initial vs total timing

[don izzo]

I have read interesting comments on this topic posted by Duke and others and now realize just how little I know about the subject. Does anyone know of a Book or article I could get that would educate me on this subject matter?

Thanks to all
Don

[Joel Falk]

Re: initial vs total timing

Here is a site with a bunch of good articles

http://lbfun.com/Corvette/Tech/vettetech.html

I'd start with the timing 101 and go from there.

[don izzo]

Thanks just what I needed! *NM*

[Duke W.]

Re: initial vs total timing

To summarize, flame propagation speed is determined by three factors:

Mixture density - low density (like idle) low speed; high density mixture (like WOT), higher speed.

Exhaust gas dilution - low dilution, normal speed; high dilution (like idle) slows it down,

A/F ratio - lean mixture, stoichiometric and leaner slows speed; rich mixture (such as WOT) increases speed.

For any given operating condition, optimum spark advance will yield minimum EGT.

For the above reasons, idle speed requires relatively high advance - much more than typical initial timing - to minimize idle fuel consumption and EGT/heat rejection to the cooling system, and cruise load is typically a relatively low density, lean mixture, so more advance is required to attain peak efficiency than at WOT at the same speed.

Detonation is primarily a low speed issue. At 1500 RPM the "end gas" (the last of the unreached fresh charge towards the end of combustion) is subjected to higher temperature and pressure for twice the clock time as at 3000 RPM, so it has more tendency to detonate. Centrifugal advance accomodates this by delaying full advance until higher RPM is achieved.

So on vintage engines, spark advance is determined by two independent variables - manifold vacuum (which is a good surrogate of engine load) and engine revs, and if you plot this on an XYZ graph you get a surface. Thus the term "map", and the initial timing can be used to move the entire surface up or down along the Z axis.

Most discussions of "total timing" usually only involve the total timing required at high RPM WOT, but this only represents a small fraction of operating conditions, which represent only a very small operating time fraction for a road engine. There is also idle to consider and virtually an infinite number of cruise conditions. For this reason I refer to "total idle timing", total cruise timing" and "total WOT timing".

Vintage centrifugal and vacuum advance systems do a pretty good job of optimizing spark advance for nearly all operating conditions, and they are simple, reliable, and easy to change if warranted.

Most OE maps are okay, but the centrifugal is often little on the conservative side (slow or what I call "lazy") to ward off detonation under worst case operating conditons such as high inlet air and coolant temperature, and a few have poorly matched vacuum advance characteristics, which is easy to fix with a different VAC. Also, whenever an engine is modified, such as installation of a high overlap cam for an OE low overlap cam, the ignition map should be reworked to something like an OE high overlap cam including installing the same VAC as an OE engine with a high overlap cam.

Full vacuum advance should be achieved at no less than 2" less than typical idle vacuum, and ported advance should be reworked to full time advance, and a total of three aftermarket VACs "fit" virtually all OE and modified engines. All you have to do is use the above "rule" to chose the correct one.

Tuning the ignition map to the specific engine configuration and typical operating environment almost always pays significant dividends in terms of lower fuel consumption and broader torque bandwidth compared to the cost and effort to make the changes (assuming you understand the basics of spark advance requirements as discussed above).

Best wide range performance and lowest fuel consumption is achieved by getting the centrifugal in as quickly as possible with detonation being the limiting factor. This can be as easy as installing lighter springs, but sometimes the total centrifugal should be reduced or increased with an equal and opposite change in initial timing to acheive the desired WOT peak spark advance at 2000-3000 RPM range without suffering detonation at lower speed.

This can all be summarized as ranges for most SB and BB engines.

Total idle timing: 20-32 degrees
Total cruise timing (typical highway cruise speed and revs): 40-54 degrees
Total WOT timing: 32-40 degrees

The range of total idle timing is determined by valve overlap. Low overlap cams in the lower half of the range, high overlap cams on the high end.

Total cruise timing depends on how quickly you can achieve full centrifugal advance. SHP cams with late closing inlet valves can tolerate quicker centrifugal advance than the medium perfomance engines with relatively early closing inlet valves. For example, a 327/365 OE map attains 50-54 total cruise timing at 2350 PRM, but because 300 HP engines have slower centrifugal curves and typically less initial timing, they will be closer to 45 degrees total cruise timing at highway speed.

Duke

[Doug Flaten]

Re: initial vs total timing

Duke, short of pinging, does high too much timing also create high EGT? Or is range between the optimum point and pinging very narrow? I have had people tell me that high advance aslo creates high temps. Usually it is in the context where I have tried to convince people that they may want to try a manifold vac source to increase timing at idle and reduce the problematic temps at idle. I have done the experiment you have mentioned before and disconnected the vac advance and watched the temp soar quickly.
Thank you again for your insight.
Doug

[Duke W.]

Re: initial vs total timing

No. Too much spark advance will usually get into detonation before any other bad effects. Most engines are engineered to get the most out of the fuel octane they are designed for in terms of CR and spark advance, but vintage engines leave some safety margin. Modern engines with detonation sensors can run at the ragged edge of detonation under most operating conditions including part load with high compression and marginal octane, which is the most thermally efficient operating point. That's why modern engines have higher CRs than 15 years ago even though fuel octane has not increased.

From idle if you disconnect a full time VAC, spark advance and idle revs will drop. Now if you start advancing the distributor, revs will increase from the increase in static timing. The point at which speed/vacuum reaches a maximum and further increases in timing have little or no effect is the optimum idle timing, and the combination of initial and full time vacuum advance should generate near this number for total idle timing. It's an easy experiment to conduct on any engine.

If you keep advancing the distributor significantly beyond this point the engine will eventually start to run rough and eventually stall.

Also remember that detonation significantly increases chamber boundary temperatures, and this causes more heat transfer to the coolant, but it is a result of detonation and detonation is a result of too much spark advance for the operating condition and fuel octane. Significant detonation can increase heat transfer by as much as an order of magnitude, which is why heavy detonation will often lead to rapid failure of exhaust valves and pistons, because they are normally operating close to the temperature where they rapidly lose strength.

So if you take a regular fuel engine and put high octane race gas in it, you can probably advance the timing to the point where it will have little torque/power before it begins to detonate. I'm not sure it would cause overheating, but this is an extreme example.

For a typical engine the sequence of events from too much spark advance is detonation - overheating - component failure, so overheating is a secondary, not primary effect of excess spark advance.

Duke

[Doug Flaten]

Thank you Sir!

That is what I expected. There appears to me much more tendency and range to be in a retarded timing operation than in an overadvanced timing. I seem to pick up something from you everytime you get into these ignition discussions.
Doug