Why do I need to do this? If I set it to factory specifications, for my stock truck, it should already be optimized, correct? Those are great questions and conclusions and if gasoline was the same as it was when your truck was manufactured, I'd agree.
Gasoline Formulation/Octane
The formulation of gasoline has changed quite a number of instances and dramatically since sweptlines were manufactured, primarily to satisfy emission regulations. Most of the changes revolve around detonation prevention additives. Detonation is when the air/fuel mixture ignites prematurely and attempts to drive the piston downward while it is still on it upwards stroke. This creates a "knocking" or "pinging" sound from the engine. Detonation will quickly destroy an engine. Gasoline's ability to withstand pre-ignition (detonation) is graded by its octane rating.
In 1921, lead (tetraethyl lead) began to be blended into gasoline to increase its octane and prevent knock. Lead was near universally utilized as an octane booster until it was initially reduced in 1985 and eliminated from US pump gasoline by 1990. Along with engine de-tuning, other octane boosting additives were utilized briefly in the 1990s, but ethanol soon became the environmental poster child as an octane booster (anti-knock additive)
Lead was an effective and a minute anti-knock additive. It was only 0.03% component. As such, a gallon of gasoline was nearly all gasoline and with that, was able to develop a high combustion pressure. Ethanol, on the other hand, is commonly a 10% blend of alcohol. Additionally, as alcohol and gasoline will not blend, a binding additive is utilized. Emission regulations require further additives for things such as oxygenation. So pump gas today is diluted with a number of additives that greatly reduce its volatility. As such, an equal amount of ethanol will not yield any where near the combustion pressure of lead based gasoline, nearly 30% less. This is why fuel economy suffers greatly as the percentage of alcohol is increased within ethanol. Folks who purchased "Flex Fuel" vehicles soon found while E85 came at a lower cost, their gas mileage dropped by 25% when utilizing it.
The reduced volatility of ethanol, compared to gasoline, also impacts flame spread: How quickly the air/fuel mixture achieves maximum combustion pressure. The flame spread rate of ethanol is much less than gasoline. As such, ignition timing must be advanced significantly, within a sweptline era engine, to achieve maximum combustion pressure at the optimal time.
Initial (idle) Engine Timing
Sweptlines manufactured in the later production years had, at least in part, some emissions controls. Emissions were only first tested at idle. So a simple method was to lean the idle mixture, which required retarding the timing. This is why emission specifications of the day called for timing values between 0 and 5*. Utilizing these specifications with today's fuel, typically ethanol, will result in an engine that idles roughly, if at all, and has a very rich smelling, eye watering exhaust, as the air/fuel mixture did not have time to fully burn before the exhaust stroke commenced.
Ported Vacuum Advance
While the use of a ported vacuum advance saw some limited use in the 1950s with engines that did not have mechanical advance, it did not begin to see regular use until the advent of emission controls. In the years prior to microprocessors, vacuum advance provided a means to coordinate the amount of timing advance as dictated by engine load, to achieve maximum efficiency and power. Prior to emission regulations, distributor vacuum advance was served by manifold vacuum. However, as manifold vacuum is typically high at idle, the vacuum advance would advance engine timing to far to support a lean idle mixture. A means to delay the application of manifold vacuum, to the distributor, until the throttle was off-idle was required: ported vacuum. This was achieved by adding a vacuum passage that was just above the throttle plates. As such, when the throttle plates were at idle, this passage would have no or nearly no vacuum. As soon as the engine came off-idle, this port has exactly the same vacuum as manifold vacuum. So, there is no big mystery to ported vacuum. It is identical to manifold vacuum, except at idle, where it is zero or near zero.
With today's ethanol blended gasoline, lean idle mixtures and ported vacuum advance has become obsolete. Ethanol's flame spread rate is simply too slow to allow for such retarded timing.
Tools Required
*Vacuum Gauge
*Tachometer, non-inductive
*Hand Vacuum Pump
*Timing Light w/advance feature---timing tape or fully degreed harmonic balancer
*3/32 Long Shank Allen Wrench
2*12' lengths of 3/16" rubber vacuum hose
Prerequisites
If you're working with a stock engine and presumably a stock distributor, we'll want to verify that first. There will be an ID tag on the distributor with a seven digit part number. Compare this to the part number published within the Specification section of the factory service manual. My '71 D100 still had the original distributor (3438453) that a previous owner had installed a fully self contained electronic ignition conversion within. So it still had the published advance characteristics.
My '71 D100's Mechanical Advance:
Idle: -
35MPH/1500 RPM: -
50MPH/2000 RPM: 10*
65MPH/2600 RPM: 10*
I utilized the above RPM points as they corresponded to the test measurements that would be made at certain speeds. Comparing these with the published specifications, they were within the error margin of the specifications.
