Reading distributor specs and expected real world performance.

[djwarner]

Haines provides specs for the various distributors used on our cars. Specs include Idle degrees/RPM, upper and lower centrifugal advance specs, and upper and lower vacuum advance specs. In a recent thread it became apparent timing setting taken with a strobe varies considerably depending on whether the vacuum port was connected. Timing can be set with a strobe with the engine running or with a test light with the engine stopped. Since some of the distributors listed have a centrifugal advance effective at idle speed, I began to wonder just what is the appropriate method for setting timing. The factory service manual is less than explicit on the matter.

I constructed a spreadsheet to calculate the advance for varies RPMs and Manifold Vacuum based on the following assumptions. The calculated timing is the sum of the static timing, centrifugal advance, and vacuum advance. The minimum advance for centrifugal and vacuum is zero and the max advance is that given at the upper limit. Static timing is adjusted for any centrifugal advance in effect at idle.

For a stock distributor on a 240 or 260 Haines says the Centrifugal Advance is 0 at 550 Rpm and 9 degrees at 1200 Rpm. Vacuum advance at the upper limit is given as 7.5 degrees. The question is whether 9 degrees is a mechanical limit or just a point on a curve? Same thing for the Vacuum Advance.

Members of the forum are reporting numbers above 30 degrees with the vacuum line connected. This is far above the 26.5 degrees I calculated when applying the upper limits as a hard limit.

I ran my unloaded engine at various speeds and recorded the manifold vacuum with a digital manometer. I then computed the expected advance per my spread sheet for the stock distributor, the non-emission controlled distributor, and for the solid state distributor from a 79 280ZX I actually have installed in my L24. Applying the upper limits as described above, the various distributors should have maxed out between 1000 and 1200 RPM. So I am assuming the upper limit given by Haines is just a point on a curve and not a limit.

Can anyone confirm the proper method for setting the timing

and what timing should be expected?

[LeonV]

Distributor specs are given at the distributor. Spark advance is typically described in crank degrees. The distributor spins at half crank speed, so multiply your numbers X2.

[Zed Head]

In other words, the zero at 550 RPM is actually zero at 1100 engine RPM. That's why the FSM maintenance procedures and the sticker under the hood always call out an RPM at which to set timing. Another thing that is typical of factory settings is that the vacuum advance actuator is connected to "ported" vacuum. Ported vacuum is not active when the throttle is closed, so there is no need to disconnect the hose. So the procedure does not call for disconnecting the hose. Some people though, in their modification efforts, connect the vacuum advance directly to intake vacuum.

The early carb'ed engines may also run full time vacuum from the factory though, I'm not familiar with their details. In that case you would want to disconnect the hose, since engine speed and intake vacuum will cause variation. You kind of have to get in to the details of your own set-up, and whether it's factory stock or has been played with.

[djwarner]

Leon, that makes sense. I'll adjust my formulas.

Zed, the vacuum port comes off the balance tube per FSM. Chilton says for models through 1979 to leave the vacuum connected which should be incorrect as I figure it. Haines sets the point distributor with a test light and uses a strobe for transistorized ignitions with the vacuum disconnected.

No one has answered whether the centrifugal and vacuum advances are mechanically limited at the upper limit. Anyone?

[Zed Head]

Yes, they both are limited at the high end.

The Chilton and Haynes books are notorious for having sketchy, mainly generic, advice. The FSM is the way to go. But, in looking at the 1972 FSM I can see where your problem lies, they don't say much, and the idle settings are pretty low, even for a good running engine. Looks difficult.

I'll step out of the carb conversations. Looks like a messy scene.

Edit - just looked at the 1973 procedure and it makes even less sense to me. If you do things in order, timing is set with load on the engine for an automatic, shift lever at "D", and the vacuum advance actuator hose connected, then the hose is disconnected for further tune-up operations. So, my comments above about what to do with the vacuum hose were totally wrong for the 240Z's and don't match anything I've ever seen on the US market cars I've worked on. Weird stuff. Good luck

Edited November 30, 2013 by Zed Head

[240260280z]

[Stanley]

just looked at the 1973 procedure and it makes even less sense to me.

That bit refers to the insanely complicated procedure for setting the dual point emissions distributor for the '73 AT's. It's in the engine manual, but I won't pretend to understand it. A car with AT will start and run much better with the single point distributor, at least mine does.

If I understand, in the Spec. and Service Data table Blue posted, the 17 deg @650 and 17 deg at 700 (AT) refer to crank rpm like on the tach. But the advance characteristics in the same table refer to dist degrees and rpm that are 1/2 tach rpm.

BTW the table is for the D-606-52 which is the "Euro" non-emissions dist.

Now for the two graphs. The two sets of data graphed had me totally confused until I added the numbers together and divided by 2. Then you get the same start and max numbers in the Spec Table. My guess is that these tables are for mechanics testing distributors. If a distributor tests within the two lines, it's OK.

With a puzzle like this, who needs Sudoku?

[djwarner]

Here is what I've learned so far. Using what Leon said about distributor specs given in Distributor RPM rather than Crankshaft RPM, my spreadsheet made a lot more sense. Re-reading Chilton and Haynes it appears the dual point ignition systems were used on AT z's to reduce noxious compounds. The question remains whether timing was retarded for a cold engine and advanced for a warm engine or vice versa.

Complicating things is that I have a transistorized distributor from a 79 280ZX automatic. Looking at the response curves of my distributor vs the stock distributor, I found they were very close but the vacuum advance on mine had a smaller range. Both Chilton and Haynes gave the timing as 10 BTDC which is the spec for the retarded timing. However, when I compared the calculated timing with vacuum advance include ( as in normal operating conditions ) I found the automatic was 5 to 7 degrees retarded compared to the MT Distributor. This oddly compares to the additional 6 degrees specified for the advanced timing spec for AT distributor. This would indicate that the advanced set is used for a warm engine and the retarded set is used for a cold engine.

