An easy spark question....

[Michael_D]

I'm new to this "tune a car by laptop" stuff. I'm old school. Filing points and keeping a pocket full of power valves in the pits used to be pretty routine for me...

I'm dinking around with the main spark table on my personal rig, and it still shocks me every time I see the OEM table. 40 degrees timing in the idle and low rpm / low lode cells??? How can this be? I used to always set SBC initial at 8, and total to 32. Then tweak a little bit as needed. I get the reduced high load spark with these new rigs, but why so much advance in the lower rpm range? How can that be more efficient? The piston is just rolling out of bed at 800 rpm, so why so much advance? I'm actually pretty amazed it doesn't knock like crazy with that much advance.

Out of curiosity, I started moving low load values lower than I had "dared" to before, thinking I might raise EGTs too far and melt an exhaust seat, but I'm pretty sure the engine likes it. It seams to run a bit smoother. The furthest I had lowered it to before was 25 deg. This didn't really seam to help much under about 2200 rpm, so I left it at 30 (still ten less that stock). Well the other day I just said screw it, and moved it to 22 deg. It made a difference.

Before I go moving timing any lower, I figured it would be best to ask for input top see if I'm overlooking something. I'm looking for more of a theoretical explanation with respect to these throttle bodied/common plenum multi coil, EFI rigs, than a "you should do this" response.

[10_SS]

you can give almost any stock single cam low compression big engine that much advance and it will most likely be happy. But before knock sensors came in to play... it was very dangerous so reducing timing everywhere was the key to longevity. That much timing gives you noticeably more power, torque, and uses less fuel. Back in the day I would personally take every car I had with a distributor and advance the timing a bunch... and leave it. They all had more power, ran great but I had no idea if it was knocking... They sometimes didnt start good, but that's the mechanical distributors fault.. not the advanced timing (ECU's now have separate crank timing )

Seems you and I have opposite tuning techniques... I advance everything, everywhere if I can, and you reduce timing. I personally try to advance idle timing on everything I have, sometimes up to 15 degrees... however I think they keep it so reduced at idle to keep a hot exhaust to help pass emissions.. and not foul plugs. With the advanced timing, it's more responsive, quieter, makes more power, uses less fuel, etc..

The only reason the engine feels smoother is because there is basically more load on the engine... when engines are loaded, they run smoother. I would leave the timing up where it was as long as you don't have knock.

[SultanHassanMasTuning]

dont look at the main spark for idle, there is a idle spark table. which stock is around 10-13

low load high timing is common

[daniel76309]

If you are looking at your main spark table, I believe the above is correct--I don't think it has any effect at idle.

I am also new to this but I will comment, and I also have a related question.

In my experiences with older cars, advancing spark at idle helps. I always found that connecting the vacuum advance directly to manifold vacuum (rather than "ported" vacuum) would probably be 30-something degrees of advance at idle, and it would run much better than at the 12 or so degrees of initial advance. By running better, I mean higher RPM, or less throttle opening required for any given RPM, with better vacuum. I would think the same would be true for modern engines.

This brings me to something I don't totally understand... With my stock ECM calibration, I don't recall the exact limits but the idle spark table goes way beyond idle in terms of RPM and cyliinder air, e.g. what is the point of an idle spark table going to 6000 RPM? Anyway, the stock table was something like 14-18 degrees in the "normal idle zone", with MORE spark for higher RPMs, and LESS for lower RPMs. However, as I understand it, idle ADAPTIVE spark control is supposed to pull timing in an overspeed condition, so it seems to me that the two tables would be "fighting" each other. For example, if target idle is set at 850 and actual RPM is 1000, the base idle table would add to timing advance, but the adaptive table would retard in order to reduce idle speed, so they would to an extent tend to cancel each other.

I have experimented with this on my car (LS376-480 crate engine) and it seems to like about 30 degrees at idle. So I set the WHOLE base idle table to 30, and I also reduced the "aggressiveness" of the adaptive spark control. With this it idles more smoothly and with more vacuum, and I haven't noticed any ill effects from setting the whole table to 30 degrees. (I think that as soon as the throttle is opened a small amount, the ECM refers to the base table rather than the idle table, so the idle table does not have any effect.) If there is a down-side to this, someone please comment. Otherwise, to the OP, I would recommend checking what is in your idle table, and if it is in fact something in the teens, try adding to it and see if it helps.

[Michael_D]

Ahhhwwwsshiittt…. I spaced out vacuum advance. Initial was for cranking/starting and vacuum for idle, coast down and throttle transients. Mechanical is for rpm ramp up. Now I recall seeing a 30 deg idle advance farly often with vacuum connected.