Replacement and/or Aftermarket Mopar Style Distributors
If your sweptline is not a hot rod, still works for a living, and you've installed a Direct Connection, Pro Form, or salvage yard distributor; you may need to limit the mechanical advance. Performance distributors typically have a fair amount of mechanical advance that will be too much for a truck that tows or carries cargo. The crude method of limiting advance is to disassemble the distributor and partially weld close the slots the advance weight pins move in, to advance the timing. This isn't a real accurate method, requires a lot of time consuming trial and error measurements, and is difficult to reverse. Rather than go through all of that, there is a clean method to limit the advance: FBO Advance Plate This a plate that drops in over the advance weight pins. Simply select the etched labeled slots of the advance limit desired. They're kind of pricey for what they are, but you can't fiddle around welding the slots for what it costs. https://4secondsflat.com/Mopar%20Mechan ... 0plate.htm
Next, I desired to measure the levels of vacuum the advance diaphragm began to engage and what level it was at maximum. To do this, I removed the distributor cap so I could view the reluctor pickup coil or points. I connected a had vacuum pump to the advance diaphragm and slowly began to pump vacuum until the reluctor coil just began to move and noted the vacuum level. I continued adding vacuum and noted the level where the reluctor pickup coil ceased to move:
Vacuum Advance Begins at: 8 inches
Vacuum Advance Achieves Maximum at: 14 inches
Next, I left the hand vacuum pump connected to the diaphragm, released the vacuum, and started the truck. With the timing light, I noted the idle advance setting w/o vacuum applied to the diaphragm. I applied 20" of vacuum to the advance diaphragm to fully advance it and measured the amount of maximum vacuum advance:
Maximum Vacuum Advance: 13*
I compared this to specifications and found it to be within a margin of a error.
Manifold Vacuum Connection
Many carburetors will have a 3/16 manifold vacuum hose barb at the base of the carburetor. If your carburetor does not have such, you'll need to add a hose barb at the intake manifold opening for the power brake vacuum booster feed. If your truck does not have power brakes, you can simply remove the plug and install a low cost brass hose barb sourced from the local hardware store. If you do have power brakes, I would recommend replacing the existing hose barb with the multiple barb adapter, rather then teeing into the booster hose. These are available from a number of classic car parts sources. The photo below is from moparmall.com web page:
Initial (Idle) Timing Adjustment
Connect a vacuum gauge to manifold vacuum. Connect a timing light. Connect a hand vacuum pump to the distributor vacuum diaphragm. Start the engine and allow it to warm up to normal operating temperature. With the hand vacuum pump, apply enough vacuum to fully advance the vacuum diaphragm. While monitoring the vacuum gauge, adjust the distributor timing to achieve maximum vacuum. Read the timing value with the timing light and make a note of it. Retard the timing 2-3 degrees to provide some detonation headroom for temperature and fuel variations. Note this value and secure the distributor. Re-adjust idle speed. Adjust carburetor idle mixture screws for maximum vacuum reading. Re-adjust idle speed.
I achieved maximum idle manifold vacuum when adjusting the distributor timing to 38* on my '71 D100. I was quite surprised by this value. Back in the late '70s, I attended a number of Direct Connection Drag Racing seminars hosted by Chrysler and taught by their engineers. At those seminars, we were taught to adjust timing for an "all-in" value of 32*. Here, I just set my idle timing to 35* (allowing for detonation headroom) with no mechanical advance. Then again, gasoline was formulated quite differently in the late '70s.
All-in Timing Adjustment
Next, we move to adjusting "all-in" timing. As I don't have a dyno (despite being on my Christmas Wish List for decades), this will be road test measurements. As my truck does not have the HD gauge option, I placed a towel on top of the dash, to protect it, and then placed the vacuum gauge and tachometer on top of the dash. Ideally, having an assistant makes this much easier. As I didn't, I needed to have the gauges where they could be readily read while driving. I also placed a hand operated vacuum pump, connected to the vacuum advance diaphragm, in the cab. The photo below also depicts an A/F meter. This is not required for timing adjustment, I utilized this when re-jetting the carburetor, which should follow timing optimization.
In practice, while doing this without an assistant, the degree of change on the vacuum gauge was so small that I couldn't monitor the gauge, hand pump the vacuum, and stay in my driving lane, all at once. So my first step was to apply more than enough vacuum for full vacuum advance. I would then release the vacuum while monitoring the vacuum gauge. If the vacuum gauge reading increased upon release, I was adding too much advance. If it decreased, I could still add more. If the manifold vacuum increased, I would repeat the test, but this time only applying half the vacuum level range ( 8 - 14 inches), to the advanced diaphragm, required for full advance. 11 inches in case of my '71 D100. Depending on whether manifold vacuum increased or decreased, when the advance diaphragm was released, I'd split the applied vacuum amount in half again for the next test. 9.5 inches if the manifold increased in the previous test or 12.5 inches if it decreased.
