Just In The Spark Of Time

There is no lack of opinions in this hobby and they are usually founded on nothing more than an individual's experience. Theory of what makes an engine run is hardly ever applied. Nowhere is this better exemplified than when dealing with spark advance curves and ignition tuning. The epitome of a dialed-in engine is a well-defined advance curve, but trying to reach that goal usually leaves you scratching your head. A problem is determining what your particular engine combination requires for spark lead and rate of gain. All too often the requirements of the engine are oversimplified and a fixed amount of advance is set and never touched again, most likely leaving many ponies locked away inside the distributor. Since the premise of anything other than maximum horsepower does not bode well with us at HPP, we have prepared this lesson about timing. Part I will cover theory, Part II in the next issue, a practical application.

The How and Why of Advance
Any conversation about spark timing is actually a discussion of the gas-exchange process and the theory of flame speed and burn rates. During combustion the spark travels slower than the piston, which is why it is 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 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 fresh charge and vice versa are of great importance during the expansion 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 during propagation is not easily determined, the consensus of most engineers is a velocity of 10-25 meters/second (m/s). Many factors affect the speed of the flame and thus the amount of spark advance the engine requires to produce the best power without entering abnormal combustion, more commonly known as detonation or ping. A spark plug firing is always referenced against the position of the crankshaft in rotational degrees. As an example, a total advance of 42* BTDC translates to the arcing of the spark plug when the crankshaft is 42 rotational degrees prior to TDC.

The purpose of spark timing is to control and utilize the greatest amount of energy from the fuel consumed. It is accomplished by having the cylinder pressure peak in as few rotational degrees of the crankshaft past TDC as possible. This will allow for the most energy to be imparted against the piston as the flame front expands, transferring more power to the crankshaft. The problem for the tuner is the flame speed that is identified as a median velocity depends on a combination of burning speed, flow speed and expansion speed, which are impacted by compression ratio, mixture ratio, charge motion, spark plug location, cylinder head material and combustion chamber design. Additionally, the bore size and the flame-termination process when it reaches the cylinder wall come into play. During the final stage of combustion, the flame slows down as it approaches the far walls of the chamber. This exact point is difficult to detect so most travel is referenced to the 95-percent position. The quicker the mixture burns, the less head start the flame will require; conversely, the opposite applies.

A good rule is that as the ratio increases or as the spark plug moves closer to the center of the bore, the amount of advance required goes down. Most Pontiacs had the timing curve referenced against the engine's ability to produce maximum brake torque, or MBT. Occasionally this abbreviation may be used to denote maximum best timing for torque.

During combustion if the flame spreads out from the spark plug in all directions with no biased flow paths, the leading-edge burning layer takes the shape of a spherical shell, which is also known as a flame kernel. The edges of the 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 cool and slow down. During the beginning periods of combustion, cylinder pressure rise is small because the amount of charge burned is extremely minute. At this point the flame speed is unusually slow due to minimal turbulence and because an area identified as a reaction zone needs to be established, where the heat transfer from the burned-to-unburned mixture will take place.

The turbulence rises from the velocity of the incoming charge and impacts the flame speed likewise. Most spark plug locations other than centralized in the bore are in low-turbulence areas and require more lead time. If the timing of the spark is either advanced or retarded from the optimal position, the work transfer to the piston will decrease. Not measured at the enthusiast level, engineers are also concerned with the mass burned fraction and the time to burn 10 and 95 percent of the fuel. As an example, it may be determined that maximum cylinder pressure occurs at 16* after TDC, but one-half of the charge may be already burned by 10* after TDC. It would then be a normal procedure to retard the spark for a reduction of MBT of one to two percent to allow for manufacturing variations, poor maintenance and fuel quality in production engines.

Until recently with the advent of electronic engine controls and distributorless ignition systems, the method to control the ignition timing was the responsibility of the vacuum and centrifugal advance mechanisms. A street engine by nature of its camshaft profile usually produces sufficient levels of vacuum, whereas a race powerplant's lack of negative atmospheric pressure demanded the use of mechanical advance weights only. The vacuum advance unit was originally designed as a means to improve fuel economy and was deemed inappropriate for use on modified engines.

What Does the Engine Really Want?
It's hard to believe but if you are using any type of mechanical advance controls on a street engine, you're probably not even close to the ideal spark curve. The ignition curve that can be supplied by mechanical means varies drastically from what the same engine would require for optimum performance with electronic engine controls.

Mechanical advance systems work under the assumption that the spark demand is linear with engine rpm and then levels out or, in simpler terms, a total amount of advance by a given rpm. This is the byproduct of looking at vacuum and rpm only to determine the advance curve. Including the static position of the distributor, this allows for a three degree-of-freedom (DOF) ignition curve while the vacuum signal is still being produced.

In contrast, electronic-engine-controlled spark timing is able to plot the spark 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, intake 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.

Although it would be nice to use a blanket figure for total advance for every Pontiac engine, that is not possible. 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. Any dome in the piston will slow the flame speed and require more spark lead. Combustion chambers vary beyond the familiar open/closed and hemispherical classifications.

