The Stovebolt Site Forums The Shop Area Engine Shop Ignition advance question

This is a theoretical question, not specific to any particular vehicle, but would apply to anything with a gasoline engine and a distributor.

I think that centrifugal advance in a distributor is used to make spark plugs fire sooner as engine speed increases to make up for the fixed time it takes for the gasoline/air mixture to burn. The net result is the peak cylinder pressure aligned with the piston’s position. If I’m wrong, tell me why and this thread can be closed out.

Using specs for my ’52 GMC (same distributor is used in same year Chevy trucks.), centrifugal advance actually starts coming in just above idle, is 22 degrees at 1700 RPM and is “all in” at 32 degrees at 2600 rpm. So, if increasing the timing advance between the advance up to 2600 RPM is a good thing why isn’t continuing to advance timing above 2600 RPM not a good thing?

My question is even more obvious when you consider a more high performance engine such as a 1969 Camaro Z28 302 V8. Specs for that engine shows an “all in” of 36 degrees at 3000 RPM. Factory red line is 6000 RPM, so no additional advance for the upper half of the engine’s RPM range.

Just like to know.

Over the span of approximately 200 dyno runs I have done over the past 40-something years, regardless of whether an engine is designed for street use or all-out performance, centrifugal timing advance numbers are remarkably similar between engine manufacturers. Vacuum advance can mostly be ignored, as it only comes into play at light throttle/high vacuum situations, and mostly affects fuel economy, not horsepower or torque figures. Vacuum advance disappears almost instantly as soon as the engine gets a little torque load. To do a dyno pull properly, a specific RPM is selected, and the throttle opening is increased to try to maintain that RPM as load is applied to the engine. There will come a point where no more load can be applied at full throttle without dropping RPM. At that point, the torque reading is recorded and HP is calculated using the following formula:

TORQUE X RPM
5252

"Fudge factors" can be applied to the raw HP figures to account for temperature, altitude, barometric pressure, and relative humidity changes.

RPM is then increased (usually in increments of 500) and a new torque pull is made, until a predetermined max RPM is reached, or HP begins to decrease. Any alteration to operating conditions such as carburetor jetting, cam timing, intake/exhaust configuration, or ignition timing requires a new sequence of pulls. ONLY CHANGE ONE THING AT A TIME!

If we're evaluating centrifugal advance at a particular RPM, for instance, find the maximum torque developed at a particular WOT speed, and start advancing timing one degree at a time and recording the torque change. You'll find the "sweet spot" where torque is at a maximum. Advancing the timing further results in a loss of torque, due to the engine trying to run backwards as the flame front reaches the piston crown too soon. Record the torque loss from the over-advance. Go back to max torque, and start retarding the timing, recording the torque loss as the timing is retarded. You'll find that it takes approximately 5 degrees of timing retard from the max power point to equal the torque loss from ONE DEGREE of over-advance. It's always a good idea to err on the side of "not enough" centrifugal advance, as the law of diminishing returns kicks in very quickly!

The above results apply to virtually every engine I've ever dyno'ed, regardless of manufacturer, or the amount of modification it's had.
Jerry

I'm pretty sure I understand what you're telling me Jerry, but you didn't answer my question.

Given a constant amount of time for the spark to get the air/fuel mix to burn completely and a decreasing amount of time (as RPM increases) for a piston to be in the proper place in it's travel in the cylinder to take full advantage of the expanding pressure of the air/fuel mix, why not continue to increase the timing advance past 3000 (or so) RPM?

There must be a reason that "centrifugal timing advance numbers are remarkably similar between engine manufacturers".

I think Jery answered your question in a slightly different way. He referenced several times the peak torque RPM/ignition advance/throttle opening balance. If you have achieved peak torque at a certain RPM with 32 degree advance, and one more degree of advance actually hurts the torque output, you have gone too far with advance. Torque is the goal, if you light the fire too soon it will effectively try to push the piston backwards down the cylinder and drastically hurt the torque/hp/economy/drivability/engine longevity. Ignition "ping" is the sound of the explosion rattling the piston assembly, it can and will destroy the piston. No doubt the manufacturer wanted to get as much performance as they could but they also wanted the engine to live a long life, that is where vacuum advance comes into play, it increases advance only when the engine can allow it without damage. P.S., my '57 2 ton truck with the original 283 has no vacuum advance, only centrifugal. They understood that it would be used for heavy work and most likely would not see much part throttle high vacuum use. The centrifugal/vacuum governor limits the RPM at 4000, static timing is 4 degrees, I didn't look up the total advance but it probably was on the safe side so as not to burn holes in the pistons or bend the rods, they wanted the truck to live forever under heavy load.

I think another factor that enters this discussion is the amount of time that the ignition induced pressure increase has to apply its force to the piston. At lower engine speeds (say 1000 rpm) the time is relatively long when compared to time at higher engine speeds (say 3000 rpm) and even less at maximum engine rpms. As a result there are diminishing returns to the earlier initiation of the pressure increase and as noted 1 degree to early has significantly more influence than 1 degree to late. The pressure build after spark starts the pressure build takes time, milliseconds but still time, so at some engine speed that earlier spark timing doesn't generate any benefit.

I don't have multiple engineering degrees to be able to address the theoretical reasoning behind the limit to advancing ignition timing beyond a certain point- - - - -you'd need to talk to my brother, who has spent 40+ years doing computer modeling for automotive manufacturers to have that conversation. What I can offer is the results of several decades of hands-on dyno run experience. Just about any engine stops getting a power gain when the centrifugal advance goes beyond about 36-37 crankshaft degrees at maximum torque. Many factors get involved, such as compression ratio, cam timing, combustion chamber shape, the number and arrangement of valves, and computer control of cam timing, fuel/air ratio, and the design of the intake and exhaust systems.
Jerry

Turbulence within the combustion chamber accounts for much of the static advance above a certain RPM. That turbulence will cause the flame to complete its travel from the spark plug to the far corners of the chamber in less time as RPM increases. Combustion chamber shape determines much of the turbulence. Not sure I've worded that correctly but some engines run fixed advance.