UNDERSTANDING SPARK ADVANCE
There's certainly no lack of opinion in this hobby, and they are usually founded on nothing more than an individual's personal experience.
By Ray T. Bohacz
The engineering theory of what actually makes an engine run is rarely applied. Nowhere is this more apparent than in dealing with spark advance curves and ignition tuning. The epitome of a dialed-in street or race engine has a well-defined advance curve, but trying to reach this goal often leaves you scratching your head.
The problem lies in determining what your particular combination wants for spark lead and the rate of this gain. All too often, the amount of ignition advance is set and never touched again, most likely leaving many ponies locked inside the distributor. Since the premise of anything short of maximum power does not sit well with us here at Street Thunder, we have prepared this lesson about ignition timing.
THE HOW AND WHY OF IGNITION ADVANCE
Any conversation about spark timing is actually a discussion of the gas exchange process, and the theory of flame speed and burn rate. During combustion, the spark travels slower than the piston, which is why it's necessary to give it a head start. Combustion in an engine is dependent upon the ability of the flame front, originating at the spark plug, to travel into regions of unburned mixture. This occurs by means of conduction, diffusion, radiation, and the convection of heat. The unburned mixture portion is then heated and compressed, and it ignites. Conduction and the diffusion of heat from the burned charge into the fresh charge, and vice versa, are of great importance during the expansion, or power, stroke.
This sheds a different light on the old saying that airflow in an engine is king. All the airflow in the world will do nothing without an efficient and powerful combustion event.
Although the exact speed of the flame front during ignition is not easily determined, the consensus of most engineers is a rate of 10 to 25 meters-per-second (m/s). Many factors affect the speed the flame, and thus the amount of spark advance the engine requires to produce the best-possible power without entering an abnormal combustion (like detonation or pinging).
The firing of the plug is always referenced against the position of the crankshaft in rotational degrees. For example, a total advance curve of 42 degrees BTDC translates to the arcing of the spark plug when the crankshaft is 42 rotational degrees prior to top-dead-center (TDC).
The purpose of spark timing is to control and utilize the greatest amount of energy from the fuel consumed. This is accomplished by allowing the cylinder pressures to peak in as few rotational degrees past TDC as possible. Doing so allows for the greatest-possible amount of energy to be utilized in pushing down the piston as the flame front expands, thus transferring more energy to the crankshaft.
The problem for the tuner lies in the wide range of factors at play inside any given engine. The rate of flame travel is dependent upon burn speed, flow speed, and expansion speed. These three rates are reliant upon mechanical factors such as compression ratio, air/fuel mixture ratio, the motion of the incoming air/fuel mixture, the location of the spark plug inside the combustion chamber, the material the cylinder head is crafted from, and of course the engineering design of the combustion chamber itself. Additionally, the diameter of the cylinder bore and the flame termination process (defined as what happens when the flame front reaches the edge of the piston and contacts the cylinder wall) are also factors. This last point is difficult to detect, so most flame travel is referenced at 95 percent of the burn by researchers.
As hot rodders, we understand the fact that the faster the compressed air/fuel mixture burns, the less of a head start (advance) the flame will require. Conversely, the opposite is also true. A good rule of thumb to remember is as the compression ratio increases, or as the spark plug moves closer to the center of the combustion chamber, the amount of timing advance required for maximum performance will go down.
With factory (unmodified) engines, the timing curve is usually referenced against the engine's ability to make maximum brake torque, called MBT. The MBT acronym is sometimes used in reference to "maximum best timing" for torque, but the meaning is essentially the same.
HOT KERNELS
During the combustion process, if the flame front spreads out from the spark plug in all directions with no biased flow paths, the leading edge of the burning layer takes the shape of a spherical shell, which is also called a flame kernel. The edges of this sphere will be ragged due to convective currents in the highly-turbulent mixture. Once the flame reaches the vicinity of the combustion chamber wall, it will be cooled and slow down. During the critical beginning periods of combustion, the cylinder pressure rise is small because the amount of air/fuel charge burned is relatively minute. At this point, the flame speed is unusually slow due to a minimal amount of turbulence and because an area defined as a "reaction zone" has yet to be established. The reaction zone is where the heat transfer from the burned to the unburned mixture will take place. The turbulence rises from the velocity of the incoming air/fuel charge and impacts the flame speed in a likewise manner.
