127 | Spark Plugs Essential Knowledge
Spark plugs are often an item that doesn’t receive a lot of thought in the specifications and assembly of a modified engine package. In this webinar we’ll discuss what you need to know about spark plug heat range, what effect the sparkplug will have on your tuning, how to decide on plug gap, and what you should be looking for when your examining spark plugs.
It's Andre from the High Performance. Welcome to this Webinar. We're going to be discussing everything that you need to know or should be knowing about spark plugs for those of you getting involved in EFI tuning. The spark plug is one of those components that I think is quite often overlooked in the larger scheme of engine tuning. Particularly when we're talking about run-of-the-mill road cars, maybe something that is a mild power increase over stock, and we are particularly in a situation where we're dyno tuning for a living.
It can be very easy to roll the car into the dyno bay, put it on the dyno, tune the car, and send it out the door without giving a second thought to the spark plugs that are fitted to it, the condition of those plugs, or anything else. Really often, the only time we start thinking a little bit more about the spark plugs fitted to the engine is when we start suffering from problems, either an ignition misfire is a good place where we might have a problem that gets us sort of looking or thinking about the spark plugs. The other issue is obviously if we end up damaging or breaking a spark plug, and that's going to, again, require those plugs to be removed. So, there is a little bit of knowledge here that we need to know, need to understand, in order to sit us in good steed when it comes to EFI tuning. What we're going to do is break this Webinar up into two sections, really.
First of all we're going to talk about spark plugs in general terms, and basically I'm gonna list the things that you need to be considering when it comes to the spark plug. Now, I'm not a spark plug engineer. My knowledge that I'm going to provide you today really comes from 15 years of just what I've learned, what I've needed to know, and what I've picked up along the way, so we're going to be focusing on the important things that you really need to understand rather than getting too sort-of stuck in the detail of how the spark plug is constructed and every sort of single component that goes into that spark plug. Now, following on from that, we're going to cover off in brief terms some of the aspects to do with the spark plug reading. We are going to cover that shortly, but I'll just say right at the start that there's a lot of misunderstanding around spark plug reading, and probably misunderstanding around what is expected or required in this regard when we're dealing with an EFI system.
So, I will get into that after we've learned what we need to know about the spark plug. So, let's start by dealing with what the outcome of a problem with the spark plugs can be, or how this can manifest or show up when we've got a car on the dyno. So, the first one would be an ignition misfire. So, this is a really common one. The result of a spark plug that may be gaped too wide or of the wrong plug construction, where the spark plug gap is projected way out into the combustion chamber.
So, this is probably the most common scenario we're going to find when it comes to minor problems with our spark plugs. What we'll tend to find is that the car will tune up perfectly normally, and the idle, light throttle cruise, moderate load, and particularly with turbo charged engines, what we're going to find is that as we increase the load on the engine, as we increase the boost pressure, we're going to find that it becomes difficult if not impossible to ignite the fuel-air charge. So, this is where the problem sort-of manifests as an ignition breakdown. So, this is going to require us to get our hands dirty, physically remove the spark plugs, and find out what's going on. Now, the other aspects are where we might have a spark plug fitted to an engine that probably isn't suited to that application, and a really common scenario here is where we've got a factory specification spark plug fitted to an engine.
I'm gonna use a turbo charged example here, although the same can be said for naturally aspirated engines, just everything tends to get amplified in a turbo charged application. So, let's say we've got a factory production engine. Maybe it's a Mitsubishi 4G63, and it's got its factory spark plug fitted to the engine. Chances are, it's going to have a heat range somewhere around about six, and it's probably going to have a plug gap somewhere around about 40 to 50,000ths of an inch. So, somewhere just a bit over one millimeter.
Now, that's obviously totally fine for a standard production engine producing somewhere around about 15 to 18 psi of boost and probably somewhere in the region of about, maybe, let's say 180 to 200 kilowatts at the wheels. Now, what often happens is that the owner of the car gets carried away. They modify the engine. Maybe they fit better pistons and conrods. More often than not, they're going to go with a larger turbo charger, a different exhaust manifold, and all of those bolt on supporting modifications, including the fuel system upgrade, completely forgetting about the poor spark plugs.