Today it's more accurate to rate a cylinder head based on its ability to generate mixture motion, which can then be broken down into internal charge acceleration and externally induced motion. As the piston reaches TDC and the area in the cylinder is decreased, a cylinder head with a higher ratio of squish-to-bore will accelerate the charge as it is forced to vacate the quench region. This is identified as internal charge acceleration. A properly designed cylinder head will have the ability to stratify the mixture nearer the spark plug during this process. Closed chamber heads accomplish this with the use of squish pads, the area that is not open to the bore.

Externally induced mixture motion quantified as either swirl or tumble homogenizes the charge, making it more uniform while adding turbulence and increasing the burn speed. Many newer Pontiac engines such as the LS1 and 3800 V6 are excellent examples of using high squish/bore area relationships and external mixture motion, respectively, to decrease the required amount of advance.

The aftermarket has also recognized this and the best current example is the Feuling Center-Fire cylinder head. A high compression big-block Chevrolet fitted with these cylinder heads and tested by the University of Northwest Ohio at Lima required only 19* of advance to produce MBT. Also, the excellent GM L-31 Vortec heads historically require 29* of total advance when used with a short camshaft.

Other factors affecting the ignition curve are the efficiency of the engine coolant and air/fuel ratio. Rich mixtures require less spark advance than lean ratios, and cooler cylinder head metal temperatures allow for more advance before entering detonation.

Whenever the timing is retarded beyond the optimum setting, exhaust gas temperatures rise since the burn is being completed in the exhaust manifold or header during blowdown. Obviously, this is a waste of energy since it is not being used to expand against the piston. Very early initiation of the spark will create excessive cylinder pressure as the piston is sweeping toward TDC, trying to force it back down prematurely and in extreme cases bending the connecting rod.

Accepting the importance of the proper timing is fairly easy to grasp, but to create a correct advance curve you need two things to happen: accurately identify TDC on number one cylinder, and have a repeatable primary switching signal or interrupt.

Referencing from timing marks that have error due to a stack-up of tolerance will lead to erroneous amounts of spark advance, making the tuning job harder. The fact that most engines trigger the primary ignition from a distributor that's connected to the crankshaft via a timing chain leads to inherent error due to stretch and the meshing of the timing gears. It makes no sense to try to determine the crankshaft's position through these intermediate components and not from the crankshaft itself. Recognizing this, aftermarket ignition companies such as ACCEL and Electromotive among others offer electronically programmable ignition systems that are referenced from a very accurate crankshaft-located sensor, eliminating this error.

Wear in the distributor gear and shaft bushings adds play and timing error as well. This will vary the primary switching point on each cylinder, yielding different timing on each bore. For this reason it's 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 that is supported by bearings instead of bushings should be employed.

Getting Started
Before you tune a mechanical advance curve, the ignition and fuel system should be in satisfactory working order. The ignition needs the ability to not only produce a high-voltage and high-ampere spark, but keep the plug arcing for as much of the crankshaft's rotation as possible. Its advance mechanism should be clean and work smoothly, and the vacuum advance diaphragm should be checked for leakage with a vacuum pump. The timing marks need to be cleaned, and if possible checked to verify TDC.

Either a timing tape or dial-back timing light needs to be used. Starting with the static setting, the advance curve should be checked and recorded individually in 100 rpm increments.

Base: Initial advance read at the crank at idle with the vacuum advance disconnected.

Mechanical: Centrifugal advance determined by rpm acting upon weights and springs in the distributor measured at the crank with the vacuum advance disconnected.

Vacuum: Advance added during high vacuum light load cruise usually between 7* to 10* depending upon the canister used. It must be recognized that at WOT the restriction of the throttle plate is no longer present and manifold vacuum drops to near zero, eliminating any advance from the vacuum canister. When using an OE distributor you will need to locate an aftermarket advance kit. Depending on the engine family, it will be either a weight and spring kit or some variation on that.

Some companies offer adjustable vacuum advance units. If possible one should be used to dial in the part throttle timing curve. Additionally, a vacuum gauge in parallel with the vacuum advance canister will allow the amount of advance versus vacuum signal to be charted.

When dealing with a street car, the tuning session should be done with the fuel that is normally used and all other ancillaries such as the air cleaner assembly intact. This should be performed while the engine is at operating temperature. Keep in mind that as the timing is advanced at idle, the engine speed will increase, requiring the carburetor to be readjusted. For the best driveability, use as much advance as possible without entering detonation, and for power, either tune at the track or on a chassis dyno.

Though the art of dialing-in your Pontiac's spark advance is not complicated it can very easily become a consuming and frustrating process. Eight cylinder Pontiacs are relatively easy to tune since the distributor weights and springs can be accessed by just removing the distributor cap and rotor. The author has found that the rate of advance is the hardest to get correct, while the total timing is the easiest to nail. But getting the spark timing correct will reward you with a Pontiac that performs like never before. Calibrating the advance curve is probably the best investment in time you will ever spend tuning your engine. Stay tuned for Part II to learn how this theory is applied when we fine tune the timing curve on a Pontiac V8 on a chassis dyno.