Ignition and Combustion - The Spark Plug Story

[Rednaxs60]

The forum is quiet, so I have had time to do more research on what makes the engine tick, makes the ECU replacement project that I am undertaking easier to understand.

This thread is about the spark plug that has a tale as old as time, how it impacts on the ignition system and engine combustion.

There are many claims about performance improvements just by purchasing what appears to be a high end replacement spark plug. The basic three types of spark plugs are copper, platinum, and iridium. These can be further broken out. Platinum can be single or double platinum, iridium - single or double.

What spark plug do you need. The best source for this information is the owner's manual. The OEM has the engine tuned for a specific spark plug type. Older motorcycles such as the GW 1200/1100/1000 models use a standard copper spark plug and the ignition system is designed to use these specifically. This is not to say that premium platinum or iridium plugs will not work just as well; however, most of us will not notice an engine performance improvement with a well tuned engine.

What affects spark plug longevity? There are not a lot as follows:

Fouling with oil or fuel
Carbon deposits
Erosion and wear
Cracking due to vibration or physical damage

Spark plugs should be replaced according to the maintenance guide in your owner’s manual. Using the recommended OEM recommendation, a new set of plugs every year is a small price to pay for peace of mind riding.

What happens in the engine cylinder? The cylinder goes through the four stages of the four stroke cycle. The stages we are concerned with are the compression and power strokes.

When the intake valve closes and the cylinder stroke changes from the intake to compression stroke, and the piston starts to travel up the cylinder to TDC, the cylinder volume decrease and cylinder pressure increase. When the cylinder reaches the required ignition timing BTDC, the ECU triggers an ignition event and the coil charge is discharged through the spark plug. For example, using an ignition timing of 30 degrees BTDC, the combustion process is started. At an ignition timing of approximately 20 degrees BTDC, the air-fuel mixture starts to burn more rapidly causing the cylinder pressure to be greater than if there was ignition. It has taken 10 degrees of prank position to generate this pressure level, known as the ignition-delay period.

The air-fuel combustion progresses rapidly past this point such that the peak cylinder pressure (PCP) occurs between 10 and 20 degrees ATDC. The combustion process is 100% completed by about 20-25 degrees ATDC. Just like engine power, peaks than drops off. The 1200 GW engine reaches its peak HP at approximately 5500 RPM then levels off and decreases a bit up to max engine RPM.

The venerable spark plug can be expose to cylinder temperatures of up to 5000 degrees F, and pressures from 300 to 2500 PSI depending on the engine.

Engine knock and detonation is when the ignition event is started too early, and the PCP occurs before TDC and results in reduced engine power and performance.

If the ignition event is started too late, engine power and performance suffer as well.

This topic has a lot of information available to correlate and post, and as such will be doing several posts regarding this topic. This is the first.

I will end the first post with my usual disclaimer. I am not the author of this information, just reporting on it. It is also mostly for my edification, but I hope that others who may follow this thread will find it informative and a good read.

Cheers

 

[Rednaxs60]

Thank you for the "like". The Mrs asked if I was writing a book. Mentioned no, but there is a lot to be known about this small innocuous item that is the only readily available removal item on the engine that you can view and it will give indications of what is happening inside the engine cylinder. There is nothing else on the engine that is removable that can provide any sort of engine diagnostic. The recommendation is to learn how to properly read an engine spark plug. You may not be able to change what is happening once you make a determination based on what the spark plug is indicating, but at least you will have a basis for any upgrades you do in the future. Cheers

 

[pidjones]

Many automotive service manuals contain a page of plug photos. They can guide you in "reading" the plugs. I was taught by an old mechanic that would pull an old plug from a box, point to it, and tell me what he read from the plug - too hot, cold, rich, lean, water leak, etc.

 

[Rednaxs60]

What can the spark plug reveal? Ignition timing, spark plug heat range, and air-fuel ratio are the three engine operating parameters that can be revealed.

This is not a black art, but it is more than just taking a spark plug out of the engine.

Reading a spark plug is a tried and true investigative method to determine how the engine is operating. I have posted this information in my other threads on EFI, but will do a Reader’s Digest version here.

Most information regarding this mentions that to do this properly, you should install a new set of plugs, do a road/track trial up to full power, find somewhere to pull over, remove the spark plugs before the plugs can be contaminated form idling, low power operation. Quite the mouthful, but for us street riders, this is not practical.

What you the tuner can do, is to remove the spark plugs immediately on returning to your house, place of residence. The amount of spark plug contamination should be minimal.

