Lean mixtures need more advance, not less. A truck at highway cruise speeds only needs about 50 horsepower and lean fuel mixture -the total combined advance of initial, mechanical, and vacuum advance will be approximately 50 (fifty) degrees BTDC.

Agreed, without a timing light there is no way to know what's what. A vac gauge is a great way to set the initial or crank static timing but most stock distributors are not going to allow for much advance. And the fuel today is not going to allow for a lot of advance with high compression.

Generally, ping or spark knock on full throttle acceleration - from a dead stop - points to excess initial advance.

Part throttle ping or, knock when cruising around, (passing cars, etc, though, points toward excess vacuum advance. Two totally unrelated and independent advance methods. Racing engines have one purpose - full throttle, all the time. They don't care about mileage, so there is no purpose for vacuum advance and it's important to make that distinction if you want something that will have good driveability around town and the best possible fuel economy.

Sometimes you'll hear people say that vacuum advance and ported vacuum is a legacy of emissions requirement, but the 1950 Motors Manual (has tuneup specs for dozens of makes going back to 1930s) talks about ported vacuum and long before smog became a critical issue.

Of course all this is done by computer today, constantly adjusting and making allowances for fuel octane, temperature, elevation, etc - but careful tuning with a good basic understanding of the requirements will reap big benefits. Best performance and tune means best possible fuel mileage, and very gratifying to have the old gal run right.

 

oneowner83,

Yes, 4 speed manual OD with 3.08 rear gears.

If timing [static] is too far advanced, the sparkplug fires too soon as the piston is rising on the compression stroke and tries to push the piston down before it get to the top. This works against the starter motor. The original Duraspark ignition module was designed with a feature where the timing was momentarily retarded during starting. I am using a MSD ignition module, which does not have this feature.

Your thoughts regarding the "topping off the tank" with 6.9 gal. are valid. I normally refill at approximately 200-225 miles of my normal "town" driving, but this time I wanted to see what the mileage was like when driving on the highway, as described, so I filled up prematurely. The "trip" was relatively short.

Thank you for your input, Steve!

David

 

Nope. Lean mixtures are more likely to knock. More advance is also more likely to knock. If you go too far on one you have to go the other way on the other.

 

This is an example of how forums like this can be useless... one guy says "do it this way" and the other says "no, that's wrong, do it the other way."

But neither offers any sort of proof (hyperlinks, quotes, etc.) to back themselves up.

So, how is the OP supposed to know who to believe, and what is correct? Search long enough on the Internet and you'll find people claiming the earth is flat and moon is made of blue cheese - doesn't mean it's true, though....

 

Thanks, Chris. Sometimes, I feel my place in life is to moderate by trying to understand what someone is trying to say and then convey the idea to another. Not that I am super intelligent, but that I am blessed with patience and a desire to help.

I believe Tedster9 and Nothing Special are both trying to help and really are not too far apart in their ideas. From what I have read and experienced [gut feelings, not backed up...], a lean mixture is more difficult to ignite, so in fact, does require more advanced timing in order to be able fire at the correct point in the compression cycle. Also, lean mixtures can cause heat which causes preignition. One has to look at and determine the reason for the lean condition and correct that.

My reason for starting this thread was to share what I am working on in attempting to understand the compromises we make to get to a certain point regarding ignition timing.

What I have stumbled on [new to me] is that it is possible to advance timing so far that under certain limited conditions, one can get very good mileage, but not be practical for everyday driving conditions. I did not know that, before.

Currently, I have over-corrected and retarded the timing too much [This is an assumption based on the fact that at no point do I get the slightest hint of pinging]. The engine starts and runs well, but gas mileage has dropped, moderately. After I get the chance to readjust the timing [split the difference], I will report, again.

I welcome input!

 

David - I really do think you are leaving mileage on the table if you aren't getting pinging. Apparently the vacuum advance isn't giving the engine as much as it could take. The beauty of that simple device is that it drops the advance as the throttle or the load on the engine increase, and those are the things that increase pinging. (Actually, it is brake mean effective pressure that is going up, which increases the chance of pinging.)