If you max out both mechanical and vacuum advance, you may be able to obtain more vacuum advance by adjusting the limit screw of the advance diaphragm, through the diaphragm hose barb, with a 3/32" Allen wrench. Additional mechanical advance can be obtained by changing advance components (springs, weights, etc)
With my '71 D100, with a vacuum applied that would provide full advance, I was "all-in" at 48*. Releasing the vacuum yielded a small increase in manifold vacuum. So I reduced the vacuum level to the advance until manifold increased, as described in the previous paragraph. I extrapolated the reduced vacuum level to a decrease of 3* of advance. I returned to the shop and adjusted the vacuum advance, with a 3/32" Allen wrench through the diaphragm hose barb, to reduce the vacuum advance by 3*, from 13* to 10*. I repeated the road test. Releasing all the vacuum did not result in a decrease of manifold vacuum. I slowly hand pumped vacuum and the manifold vacuum, initially, slightly increased but remained relatively constant with what I'm estimating, via extrapolation, was the final 5* of additional advance.
I repeated the above test at my other test speeds of 35 and 50MPH. Manifold vacuum either remained constant or slightly increased.
I reconnected the vacuum advance diaphragm and performed several hard accelerations from dead stops and from my test speeds and detected no audible signs of detonation. I was satisfied with a maximum, all-in, timing adjustment of 45*.
Reducing the vacuum advance 3* also reduced my initial timing adjustment. The only method in which I could recover this is to reduce the mechanical advance by 3 degrees, rather than the vacuum advance. This is not practical to obtain. So the best compromise is to accept the degraded idle timing.
How Much Mechanical? How Much Vacuum?
We've established our upper and lower base lines: All-in Advance and Initial (Idle) Advance. We can further fine tune by adding mechanical advance, which requires reducing vacuum advance or visa-versa. Race engines will typically rely solely on mechanical advance, as they are typically operated at or near wide open throttle where manifold vacuum levels are below the level where the diaphragm begins to advance. Street rods will use both, but with more emphasis on mechanical advance than vacuum. Trucks that haul or tow heavy loads will have their emphasis on vacuum advance, as this is utilized to "sense" engine load and reduce advance to prevent detonation when under heavy load (less manifold vacuum).
Bear in mind, whenever we change vacuum advance in the mobile RPM range, it has an impact on our Initial (Idle) Advance. Compensating for reduced vacuum advance by increasing distributor advance has its limitations. At some point of increasing distributor advance, we will begin to experience hard starting. The air/fuel mixture is igniting at the optimum point of advance when at idle RPM, but not the extremely slow RPM of the starter motor. So the piston is traveling upward when the ignited air/fuel is attempting to drive it downward. A significant amount of vacuum advance prevented this, by providing a retarded timing during starting. The engine does not develop enough vacuum during starting to pull the vacuum advance diaphragm. As such our starting timing is retarded by whatever the maximum amount of vacuum advance is configured for. Utilizing my '71 D100 as an example, the Initial (Idle) timing was set to 32*, after we reduced vacuum advance form 13* to 10*. As there is not enough vacuum during starting to pull the vacuum diaphragm, starting timing is actually 22*.
How quickly mechanical advance increases with RPM is regulated by the mass of the advance weights and their return springs. A spring can be removed to provide early advance or its tension changed to impact how quickly mechanical advance begins and at what RPM it is at maximum. Advance weight tuning spring kits are readily available at speed shops. Where you configure this will depend on the application of the truck, camshaft grinds, and other engine performance factors. It can be a fair amount of tedious trial and error. If your truck is a hot rod you'll be operating at higher RPMs and will require more advance at upper RPMs. If you have a street/strip application, you'll be moving your operating RPM range higher, but you'll still need to maintain drive-ability and enough manifold vacuum at low speeds/idle to operate creature comforts, such as power brakes. Ultimately, you'll likely find you'll compromise one component adjustment for another and there will be trade-offs. These trade-offs could become more pronounced the more you attempt to multi-task your sweptline.
No Compromises?
So you want to have your cake and eat it, too? Well, you can. There is a distributor that pretty much addresses and removes all the limitations and compromises encountered with a conventional distributor: The Progressive Ignition Distributor
https://progressionignition.com/
The Progressive Ignition Distributor is a fully self contained microprocessor based ignition in which you can create your ignition timing map practically per RPM point. This is accomplished by a plain English app operated from a smart phone or tablet via a wireless connection to the distributor. The app also displays real time RPM and manifold vacuum, so adjustment setup configuration is automatic. Parameters can be changed while the engine is running. Additionally, multiple timing maps can be created and saved within the distributor for recall and loading on demand. This allows those who multi-task with their sweptlines, such as Wildergarten, to have a timing map he can load instantaneously when performing a heavy haul, another for unloaded highway cruising, another for PTO operation, and yet another gentle mode for the bride, when she has to rescue him.