Most factory spark plug locations are not centralized in the bore, with a few exceptions (like the Chrysler Hemi). These non-centralized plug locations are typically in low-turbulence areas inside the chamber, and therefore require more ignition lead time. If the timing of the spark is either advanced or retarded from the optimal position, the amount of work energy transferred to the piston, and therefore the crankshaft, will decrease.
Factory engineers are concerned with the percentage of fuel burned and the time it takes it to burn. For example, the time it takes to burn both 10 percent and 95 percent of the fuel are common figures used by the OEM engine designers. Let's think like OEM engineers for a moment, and then we'll look at the same issue as the hot rodders we are.
In a given engine, it may be determined that maximum cylinder pressure occurs at 16 degrees after TDC, but 50 percent of the air/fuel charge may already be burned by 10 degrees after TDC. It would then be normal procedure to retard the spark for a reduction in MBT by 1-2 percent to allow for manufacturing variations in production engines.
Until recently, with the advent of electronic engine controls and distributorless ignition systems, the means of controlling ignition timing was the responsibility of engine vacuum and centrifugal advance mechanisms. A street engine, by nature of its camshaft profile, usually produced sufficient levels of consistent intake vacuum. A more aggressive powerplant with a serious camshaft design, does not produce steady and reliable vacuum, so a reliance was built on the centrifugal weights exclusively. The vacuum advance system was seen as a "smog" control and its use was initially deemed inappropriate for use on modified engines.
So, what does the engine really want?
It's hard to believe but if you're using any type of mechanical controls on your ignition advance, you're probably not too close to the ideal spark curve. Mechanical advance systems work under the pretense that spark advance demand is linear with engine rpm, and then will level out at a given point. The "total advance, in by a given rpm" theory is deeply rooted in street performance engines, and this is a byproduct of looking only at vacuum and rpm to determine an advance curve. Including the "static" position of the distributor, typical tuners are looking at a total of three points in determining the entire advance curve (static, initial, and total). Naturally, this is while vacuum signal is being produced, and if vacuum is part of the advance system, this can also be factored in. By comparison, an electronically-controlled ignition system is capable of plotting the advance curve based on load, rpm, and coolant temperature at very finite intervals, offering almost infinitely variable control. In practice, an engine requires a varied amount of spark advance as load and speed are changed, due to variations in volumetric efficiency, cylinder pressure, air density, manifold tuning, EGR dilution, and coolant temperature. Unfortunately, even after gaining a thorough understanding, a successful spark table is usually determined by trial and error. There are established guidelines however, and if these are followed, the plotting of an effective timing curve can be greatly expedited.
MORE FACTORS TO CONSIDER
It sure would be nice if we could publish a "blanket" figure for total advance requirements for every engine of a particular make, but this is simply impossible. The combustion chamber shape and spark plug location (in relation to the bore center) will be the most significant factor in flame travel and necessary spark lead. This includes the piston crown design, since any domed piston slows the flame speed down, and requires more spark lead. Combustion chamber designs vary greatly from the common "open," "closed," and "hemi" designs we're familiar with. Today, it's more accurate to rate a cylinder head on its ability to generate mixture motion than to simply refer to the shape of the combustion chamber. One trend we've seen is the shrinking of the modern chamber, as smaller displacement chambers are put to work with creatively-dished pistons to create more-optimal and burn-friendly combustion chambers. A high squish/bore ratio teamed with effective mixture motion can seriously reduce the amount of ignition advance required, and make more power too! Fans of the small-block Chevy know that typical total advance numbers for this engine range in the 34-38 degree range, with most happy at about 36 degrees of total advance. The new L-31 "Vortec" cylinder head from GM is a known performance add-on, and it's not unusual to see only 29 degrees of total advance using this head.