So, the car gets brought in. It gets put on the dyno, and one of the problems we're going to find is that when that engine is tuned to what its capable of, we're going to potentially be running a lot more boost. We're going to have much higher cylinder pressure as a result of that additional boost. The engine's obviously going to be producing more power. That's the desired result of all of these modifications, and the other upshot is, it's going to be producing a lot more temperature, a lot more heat inside the combustion chamber.
So, these are all things that a factory spark plug is simply not designed to deal with. So, if we're lucky, how this factory spark plug is going to show a problem is simply with a misfire, as we've already talked about. However, a more serious problem can come from the heat range of the spark plug being too hot to handle the increased combustion chamber temperatures that that plug is now being exposed to, running higher boost pressure and the engine physically producing more power and more heat. So, what this can result in is physical damage to the spark plug. The spark plug may end up melting.
I've got a spark plug here. I'm going to talk in a little bit more detail about the signs we should be looking for in a spark plug, but one of the key elements that we're going to see with a plug that's way too hot is the ground strap can end up being melted away. We'll actually physically see that it will be eroded. So, it's physically just being melted. It cannot get rid of the heat out of that ground electrode fast enough.
Now, this can cause knock-on effects as well. If we get to a point where that spark plug is glowing red, the ground electrode is physically glowing red, this can end up becoming an ignition source inside the engine, which can lead to pre-ignition. Pre-ignition is quite dramatically different to detonation, which I've talked about often in these Webinars. Pre-ignition is a situation where the fuel-air charge is ignited prior to the spark event, and this actually can be far more damaging than detonation. So, the fuel-air charge essentially gets ignited by some glowing ember or hot element inside the combustion chamber prior to the spark event occurring.
So, that can be a problem there. So, these are the sorts of issues that we're likely to see if we have a spark plug that has a heat range that is too hot. So, essentially, a production specification spark plug fitted to a heavily modified engine, making perhaps double the factory power. These are the sorts of problems that we might have. On the other side of the coin, though, if we've got a spark plug that is too cold, perhaps, in the heat range, then once the engine's up and running, we're probably not going to notice this too dramatically in terms of the engine's power production or how the engine operates, but an important aspect of that heat range is that the spark plug needs to be designed with a suitable heat range for our engine, and if the heat range is too cold, it won't allow the deposits to be burned off that spark plug, and over time, what this leads to is that the spark plug may become fouled, and this results in difficulties with getting the engine to cold start.
So, these are some of the problems that we're likely to see and how they may manifest out themselves when we've got the car in front of us on the dyno. So, let's choose. Let's look at what we need to understand in order to choose the correct spark plug, if we are speccing a spark plug for a modified engine. And one of the first things we need to understand is that spark plugs come in a huge range of different fitments, so I've got two here, and you can see that they're completely different. First of all, the thread diameter on the two plugs is completely different, as is the reach of the two plugs.
So, fortunately, in those two examples, we couldn't fit the incorrect plug to the wrong engine. It just simply won't go in. However, there are applications where there may be multiple plugs that will actually screw into the cylinder head, but particularly if you end up with the wrong reach of spark plug fitted to the cylinder head, obviously that's a recipe for disaster, and you're asking for a hole to be punched through the top of your piston next time you try and start the engine. So, the very first aspect of choosing the correct spark plug is ensuring that the plug will physically fit. So, this includes the thread diameter, the thread pitch, and also the reach of the plug.
We also need to consider the size of the hex that we actually use when we're installing the spark plugs into the engine, and this will depend on, again, the cylinder head design, particularly with a lot of the late model Japanese engines. We're getting smaller and smaller engines with very, very tight spaces to work around, and some of the large hex spark plugs simply won't fit into these sorts of engines. So, those are the basics there, making sure that the physical aspects of the spark plug are suitable for the engine that we want to fit it to. The next thing we're going to talk about, and this is probably one of the more advanced aspects. We're going to talk about this in a little bit more detail later on as well, is making sure that you've chosen a heat range that is suitable for the application, and this really sort-of covers back to what I just talked about, a heat range spark plug that's suitable for a production engine, making maybe 200 kilowatts at the wheels, is probably not going to be a very good selection for a heavily modified engine producing maybe 300 to 400 kilowatts at the wheels.