I had to review this old document: It’s a good read.

Getting the Ignition Timing Right
Spark advance numbers.
by Julian Edgar

People always talk about air/fuel ratios, but setting the correct ignition timing on programmable management is at least as important to getting good performance, economy and responsiveness.

When to Fire
The period between the spark firing and the complete combustion of the fuel/air mix is very short - on average only about 2 milliseconds. Ignition of the fuel/air mix must take place sufficiently early for the peak pressure caused by the combustion to occur just as the piston has passed Top Dead Centre, and so is on its way down the cylinder bore. If the ignition occurs a little too early, the piston will be slowed in its upward movement, and if it occurs too late then the piston will already be moving downwards, so reducing the work done on it. If the spark occurs much too early, the ignition pressure wave can ignite the mixture in various parts of the combustion chamber, causing detonation.
If the composition of the mixture were constant (and it isn't!), the elapsed time between ignition and full combustion would remain about the same at all rpm. So, if the ignition advance angle were set at a fixed angle before Top Dead Centre, then, as the engine speed increased, combustion would be shifted further and further into the power stroke. This is because the faster moving piston would be further down the bore by the time combustion actually occurred. To prevent this, the ignition advance must increase as engine speed rises.

In addition to engine speed, the other major factor affecting the amount of advance required is the engine load. At light loads (ie when lean mixtures are used) the speed of combustion is slowed and so more ignition advance is needed.

But unfortunately not only does engine speed and load determine the best timing for the combustion of the mixture, but the following factors are also relevant:

the design and size of the combustion chamber
cam timing, especially in variable valve timed engines
the position of the ignition spark(s) in the chamber
the fuel characteristics
the emissions levels required
engine coolant and intake air temperature
the safety margin required before detonation occurs

The emissions of an engine will be affected by the ignition timing that is used, in addition to the air/fuel ratio. Oxides of nitrogen increase as ignition timing is advanced. Running light-load advances of 40 or more degrees is common, giving good responsiveness off load, but if emissions standards need to be met, this advance may have to be reduced. On the other hand, the emission of carbon monoxide (CO) is affected very little by ignition timing, being much more influenced by the air/fuel ratio. At stoichiometric and lean air/fuel ratios, increasing the ignition timing can reduce specific fuel consumption substantially. Finally, the emissions of hydrocarbons at stoichiometric and rich air/fuel ratios increase with advanced timing, but timing has little influence at very lean air/fuel ratios such as 19:1.

It's impossible to ascertain the best ignition timing by juggling all these interrelating factors on paper. Instead, making real-time dyno changes to the ignition timing while using an exhaust gas analyser or air/fuel ratio meter and a means of detecting knock is the only practical way of seeing how the ignition timing being used influences emissions, power and fuel economy.

[Michael_D]

Cont....

1. Cranking and Idle
Some programmable engine management systems have a default cranking advance of 15 degrees, a value about midway through the range of appropriate cranking advances. Smaller engines with faster cranking speeds need a greater ignition advance (up to 20 degrees), while slower cranking speeds of a high compression engine will require less advance (down to 10 degrees). The compression ratio of the engine will also determine the likelihood of kickback on starting. Engines with a low static compression ratio of 8:1 will accept an ignition advance of anything from 0-20 degrees without kick-back. A 10:1 compression ratio will reduce this to 15 degrees, 11:1 to around 10-12 degrees, while race engines using very high compression ratios of 12-13:1 can sometimes tolerate no cranking ignition advance at all.

Most engines will idle happily with an ignition advance of 15 - 32 degrees. This is a very wide range - some engines will certainly not be happy at 32 degrees and others won't be at 15 degrees! An overly high amount of ignition advance for a given engine will result in lumpiness at idle, excessive hydrocarbon emissions and sometimes exhaust popping, while too little advance will also cause lumpiness. If the engine runs closed loop fuel control at idle, too much idle timing advance can disrupt the oxygen sensor reading, causing the self-learning process to overly enrich the idle mixture. Setting the optimal ignition timing can therefore best be done by trial and error variations.

Timing that is more advanced at slightly lower engine speeds than idle is sometimes used to help stabilise idle. This is effective because, when the engine starts to slow down, the greater ignition advance causes the engine to produce more torque, so increasing engine speed. Many factory management systems use ignition timing as a major element in controlling idle smoothness, with an increase or decrease in rpm at idle responded to by a change in timing advance.

