Engine Design
Call Us: 1-949-646-7461

Spark Plugs

Reading Your Spark Plugs Could Save Your Engine


Why? Because they alone tell what is going on inside your engine’s cylinders without any added influences. Yes, there are modern systems that show your air fuel ratio and if any detonation is being measured, but these are affected by other influences that can skew your opinion of a safe running engine. The spark plug will tell you exactly what is happening inside the cylinder. Unfortunately, reading spark plugs is a lost art and today many of the tuners out there rely upon lambda sensors and knock sensors only.


In a normally aspirated engine, to read the fueling in each cylinder requires a sensor on each primary exhaust pipe. This is typically never done and a collective reading is done. Any adjustment is a global one even in EFI systems that offer individual corrections. Same for knock detection. In a turbo charged engine, the lambda sensor is post turbocharger after any secondary combustion takes place.


To read a plug correctly the car must be driven on a chassis dyno, on the road or track to full power or throttled fully on an engine dyno. New plugs must be fitted because previous running can skew your readings. It should be noted that a safe ignition timing is used even if it has to be retarded some. Tune the engine for what appears to be safe, best power and then fit some brand-new plugs. Run the engine up to its max RPM and cut the ignition clean. Do not idle the engine. If you are on the road or track, fit another plug or do the cut before the pit road entry so your cab coast in under no power. The weather can change the plug readings in a small way as well, so take into consideration the weather. If it is really hot the engine will run hot and the plug will show this too.


When removing the plugs it is always a good idea to blow out the plug recesses in the cylinder first so you clear dirt and debris. This junk can drop into the cylinder if you are not careful. Also, a good inspection light with some sort of magnification is required. Another tip is to have a rubber tube for installation of the plugs. This way you will never cross thread the plug. Do not use anti seize of any sort on the plugs. This will skew your readings, act as a heat barrier and can coat the porcelain area of the plug. If you have to, use a little light engine oil , EPL grease or similar. Some plugs like NGK, come with a special coating already that require no lubrication at all.


If the base timing is safe, the first reading is for the correct fueling. The porcelain around the center electrode is what you will be reading for this. You want to see some color around the bottom of the porcelain near the base of the plug. If the fueling is correct you will see a thin line of color at the base of the porcelain. As the engine gets richer this will move up towards the firing end. Any color at the firing end is not indicative of best fueling, so do not get fooled by what you see up at the firing end. Many think a good initial check is the color of the porcelain with a good tan color they assume that the engine is running in a safe region. However, different fuels today can skew this too.


To check for best ignition timing the ground strap is what you look at. When you add timing or advance the timing you will typically generate more power and therefore more cylinder temperature. This temperature will show up on the most exposed part of the spark plug, The ground strap. It is quite simple really when you think about it. As the temperature rises, the ground strap will absorb more heat and then that heat gets transferred back into the plug body and hopefully into the cylinder and to the coolant. As the ground strap heats up more, more discoloration will show and travel towards the point where the ground strap is welded to the body of the plug. If the color is all the way to the body of the plug you need to retard the timing if you are doing any sort of circuit racing. If you are drag racing where the engine gets short bursts, you can sometimes run the plug that hot.


The last part of reading plugs is to check if the heat range is suitable for your use. Heat range has nothing to do with the engines performance unless you are running so hot the plug fails. It has nothing to do with the temperature of the engine either. It has to do with the plugs ability to remove heat from the plug into the head and finally into the coolant. Plugs with more exposed center electrodes are generally on the hotter side of the heat range, conversely plugs with the center electrode more covered run cooler. Firing ends also play a part here. You can have a plug with a center electrode covered more but with an extended firing tip. This plug will run hotter than the same plug construction with a recess firing end. So, how do you tell what plug to use. First pick the firing end that will work in your engine. If you have a piston clearance issue, a firing end more recessed will be required. If the engine is turbocharged, typically recessed firing ends are required as well. Engines that have a response or laziness to them may require an extended firing end to place the flame kennel more into the chamber.