So, what do we actually mean when we're talking about the heat range? It's a term that a lot of people use without really understanding what that terminology means. So, again, we'll just grab one of our spark plugs as an example here, and what we're talking about with the heat range is how the path, if you like, for the heat to get out of that ground electrode and transferred back into the body of the spark plug. So, this is a particularly cold plug. This is an eight on the NGK heat range, and we can see that the path from the ground electrode into the body of the spark plug is quite short. So, this means that it can dissipate that heat back into the body of the spark plug very, very easily.
We also see that the spark plug gap is kind-of almost recessed down into the body of the plug, and this is referred to, we'll talk about this again in more detail. This is referred to as a non-projected spark plug. So, the flip side of this, if we look at a hotter plug, this is, we can see, the electrode is much longer. It's projected further out into the combustion chamber, and the path for heat transfer through that ground electrode down into the body of the spark plug is much longer. So, what this really relates to is how quickly it can dissipate the combustion chamber temperature that it's exposed to.
So, if our heat range is too hot, and our ground electrode is too far out into the combustion chamber, exposed to too much heat, and that path for the heat transfer is too long, what can happen is that that ground electrode becomes melted. It physically starts to glow and can be eroded or melted away. And the flip side of that is what I also talked about just recently, with if a heat range is too cold, we're not going to be able to burn off the deposits or contaminants that are likely to gather on the electrode and this results in problems with the spark plug fouling over time. So, it is important to choose our heat range based on the sort-of power level that we're expecting. So, how do we do this? What do we do if we don't know any better? Some of this knowledge really does come from experience, physically trying different heat range with different spark plugs in a particular application, and seeing what works, seeing what the results are, what they look like when we pull them out.
But obviously, if you don't have that experience, where do you get that? Trial and error. So, the best place to start is by using a factory heat range spark plug or factory designed spark plug as a bit of a guide. This is going to give you the factory heat range, and, just as importantly, it covers off the physical fitment aspects that I've just briefly mentioned before. And then, if I was building a car that was just mildly modified, so let's say a naturally aspirated engine, where we were perhaps tuning it, or we were perhaps fitting an exhaust system in an intake. So, I call these mild modifications.
I would in that case be quite confident running with either the factory heat range spark plug or moving one heat range colder. The bigger issue is, we were talking about very, very, very heavily modified naturally aspirated engines. We were talking about very large cam profiles, Where we’re talking about much higher compression ratios. Engines modified so that the volumetric efficiency is dramatically increased. All of these add up to increased combustion chamber temperature, and in that case, I would probably be inclined with a naturally aspirated engine to move two heat ranges colder than stock and see how that works in the application.
Turbo charged engines are a little bit more serious, because we obviously have the ability to make a lot more power from these engines than what they were originally designed for. Again, for a mildly modified application where maybe we are running the factory turbo charger at a high boost pressure, or we have gone to a slightly larger turbo charger, and we're running a little bit more boost and producing maybe somewhere in the region of, maybe, another 50 to 75% power, I again would be looking in the range of about two heat ranges colder. As we move into some of the more specialized applications, where we're running on alcohol based fuels or E85, we're running very, very high boost pressures for specific applications such as drag racing. It's not uncommon to end up moving three or four heat ranges colder or even further. Quite often, with the drag cars I was involved with, we would be running around the 10 on the NGK heat range scale.
This brings with it its own set of problems because often when we're running heat ranges that cold, it can be beneficial to actually use a hotter spark plug to get the engine running, warm it up, get it up to temperature, before swapping to a colder heat range plug for actual competition. The other thing that's really important to understand here when we're talking about heat ranges is knowing how the heat range numbers work for the type of spark plug or the manufacturer's spark plug you're dealing with. In particular, I probably seem to use NGK more than most other plugs, and another common type of plug, manufacturer spark plug, is Champion. The heat ranges, just to confuse things, work in the opposite way. So, with NGK, a higher number is a colder spark plug, so a 10 is dramatically colder than a six, for example.