Engine timing can be found on the ground electrode. There will be a line on the ground electrode. The closer this line is to the spark plug threads, the more advanced the timing is. The closer this line is to end of the ground electrode, the more retarded the engine timing. This line should be somewhere near the bend in the ground electrode.

Spark plug heat range is based on the colouration of the spark plug threads. A correct spark plug heat range will show colour in the first two to three spark plug threads. More thread discolouration and the spark plug heat range id too “hot”. Fewer threads discoloured and the spark plug is too “cold” for the application.

Air-fuel ratio is indicated in two places on the spark plug. The base of the spark plug threads, the flat spot, should be a golden-brown colour. The second place is at the base of the porcelain insulator. This is called the fire ring. To properly view this, you need to take the plug apart, never to be used again. There should be a small black ring about a ¼ inch in thickness at this location. A thinner fire ring and the engine is operating in a lean fuel condition, a thicker fire ring and the engine is operating in a richer fuel condition.

An on-line search regarding reading spark plugs will result in finding quite a few articles and YouTube videos that provide further information.

As pidjones mentions, there are spark plug guides that can be used, and are a good place to start.

 

[Rednaxs60]

Spark plug gap. What should it be? For most of us, the OEM recommendation is the best spark plug gap. For the adventurous, the spark plug gap can be experimented with. Wider, narrower, all requiring experimentation and a good plan to determine what spark plug gap works best. This is much like tuning the VE (fuel) table. Start at a good compromise. Have sets of spark plugs with the spark plug gap noted.

You could start with the OEM recommendation, generally 0.035", have other sets gapped at 0.040" and 0.045", and smaller at 0.030", 0.025". Note how the engine operates with each set of spark plugs, this will be seat of the pants tuning. Give each set of spark plugs a good chance for success. Operate the engine over a one or two week time frame and assess the engine performance. If the engine performance is less than expected before the end of the trial period, change out the spark plugs, use a different set.

A wide gap creates a longer spark and may lack sufficient oomph to adequately ignite the air-fuel mixture. A smaller gap creates a short intense spark and is regarded as a better option, especially when dealing with richer fuel mixtures.

Spark plug gap orientation. Another tried and true tuning issue. This is where you orient the spark gap so the open section between the ground electrode and centre electrode is oriented such that the spark plug flame is more accessible to the air-fuel mixture. To do this you mark each spark plug so that you know the orientation of the spark plug gap. Install the spark plug to orient this gap with the position of the intake valve. If the spark plug gap requires adjustment, add or remove washers to suit.

 

[julimike54]

Spark plug gap orientation. Another tried and true tuning issue. This is where you orient the spark gap so the open section between the ground electrode and centre electrode is oriented such that the spark plug flame is more accessible to the air-fuel mixture. To do this you mark each spark plug so that you know the orientation of the spark plug gap. Install the spark plug to orient this gap with the position of the intake valve. If the spark plug gap requires adjustment, add or remove washers to suit.

Interesting idea, don't think I've ever checked to see what the orientation is on the bike. I guess I'd have to look at where the intake is....

Some plugs are made with a taper and may not allow for 'washers' to be added, since as I understand they seal on the 'taper'.

 

[Rednaxs60]

Interesting idea, don't think I've ever checked to see what the orientation is on the bike. I guess I'd have to look at where the intake is....

Some plugs are made with a taper and may not allow for 'washers' to be added, since as I understand they seal on the 'taper'.

True enough, the GW spark plugs are not tapered, use a flat washer for sealing.

These little tuning quirks are for the adventurous types, for hard core performance, or racing enthusiasts. For most of us, trying to eke out an HP point or two is not worth the effort, but it can make a difference on the track.

The best way to increase the HP output of an early GW engine is to research and install a turbo system in conjunction with an upgrade to an EFI system, being careful not to blow up the engine from adding too large a boost system. There are some examples of this on the various forums.

Another issue to consider is whether you have access to a dyno for tuning. If you do and have deep pockets, go for it. Most of us do not have this luxury and do most if not all our engine tuning by feeling the engine operation with the "seat of our pants" tuning technology. This is even after an engine road trial is analyzed in a program such as MegaLogViewer (MLV) and parameters/settings are tweaked.

 

[Rednaxs60]

julimike - I'm thinking that you would orient the plug so the gap is at the 12:00 position and pointing up. Haven't looked but that would be good starting point. Need to know where new washers can be bought.