Comparing the vacuum advance curve differences, my distributor could allow an additional advance when developing less manifold vacuum (something less than 3 degrees). When opening the throttle, the vacuum advance actually decreases thereby retarding the timing. This took a few minutes to sink in. Aircraft instruments measure manifold pressure rather than manifold vacuum. A stopped engine will read 29.92 inches of Hg. At idle, the throttle plate restricts airflow and a partial vacuum is created (perhaps 14 inches of Hg). As the throttle is opened, more air flows in and the partial vacuum is eased. At full power, a normally aspirated aircraft engine will have a manifold pressure in the 23-28 inch range. In our ignition systems we measure the vacuum, not the absolute pressure. So as we open the throttle, the vacuum reading falls. As the vacuum falls, the vacuum advance applied to the distributor decreases, retarding the ignition.

If I were to re-time the engine to run at the advance setting spec and had the additional advance contribution from the different vacuum curve, I wondered if it could lead to detonation. This is where the non emission controlled distributor specs helped. Even with my 79 280zx distributor at the advanced setting, the timing would never be more advanced than the non emission controlled one.

So I advanced the timing to 16 degrees BTDC and went for a spin to fully warm the engine. The difference in performance was remarkable. When I got back in the garage, the temperature switch for the timing relay was 25 ohms compared to the 220 ohms I found at room temperature. Concerned that the system may have doubled the 6 degree advance, I hurriedly took another timing check only to find the timing still at 16 degrees. I then stopped the engine and pulled the lead off the temperature switch and found the timing still at 16 degrees! So either the transistorized ignition systems is broken or it was not wired in properly when it was installed. In either case, I'm happy with the significantly improved performance.

According to Chilton, a 6 degree advance in timing would only supply about a 2.5% increase in power but dramatically increase spark plug temperature. Since my plugs have been sooting up, I would expect some better cleaning with the higher temperature. I will have to see. As for the 2.5% power increase, that may have been compared to a properly timed engine. If I had only seen a 2.5% increase, I doubt that I would have noticed it. So I suspect my previous timing setting based on the 10 BTDC was causing a significant power loss.

[240260280z]

Here is my understanding:

There are 3 timing variables for our Z cars:

1. Initial static advance. It is the "clocking" of the distributor housing. It is adjusted by rotating the distributor.
   
2. Mechanical advance (Centrifugal Advance). It is the additional dynamic advance caused by springs and weights that are spun outward as the rpms increase. This outward movement in turn, rotates a plate that advances timing. It is adjusted by altering weights and springs and the slots that control how far the plate rotates.
   
3. Vacuum advance. It is the additional advance caused by engine vacuum. (there are typically two vacuum sources. One is "manifold" vacuum which is on the manifold side of the throttle valve. It is highest at idle (and also at so-called "over-run".... (which is what happens when your rpms are high and you see the police and suddenly take your foot off the accelerator) the other is ported vacuum (which is venturi-effect vacuum) and is caused by air flowing through a passage and "drawing" from the wall as the air flow increases. Ported vacuum is typically used as the source for Z vacuum. However, a port (hole) can be placed near the throttle valve that can have both manifold and ported vacuum. Vacuum advance is adjusted by an epoxied stop screw in the vaccum housing on the side of the distributor. The size and shape of the vacuum housing also affect how fast and how far the vacuum advance is.
   

The purpose of changing advance is 2 fold:

1. maximizing power
   
2. minimizing emissions
   

A car idles best with lots of advance, however there are a few more considerations:

• fuel takes time to burn
  

• the rpms increase when you clock the distibutor (cw for Z's)...due to more efficiency/energy from a better burn
  
• too little advance starts burning the fuel too late and it ends up burning leaving the chamber as it passes the exhaust valve. It will continue to burn outside the engine in the exhaust header. You can make them glow red from late ignition of the charge.
  
• too much timing and the NO emissions are higher
  
• too much timing and it is difficult to crank the car to start it
  

Mechanical Centrifugal Advance allows the timing to be lower at start and at idle (to make starting easier and to lower emissions) however it increases rapidly to allow better fuel efficiency and power when running

Vacuum Advance is designed to provide more advance when cruising under a light load to increase fuel efficiency.

The purpose of varying the ignition timing is to optimize fuel burning and have the pressure press the piston at the correct time. This varies at idle, cruising and under wide open throttle.

Too much advance and the pressure from the fuel burn presses on the piston as it is trying to rise. This causes pinging of the piston's aluminium surface.

Edited December 1, 2013 by Blue

[Stanley]

It's in the engine manual,

Nope, nothing about setting timeing for dual points in the engine manual, it's in Chilton.

[Stanley]

Some people though, in their modification efforts, connect the vacuum advance directly to intake vacuum.

The engine pings worst when climbing a hill at full throttle and low RPM. As I recall from an old truck that had a stock vac gauge, that's also when vacuum drops. So it seems (zero experience) that you could set total advance higher with the vac can hooked to the balance tube, since it would reduce advance lugging up the hill for example. But on the other hand, acceleration might be worse. And Nissan must have had good reason to build it like they did.

Wonder how that mod worked out?

[beermanpete]

The "ported" vacuum signal is manifold vacuum but is sourced on the venturi side of the throttle valve. This allows the vacuum signal to be removed (or reduced) from the distributor at idle. At part-throttle the vacuum port is revealed to manifold vacuum by the movement of the throttle valve and the timing is advanced (assuming a light load and high manifold vacuum). At full-throttle it is equivalent to the vacuum signal available from the balance tube.