Have you checked to see if your vacuum advance unit is adjustable?

 

No, but I will.

 

Even if it isn't, and I think some were, Crane may sell an adjustable unit.

 

Does that mean that each engine that has low vacuum will have incorrect advance?

 

No. We are talking about one of the three pieces of total timing. Here they are in the order that they should be determined:

1. Initial/Static: This is mostly important in getting the engine started and to give it a good idle and low RPM performance.
2. Mechanical/Centrifugal: This is a function purely of the engine's RPM and should be determined for max power at a given RPM - short of pinging/detonation.
3. Vacuum: This is an economy feature and is purely a function of the engine's vacuum, which is indicative of the load placed on the engine. Lower vacuum, as compared to that engine's maximum vacuum, means higher load.

The vacuum advance should be adjusted for that engine's vacuum characteristics, which are determined by many things inc the cam, the condition of the engine, the carb tune, etc.

 

Betcha a soda?

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

Source: http://w2ner.com/distributors.htm

That's just one source, if you search for terms like "vacuum advance highway cruise lean fuel mixture" etc, should keep busy for a while.

It really is an interesting subject, one of the important things to realize is that vacuum advance and mechanical advance are completely unrelated and independent of each other, and what is desirable from a drag racing standpoint is at cross purposes for what most people want from a daily driver.

Mechanical advance is RPM driven, Vacuum advance takes up the slack depending on engine load. It's remarkably ingenious to use this feature to improve driveability and economy.

 

Thanks, Ted...

Here is one place where the entire source of that text can be viewed/downloaded (it's a PDF file here):

http://www.camaros.org/pdf/timing101.pdf

 

That's basically the issue, maximum efficiency is always met by having the timing advanced just to the point of detonation at all times, under all conditions.

But in real world conditions this characteristic must be compromised, a stock distributor for a pickup truck cannot operate at this maximum efficiency under the range of different conditions or folks would grenade their pistons everytime they loaded up the bed with fill dirt and headed up the mountain. Or they got a slug of substandard fuel. Remember these are systems setup before computers utilizing mechanical weights and springs, dependent on engine RPM.

Simply adjusting the initial or crank timing is basically useless and of no real benefit unless the amount of mechanical timing is ALSO adjusted with respect to both "when" (RPM) and "how much" (total timing).

 

From personal experience Ive used vacuum as well as a timing light on my Mercury and was able to get my 351W up to 18 mpg city with the A/C on in the city. With a mix of city and highway driving I saw a average of 21 mpg.

Base timing with no vacuum advance and with the vacuum line disconnected and plugged to the optional economy booster, was set at the oem emission spec of +/- 4* of 15* BTDC. If I remember correctly I have my timing set at 18* BTDC which is within the +/- 4* spec. With the economy booster hooked up the timing at idle with no distributor advance jumps up to 22* BTDC.

Runs great, starts great, and has all the power one could want. Like wise vacuum is holding a steady 18in at idle. Before when I had timing set at 15* BTDC with the economy booster hooked to vacuum I was really running at around 10* and vacuum was only at 14in.

 

This is a subject I have a passion for as I think a lot of both performance and mileage is left on the table due to a wide misunderstanding of how important ignition and it's timing is.

To start with the subject of finding sources. I can find thousands of "reputable" sources that say the world is flat, OJ is innocent, global warming is real, ported vacuum is better and lean needs more timing. That thousands of people have said incorrect things doesn't make them correct. I'll say what I have to say and why, believe it or not, ask why if you don't believe me, or better yet try it out.

So first I'm going to start with this complete falshood;

This is COMPLETELY mixing the interrelated effects of lean and low throttle. YES it is entirely true that "idle and steady highway cruise, take longer to burn". This however has NOTHING to do with those being lean conditions, in fact idle is often rather rich. It has EVERYTHING to do with those being high manifold vacuum and thus low cylinder pressure conditions. Low cylinder pressure means molecules are further apart which means the flame front is slower and thus more timing is needed.