Start off cold and work your way up the heat range until you see black specs on the porcelain. These black specs are caused by the porcelain boiling when the plug is too hot. You can look down inside where the porcelain extends out of the body of the plug. Often you can see signs of the porcelain boiling down there. You can also look at the inside of the body of the plug. Often discoloration can be seen there too. Look at the center electrode for signs of erosion. This can be evidence of detonation, and also the heat range is too hot. The precious metal electrodes make this almost impossible to read, especially the fine wire firing ends. Always look for signs of piston material stuck onto the porcelain. This shows damage has already occurred and that your initial settings were too lean and or too advanced.


You may have to repeat the procedures serval times to get the plugs to read the way to want them to. What you are actually doing is re tuning your engine.


In air-cooled Porsche heads where the spark plug is centered over at the side of the valves, the plug can be affected by the incoming air fuel mixture. The mixture is turned around either side of the valve stem and directed towards each valve outer edge. One side of the directed mixture will go towards the spark plug and can change the reading some. Twin plug these engines and the same will happen on the other side of the valve to the 2nd spark plug. This requires you to take this into consideration when reading the plugs. They may look on the rich side when in fact the cylinder is running lean. Be careful here. You may also see the plugs different from one cylinder to another. The Intake plenum and air flow through it can be affecting this. So too could the condition of the cylinder. If you have some cylinders that have poor sealing and the pressures built up are lower, this will affect the plug reading. Same with the exhaust system. Maybe one pipe is scavenging the cylinder better than another. In these cases tune for the worst and accept the compromise on the others. If you have an EFI system that can change the fueling and Ignition timing per cylinder, then tune to each plug.


Gapping is another issue that is often talked about. In most plugs today, you can buy plugs that are already gapped to different settings. For example, NGK BKR7EIX-11 is a plug with their 7 heat range and the gap is wide at 1.1mm. Sometimes gapping is necessary especially in engines that are boosted. First, try to pull the firing end back into the plug before re gapping down. If you have to gap, do it carefully and always measure the gap with wire gauges not flat feeler types.


Spark plugs come in many different shapes and sizes. Always fit the right plug for the cylinder depth or reach as it is known. Most air cooled Porsches have a 14.0mm thread by ¾”. They come with different hex sizes ranging from 5/8” to 13/16”. There are non-resistor plugs and resistor plugs. Today, non-resistor plugs are becoming hard to find, but the resistors can be removed. Precious metal electrodes are common now. Their wear factor is far better than  the older style plugs. Today Iridium is very common for its ability to withstand detonation, high resistance against wear and offer long plug life. Thin wire electrodes require less energy to fire and with most modern Ignition systems being Inductive, the condition of the battery is so important to the level of energy released at the plug. Fouling at lower battery voltages is less with fine wire plugs. They also open up the area around the electrode for the gas to ignite which increases the flame kernel.


What brand of plug do we recommend?  We do not but we do recommend you understand the nomenclature of each manufacture. Especially the heat ranges, as some are backwards to others. NGK for instance, the higher the heat range number the cooler the plug.


We do recommend using a CDI ignition system over Inductive in an air- cooled Porsche. These engines have large combustion chambers, and sometimes a dished piston along with low static compression ratios. Inductive Ignition systems can only produce the amount of energy the coil can generate within its windings. It is also affected by battery voltage. Lower the battery voltage and the output will be lower as well. We highly recommend CDI Ignition as it will release far greater ignition energy, make the engine run more efficiently and will help lower the tendency for knock as the flame travel will be faster and have more temperature helping ignite the end gases. Check under our parts area for the Ignition systems we offer.


Finally, this a cost management, risk management decision. I am not disputing the power of AFR measurement devices or knock sensing devices, but suggesting that these can be influenced by outside factors and that reading the spark plugs condition will tell you exactly what is happening inside the cylinder. Today we can measure the pressure inside the cylinder with sensors, the temperature of the exhaust gases, the position of the piston in the cylinder amongst many other parameters. In most cases this technology is so cost prohibitive that you are left with the more common ways to try to estimate and understand the condition of the combustion event. Miss this and you are into rebuilding the engine with many expensive parts needing replacing. Reading the spark plugs is a fail safe way to know for sure that your engine and investment is safe and should give many hours of reliability.