On the Champion range, it works the opposite way. So, their lower numbers are colder. Very important to understand, otherwise you could end up fitting the wrong type of plug or going the opposite way to what you think you're going. Now we're going to talk a little bit more in the reading spark plug section of this Webinar about how we can tell when our heat range is in the right range. Next we're going to move on and talk about our spark plug construction, and by this, what I'm talking about is the core or electrode element.
Obviously, we've got the older style of spark plugs which used to be very common, which run a copper style core. We're now seeing more and more of the rare metal spark plugs, such as platinum and iridium, etc. Now, I've had good results with both spark plugs, so I can't say that a platinum or iridium plug is a hands down winner. There is one theory that with the iridium or platinum material, some of these more specialized materials, they are more resistant to heat, so what this means is that they can be made a lot thinner than a conventional spark plug electrode, and this can aid in the spark jumping across that plug gap. So, this can result in a better spark.
I've personally never really seen any benefit jumping from a conventional spark plug to a platinum or iridium in that respect. I haven't found that to be a sole aspect that has fixed a misfire problem for me. One of the bigger considerations here is that the life expectancy of the platinum and iridium spark plugs is much longer than a conventional spark plug, and are also much, much more expensive, perhaps four or five times more expensive. What we quite often find is that in some of the more modern engines that are coming out, where we've got intake manifolds that are running across the cylinder heads, making spark plug replacement quite tricky. Often these will be specified from the factory to use an iridium or platinum spark plug, so we get a longer service life between changing spark plugs.
That's one consideration. Generally, I find in the performance world, probably a smaller consideration, since we are likely to be changing spark plugs more frequently than in a standard road application. Next I'm going to talk about the tip styles. So, this, to a degree, does relate to the heat range of the spark plug. What we tend to find is that as we go colder in our spark plug, we tend to naturally find that the tip starts to retract back into the body of the spark plug, because we need that shorter ground electrode path to get rid of heat.
Now, again, I'll just hold these two spark plugs up, because these are a great example of two dramatically different tip styles. Now, on the right hand side, we have what I refer to as a projected tip, and on the left, it'll be the other way around for you guys, sorry. We've got the non-projected tip. Now, what we quite often find is that in a production application, the drivers behind the plug selection are very different. What we may be considering, and obviously for an OEM manufacturer, what they are interested in, is getting good fuel economy and good emissions, and this requires the ability to really, consistently light off the fuel-air charge, particularly under idle and cruise conditions.
That's really important, and a projected tip plug works very well for that. It gets the spark right out into the combustion chamber, and it gets it right into the middle of that fuel-air charge, so this helps make sure that the fuel-air charge does ignite correctly. However, when we start talking about a performance application particularly on the high load, what we've done now is, we've increased dramatically the pressure inside the cylinder, and this makes it much harder for the spark to jump the spark plug gap. We're going to talk about the plug gap shortly, so don't let's not get ahead of ourselves. What this does have the effect of doing, though, is we find that a projected tip plug does tend to be more prone under those instances to suffer from a misfire than if we're talking about a non-projected plug tip.
So, what I mean by this is, if we've got two plugs that are, for all intents and purposes the same other than one is a projected style and one is non-projected but the same plug gap, we're likely to have more problems with misfires with the projected tip. So, again, when we are going to a heavily modified engine, it pays to run a non-projected plug, but as I've said, this often goes hand-in-hand with the colder heat range that we'll also want to be running. Another type of plug that we may see in some applications is a surface discharge plug, where essentially there is no ground strap so to speak, and it's a surface discharge straight to the body of the spark plug. Right now we'll talk about plug gap, and this is probably one of the most critical aspects that you need to worry about with spark plugs. This has been, without doubt, the biggest cause of problems when I've had cars on the dyno, is an excessive plug gap that results in misfires once we start increasing the load or increasing the boost on the engine.