2. Cruise
At light loads - as are used in normal everyday cruise conditions - an ignition advance of 40 degrees or more will improve responsiveness and economy. This advance can be used successfully on many engines - even those with an 11:1 compression ratio, if they are being run on high octane fuel. One factor limiting the cruise ignition advance that can be used is the maximum ignition timing attack rate provided by the ECU - that is, how fast the timing can change. If very advanced timing is being used with light loads and the attack rate is not high, there may be slight detonation when the engine load suddenly increases.

The hotter the camshaft(s), the less advance that will be able to be used in light load conditions (the limiting factor being driveability rather than detonation in this case), however timing in the range of 35-40 degrees is still usually used. Engines with good combustion chamber design will be able to run up to 45 degrees in these conditions. Fuel economy and engine responsiveness are both very much affected by light load ignition timing.

3. High Load
The torque output of a given engine is proportional to average cylinder pressures, so the full throttle ignition timing advance that is used should relate to the torque curve rather than power curve. The maximum ignition timing that can be used at peak torque is usually limited by the occurrence of detonation. A detonation limit is always the case in forced aspirated engines, but not always the case in naturally aspirated engines. As an example of the latter, one Porsche flat six developed best power with a maximum advance of 8 degrees, even though the engine did not detonate at even 27 degrees of advance! A Mercedes V8 engine was able to run 38 degrees at high rpm, peak load without audible detonation. However, best results came from a full load advance of 28 degrees.

On a modified engine having increased compression and hot cams, a peak torque advance of 28 - 36 degrees can often be used. In a factory forced induction engine using a little more boost than standard, the peak torque timing will be around 18-22 degrees, while in a naturally aspirated engine converted to forced induction without internal modifications, timing should be well back at about 10 degrees.

Because, as already indicated, most forced aspiration engines and many naturally aspirated engines develop best performance when the ignition timing is advanced close to the point of detonation, great care should be taken when setting the full-load ignition timing. To assess the maximum ignition advance that can be safely run at a given rpm and load, a dyno is a very useful tool. When the dyno is used in this manner, the engine is held under load at a single rpm and the ignition timing is slowly advanced.

If the rate of power increase tapers off to zero (or in fact power starts to decrease), the timing should not be advanced further. If the power development of the engine starts to fluctuate rapidly, the timing advance is excessive. These power fluctuations can be clearly seen when a dyno is used in a steady state, expanded power scale, bar graph mode. Note that the power fluctuations occur well before detonation is audible. The ignition timing should be retarded by 2-4 degrees from the point of power fluctuations.

The use of amplified earphones connected to a microphone clipped to the block is also a very good way of sensing when detonation is about to occur - the sound of the engine changes in a characteristic way even before detonation starts. But perhaps the best approach to detecting when an engine is detonating is to use equipment to read out the real-time output of the engine knock sensor output, or of any automatic ignition timing retard occurring as a result of knock sensor activity.

Optimal ignition timing is that which gives a lack of detonation, the lowest exhaust gas temperatures, and maximum torque.

From peak torque through to peak power, a modified naturally aspirated engine should increase in ignition advance to 36-40 degrees, a boosted factory turbo car should be running around 25-28 degrees, while an aftermarket, non-decompressed forced induction engine should be conservatively timed at around 15 degrees.

If the engine uses reliable knock sensing and the ignition timing can be retarded quickly at the onset of detonation (and then re-introduced only slowly), more advanced timing than these figures can be used at high rpm. An intake air temperature correction chart that quickly pulls off timing advance with increased air intake temperatures can also allow the main table's ignition timing to be fairly advanced. For example, with an intake air temp of 120 degrees C, the timing can be retarded by 12-15 degrees, so providing an acceptable level of safety while still allowing good cool weather and short-burst performance. The importance of using a programmable ECU that has tables for the intake air temperature correction of ignition timing can be seen from this example.

4. Acceleration
The ignition timing used during acceleration transients should have an attack rate that is quick enough to keep up with the timing requirements. This parameter is often specified as the maximum number of degrees per second change that is permitted. One source suggests that an attack rate as high as 650 degrees a second may be needed in some high performance engines. If the attack rate is not sufficiently high, mid-range detonation can occur in the transition from light load cruise (perhaps with 45 degrees of advance) to full throttle at peak torque (perhaps requiring only 15 degrees of advance). However, if the attack rate is set too high, slight changes in throttle will cause rapid, undamped jumps in timing which can cause minor detonation. This is especially the case at low engine revs - at higher rpm, the attack rate can also be higher.

5. Over-run
On deceleration (with injector cut-off working) most factory cars run retarded timing, such as 10-12 degrees. However, in modified cars this has been found at times to cause an exhaust burble, and if this is unwanted, more advanced timing (20 - 26 degrees) can be used. The amount of deceleration timing advance that is used may affect the strength of engine braking that is available.