Other factors affecting the ignition curve are the efficiency of the engine coolant and the air/fuel ratio. Richer mixtures typically require less spark advance than leaner ratios, and cooler cylinder head temperatures allow for more timing advance before reaching detonation. Whenever the timing is retarded beyond the optimal setting, exhaust gas temperatures rise quickly since the burn event is being completed in the header (or exhaust manifold) during the exhaust stroke. Obviously, this is a waste of energy since the heat and expansion of the air/fuel is not being used to force the piston downward. Very early initiation of the spark will create excessive cylinder pressure as the piston is still heading up the bore toward the combustion chamber, and will try to force it downward prematurely. In extreme cases, this could even bend a connecting rod.
THE USUAL SUSPECTS
Accepting the importance of the proper timing is all well and good, but to have a proper advance curve two things are required to happen. First, TDC must be accurately located for the number one cylinder. The other is accurate triggering of the ignition system, either through accurate primary switching or interrupts.
Referencing from timing marks that have error due to a stacking of tolerances will lead to erroneous amounts of spark advance, making the tuning job harder. The fact most engines trigger the primary ignition switching from a distributor connected to the crankshaft by a timing chain leads to inherent error due to chain stretch and meshing of the timing gears. It makes no sense to attempt the crankshaft position through these intermediate components and not from the crankshaft itself. Recognizing this, aftermarket high performance ignition companies such as MSD, ACCEL, Electromotive, and others offer electronically programmable ignition systems referencing a very accurate crankshaft-mounted sensor, eliminating the potential for error. Wear in the distributor gear and its shaft bushings add more play, and therefore timing error. This wear will alter the primary switching point on each cylinder, yielding a different timing curve for each bore! For this reason, it is advisable to check for cylinder timing variations using an ignition oscilloscope. A very high rpm engine will accentuate this problem, and a distributor with a very stiff shaft supporting by bearings (rather than solid bushings) would be highly recommended.
GETTING STARTED
Before attempting to tune a mechanical advance curve, the ignition and fuel systems should be checked and proven to be in satisfactory working order. The ignition system needs the ability to not only produce a high-voltage, high-ampere spark, but also must keep the plug arcing for as much of the crankshafts rotation as possible. The advance mechanism should be clean, and work smoothly. The vacuum advance diaphragm should be checked for leakage with a vacuum pump. The timing marks on the balancer should be clean, easy to read, and double-checked to verify their accuracy- especially at TDC. A timing tape or dial-back style of timing light needs to be employed.
Starting with the static setting, the advance curve should checked and recorded independently for both mechanical and vacuum advances. Naturally, these figures are then added together to produce a total timing advance figure. It must be recognized how at wide-open throttle, manifold vacuum will be reduced to near zero, eliminating any advance from the vacuum canister. When fine-tuning a factory distributor, an aftermarket advance kit will be required to make adjustments. Depending upon your engine, it will either be a "weight and spring" kit, or some variation on it. Some companies offer adjustable vacuum advance canisters, which are great for adjusting part-throttle timing advance. A vacuum gauge, hooked up in parallel with the vacuum advance unit, will allow the amount of advance versus vacuum signal strength to be charted.
The tuning session should be accomplished with the engine running on its normal fuel, and all of the other components (like the air filter assembly) in place and intact. The session should be performed with the engine at normal operating temperature. As the timing is advanced at idle, engine speed will increase and carburetor idle speed will have to be reduced to compensate. For best drivability, run as much advance as possible without sending the engine into detonation. For best power, we'd have to recommend a good chassis dyno session or some experimentation at the dragstrip.
Ignition timing is one of the golden keys to high performance, and knowing its secrets will open doors to hidden horsepower. There are normally many adjustable parts and pieces in your ignition system, especially if you're piloting a traditional American street machine. Taking some time to fine-tune your ride should result in the engine performing to its fullest potential, which to us simply means getting everything you've already paid for. Take some time and give your ignition system some attention, and it might pay you back with more than you bargained for.
