM increments and saw an impact from the vacuum at low rpms that disappeared or was negligible above about 1500 RPM. (Pretty inaccurate holding the test light with one hand and trying to hold the RPM steady with the other). That's what started me reading as I thought my vacuum advance must not be working. 

Now my thinking is to isolate the centrifugal impact by keeping RPM constant and taking engine vacuum out of the equation by leaving it unhooked and using the vacuum pump. I should then be able to see the full impact of the vacuum advance by pumping it to maximum ~ 18 in Hg. per the manual, which I think should show me the full 18 degrees of vacuum advance if the unit it working.  Seeing the breaker plate move when I pump it up doesn't tell me much as the movement is so small.

Makes sense in my head, but I am a newb with this stuff, so if I'm out to lunch let me know.

Steve

 

[beermanpete]

Using the engine as a vacuum source will be too unstable to get reliable results. Use a vacuum pump so you can set the vacuum and leave it while you get the readings you want. Try to keep the engine speed constant as well, preferably at a slow idle so the centrifugal advance stays fully retarded.

[Zed Head]

I suggested engine vacuum just as a way to see what full advance would be and to verify that everything, including the breaker plate in the distributor, is functioning.  It's a simple quick test, intake vacuum at idle is very high and will max it out.  If you want to generate a curve like those in the FSM then the pump is the way to go.

Don't forget that there are two parts to vacuum advance, the diaphragm in the actuator and the breaker plate.  The diaphragms get brittle or stiff, and can split, and the breaker plates get stuck.

[Stanley]

Some interesting ideas in the responses that hadn't occurred to me.

D602-52 is non-emission spec distributor with 17 deg. initial advance, D612-53 is emission spec with 5 deg. initial. No graphs I could find in '72 FSM or '72 engine manual for D612-53, but the emission section has the settings for it, and also settings (and the complicated adjustment procedure) for the dual-point distributor that was standard in emission spec automatics, according to FSM.  Maybe those are the only three stock distributors for L24.