Now, there's two schools of thought here. The first is that what we want to do is run the largest plug gap that we can get away with, and that's going to promote a longer spark, which can promote more efficient or more thorough combustion, essentially. That spark is going to ignite the fuel-air charge more completely. The other school of thought is that we want to minimize our plug gap to make it easier for the spark to jump across that gap and make sure that we get combustion occurring. Now, particularly again with high boost applications or high compression naturally aspirated engines, under wide open throttle conditions, the cylinder pressure becomes immense, and this makes it much harder for the spark to jump across that gap, so what we can get away with at idle and cruise is completely different to what we can get away with under full throttle, wide open throttle, high boost operating conditions.
The cylinder pressures are dramatically different, and it's under that high load operating condition that it makes it very, very difficult for the spark to jump across that plug gap, and this is made obviously worse if we're running quite a wide plug gap. Now, I said there's two schools of thought on this. There's a lot of people who will say that if we run a wider plug gap, we will pick up power. I know there are studies that have shown this. I've tried this myself on numerous turbo charged engines, and I've never been able to measure anything worthwhile, but what I can definitely tell you is that if your plug gap is too wide, and you're suffering from an ignition breakdown or ignition misfire under high boost operation, wide open throttle operation, that is definitely going to lose you a lot of power.
It's going to be frustrating. It's going to affect the operation of the engine and the operation of the car. So, if we had to weigh up these two situations, I'm definitely going to opt for a tighter plug gap every time. This is going to ensure that we've got the best chance possible of igniting that fuel-air charge in the wide open throttle operations. Now, another aspect of this as well, is we quite often see factory spark plugs, specified with a plug gap of something like 1.1 millimeters, which again is quite a large plug gap, and my guess here is that the OEM manufacturers are doing so because for the factory engine, they know they can get away with that plug gap under the wide open throttle operating conditions, so they're going to get good spark under those conditions, but the wider plug gap can help, can potentially offer more complete combustion, more efficient combustion, under idle and cruise, and this is going to help the OEM manufacturers with their emissions targets.
So, that's I think one of the considerations that's often overlooked as to why we quite often see wide plug gaps on a factory spark plug. Next aspect I'll talk about is the, actually, I'll go back, and I'll just give you some numbers here to give you some comparisons. So, for a turbo charged application, particularly a drag application, where I'm running very, very high boost pressures, perhaps 40 to 50 psi on a fuel that's very difficult to combust, maybe E85 or methanol, I would not be afraid of running my plug gaps down around 15 to 20,000ths of an inch, so that's 20,000ths of an inch is 0.5 milimeters. Most people would consider that to be very, very tight, but that is what I've found works exceptionally well under those applications. I would tend to go much larger than that.
Unless I've got a very, very powerful ignition system, we start risking problems with ignition misfire. Okay, the last aspect I wanted to deal with here was the aspect of resistive and non-resistive plugs, and again, this is something that a lot of people don't really understand. What we'll find is that the majority of plugs that we would factory fit to late model EFI engines will come with a built-in resistor. They will be a resistive design plug. What this means is that there is a ceramic resistor in line in the construction of the spark plug.
That resistor is designed to reduce the ignition interference or ignition noise that is generated when a spark is created, and this can play havoc. This noise can play havoc with aspects such as computer systems in our cars. So, a non-resistive spark plug, obviously they don't have that resistor, as the name implies, and we can have problems with those plugs interfering with some of our ECU's. So, these in themselves, which can be confusing, can actually result in ignition misfires, so this becomes a problem diagnosing what's going on. You can also have problems with your laptop dropping out of communications with the ECU, and all sorts of crazy things going on with the ECU.
