First, a little history:
The Sixlitre Tune-up and Timing Bump originated in Mustang Forums centered around the 5.0L Mustang. This modification transitioned into the now famous FSB post by Sixlitre in 2005 and then follow-up sewiv technical writeup ignition upgrade and timing bump in 2006 which were also centered on their own similar 5.0L Broncos. This has now become general "common" knowledge on this forum as an ignition tune-up standard.
The tune-up portion includes new copper spark plugs, wires, coil, distributor cap & rotor on inefficient and neglected aging secondary ignition systems. The timing bump portion includes opening your spark plug gap from the factory specified .044 to .055 and advancing the base timing from the factory specified 10 degrees "up to" 13.5 degrees. This became a major motivation for 5.0L Owners to properly tune-up their engines for maximum performance and efficiency.
Soon after, many 5.8L Bronco Owners began to try the same tune-up and timing bump as well. I venture to say that much of the initial 5.8L performance and efficiency increases were also due to a proper tune-up with new components on aging 10 to 20 year old engines. Then, many posts began to be made regarding 5.8L pinging and economy decreases. While 5.0L Owners touted success, many 5.8L Owners, including myself, initially found a better idling engine but subsequently found it necessary to revert back to the stock .044 and 10 degree timing to eliminate pinging under grade or load and to provide the best fuel economy. Clearly something was different…
Next, a little technical perspective:
The timing bump was designed for a 302 which has a Knock Sensor to protect the engine from detonation. The Knock Sensor is a signal generating device called an accelerometer. It detects engine detonation (knocking), and converts this frequency signal to a voltage. The sensor consists of a piezoelectric element mounted in a threaded metal housing. Vibrating the element generates the voltage signal. Special construction makes the element only sensitive to the particular engine vibrations associated with knocking.
When spark knock occurs, the Knock Sensor produces a pulsing electrical signal. This signal is put through an amplifier and then sent to the EEC. The EEC then immediately retards spark timing until knock is no longer sensed, or up to a maximum 8° retard. The engine will return to normal spark advance after the MAP sensor detects a 3-4 in. Hg. change in engine vacuum.
In summary, the Knock Sensor assists the 5.0L PCM in allowing the timing to be advanced to the edge of detonation and then backs it off slightly for maximum performance and fuel economy.
Know that a 351 does not have a Knock Sensor and detonation caused by over-advanced timing can be harmful to your engine. Even without detonation, this can negatively impact the performance, efficiency and fuel economy of the 5.8L by throwing off programmed timing and fuel maps throughout your performance curve.
Timing the spark with EFI is done by the engine management system. It measures airflow, ambient temperature and takes input from literally dozens of sensors all over the engine. The EEC has an ignition timing map built into its memory and it cross references the input from all the sensors to determine the precise time that it should fire the spark plug, based on the ignition timing map. Again, this is not just at idle, but across your entire performance curve.
Timing advance is required because it takes time to burn the air-fuel mixture. Igniting the air/fuel mixture before the piston reaches TDC will allow the mixture to fully burn soon after the piston reaches TDC. If the air/fuel mixture is ignited at the correct time, maximum pressure in the cylinder will occur sometime after the piston reaches TDC allowing the ignited mixture to push the piston down the cylinder with the greatest force.
Ideally, the time at which the mixture should be fully burned is about 20 degrees ATDC. This will exploit the engine's maximum power producing potential. If the ignition spark occurs at a position that is too advanced relative to piston position, the rapidly expanding air-fuel mixture can actually push against the piston still moving up, causing knocking (pinging) and possible engine damage. If the spark occurs too retarded relative to the piston position, maximum cylinder pressure will occur after the piston has already traveled too far down the cylinder. This results in lost power, high emissions, and unburned fuel.
Unless you have done significant engine modifications, your timing usually does not need to be altered from factory specifications. Artificially attempting to alter your ignition timing based on numbers someone throws out, especially for a different engine, is fool hearted at best and must be done precisely. Keep in mind that changing your base timing will cause your programmed ignition timing map to be off throughout your performance curve. Hence, ping under load.
Consider that performance engines are even more sensitive to damage from incorrect timing. This is why custom timing maps for the entire performance curve are addressed in a custom tune or on-board programming. One way to determine correct 5.8L timing is by using a dynomometer to determine maximum power output at a given speed and load. With the 5.0L, one can use the existing Knock Sensor by advancing timing until knock occurs and then backing off 1-2 degrees. Otherwise, it is always safest and most efficient to retain the factory base timing settings or risk pre-ignition, detonation or misfires.
Finally, my summary:
The point is that you may not receive gains throughout your entire performance curve. And you may risk potential engine damage, as exhibited by pinging under load. Keep in mind that a 5.0L is a different engine having smaller mass, operates at higher rpms and has different harmonics which a Knock Sensor can identify. The higher spinning 302 may appreciate a car upper intake and a single throttle body, but a 351 will lose low rpm torque and efficiency. Yes, clearly something is different.
Performing a tune-up with quality components is the key. This alone can create a dramatic change in performance and economy with an aging engine. The challenge is finding the correct timing to ignite the air/fuel mixture after considering the many variables, including faster burning low octane fuel. Base timing is not something that can be simply altered, but a specific ignition point that exploits the maximum potential of your specific engine in relation to your PCM programming. Remember that the manufacturer has already made optimum recommendations through design and testing, and clearly lists the timing specification for your specific engine on your VECI Label.
Many are blinded by the significant performance increase of a simple tune-up on an aging secondary ignition system. In challenging general "common" timing knowledge, I hope to assist other 5.8L Bronco owners in making an informed decision regarding altering their factory timing. If you are compelled to experiment with your timing, be informed and know the potential ramifications.