The line "Rich mixtures, on the other hand, burn faster than lean mixtures" is very misleading and really almost completely untrue. This depends ENTIRELY on how rich, how lean, and what you consider a midpoint. The fastest and hottest burn, the one that also makes the most power and MPG is slightly lean. Very rich is also very slow burn, it ends up having the same effect as water injection. Simply put the excess fuel without oxygen to match is as inert as water and serves no function but as a heat sink, slowing the burn. That said anything near stoichiometric for practical purposes burns at the same rate. The rich/lean issue with timing comes largely from lean mixtures being hotter and often occurring at WOT. This tends to heat parts the cylinder head to a point that hot spots contribute to detonation.

EDIT;
While on the subject There are two primary forms of detonation/knock or whatever you want to call it, pre-ignition and shock. Pre-ignition is basically dieseling, the cylinder is so hot and under so much pressure that the mixture explodes prior to the spark setting it a flame. This is very damaging, and tends to be rather loud, is not timing related but is mixture related. Shock detonation is more common, harder to notice, and both timing and mixture related. Shock detonation occurs when shock waves collide causing the fuel to explode, it should be noted just to make sure it's understood that in a spark engine the fuel should NOT explode, it burns. How does this happen, as the spark plug is on one side of the cylinder the flame front starts there, and for a time there is a pocket of unburnt mixture. This mixture has several things coming at it at once, the pressure wave of the flame front, the piston still coming up and being pressed against the hot and often pointy cylinder head. All this can cause that pocket to explode. So things like too advanced timing making the flame front to early, and lean mixtures making the head too hot can both lead to shock detonation.


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Back to the OP timing by vacuum, when using a typical distributor I personally very much like using vacuum as a way to find the ideal idle timing. It is however only useful in finding that reference point. Timing set to where it has the max vacuum at idle is often way to high for both starting and the rest of the running conditions. Vacuum should only be used to find the ideal idle timing, noted for tuning adjustment purposes and then tuning needs to move on from there.

I have written my simple but very effective method for setting timing to the ideal many times in this forum, why not again.

1. Research to determine a likely ideal WOT all in timing, this is different on all engines and must be eventually found through trial and error, but a good base can be found with research on your engine, often a number between 36-42 at around 3000RPM(or your peak HP RPM). (I tend to error on the high side for both timing and RPM for a number of reasons)

2. Start and warm up engine, make any other adjustments and unplug the vacuum advance.

3. Advance timing and adjust mixture to find the lowest timing and fuel settings that nets the highest vacuum at a desired idle speed. If the vehicle has an automatic transmission this MUST be done in gear. Note the timing and RPM, this is the ideal idle timing and RPM. (often this number is in the mid to high 20s)

4. Shut off and restart engine, often with timing that high and the engine hot it will not start. Retard the timing until the engine starts easy and tighten down the distributor. Note this timing, this is the ideal starting timing. (often this number is in the mid to high teens and I tend to error on the low side for a number of reasons)

5. Watch the timing advance as you increase RPMS, note the RPM at which it starts advancing(must be over idle speed in neutral), the RPM at which it stops advancing, and the total timing where it stops advancing. With any luck it will be the total all in numbers you found in step one. If not adjust the centrifugal portion of the advance mechanism to make it so.

6. Plug the vacuum advance into a MANIFOLD vacuum source, this will advance the timing at idle, speeding the idle and hopefully put it idling at the ideal idle timing found earlier in step 2. If not adjust the vacuum advance to make it so. (too much is better then too little, this can be very hard to set so I tend to error high)

These 6 steps should get just about any engine with a typical distributor very close to the ideal timing curve/settings and further minor adjustments can be made when they are deemed needed through other means.

Extra tip, you'll find with the timing advanced with manifold vacuum in gear with an automatic that a rather low idle speed can be easily maintained. This is very useful in many aspects, setting timing, economy, transmission wear/clunking, etc. My mostly stock 460/C6 truck idles so low and smooth that placing it in gear from park is almost unnoticeable and it will just barely move on level pavement in gear at idle.