So, for that reason, unless you've got a specific reason not to, I would always advise using a resistive spark plug if you're changing to a different design or different construction of spark plug. Okay, we're going to cover off reading spark plugs next, and before I do that, I'll just mention, if you have got any questions, I can see we've already got a few coming in there. If you have got any questions, please ask those now, and I'll deal with those after we've gone through our next section. So, hopefully at this point, you've got an idea, a better idea of the critical aspects that we need to consider with choosing a suitable spark plug for our application, and really, again, it's not particularly difficult, unless we're starting to move out into the realms of very, very heavily modified engines. In most instances, what we want for moderately modified engine, is to fit a spark plug that's perhaps one to two heat ranges colder.
Make sure it's a resistive plug. Make sure that it's physically compatible with that engine, and that one to two heat range colder spark plug is also going to likely remove the plug gap or move it further out of the combustion chamber, and we're also going to likely gap that spark plug a little bit tighter. Now we're going to move into reading spark plugs, and this is an area that I just get so many questions about. I see it in our forum. We get email questions.
I see questions on the Internet, and I'm going to probably be pretty controversial here, and I'm going to say that when we're dealing with EFI systems, we're dealing with modern EFI technology, with all of the sensors that we have available at our disposal, reading spark plugs is not what it used to be, and it is simply not as critical, not as important as most people seem to think or would like to make out. Now, I'm going to back that statement up, because obviously there is a generation of tuners, predominantly in the carburetor or constant flow fuel injection sort of area where reading spark plugs is what these tuners live and die by. They are constantly making adjustments to their tune-up based on what the spark plugs look like at the end of a run down the quarter mile. And that's fine, because they are very limited in their application to what they can see. They don't have the sophisticated logging systems that we have available.
Now, often these engines weren't even fitted with wideband air-fuel ratio sensors, and hence, this is really the only information they have available to see what was actually going on inside the combustion chamber during a run down the drag strip. I'm talking here about drag strip operation, but of course equally we could be talking about what's going on on the dyno, out on the circuit. A little bit more complex if we're talking about on the road. So, let's start by grabbing this spark plug here. Now, hopefully you're going to be able to see it in a little bit of detail, and if someone was to post up a photo of that spark plug and ask me what I thought of it, in all honesty, I wouldn't have that much to say.
I've got a spark plug here that's done approximately 40,000 kilometers. There's not a lot that we can really do about reading that spark plug in detail, and all I could really say is that maybe it's running a little bit rich, but was it running too rich at idle? Was it running too rich at cruise, or was it running too rich at wide open throttle? I really can't give any more information than that, because we've got a plug that's done 40,000 kilometers under a variety of operating conditions, and really, our chances of getting any useful data out of that plug are just about useless. So. While I feel quite strongly that plug reading is not an essential element to EFI tuning, there is still some information that we can take from our spark plugs, and that's what we're going to focus on here. I believe strongly that reading spark plugs is not a technique that is essential material for the majority of those tuning modern EFI systems.
However, it can be used in conjunction with the rest of the data that we've got, either from our dyno or our data log or our ECU to back up what we're seeing and to also prove that the spark plug that we've chosen is the correct heat range, and we can also start seeing some aspects of potential or impeding mechanical damage coming through by inspecting the spark plugs, and the other aspect is that we can use the spark plugs and read them together, look at them together, in order to get an idea of whether an engine is operating evenly across all of its four, six, or eight cylinders. So, we can use these, in essence, to give us a visual cue as to whether our fuel and air distribution into the engine is equal. So, first thing to understand is, if we want to get any useful data from spark plugs, we're going to need to fit a fresh set of spark plugs. There's not much use trying to pull out a set of plugs that have done 40,000 kilometers after a pull on the dyno and get much useful data from them. So, this is the first thing.
We're going to be starting with a fresh set of spark plugs. Now, the other aspect here is, if we, for example, at the drag strip, we've done a pull down the drag strip, what we want to do is cut the engine at the end of the run, and then get towed back to the pits. If we idle the car back down to the pits, basically what we're going to see on the spark plugs is predominately whatever was going on as the engine was being idled back down the return lane to the pits. Likewise, on the street, likewise on the racetrack or if we're on the dyno. So, it's important to understand that we need to actually cut the engine at the point where we're interested in seeing what was going on.
Now, we don't really want to spend a lot of time idling the engine, because that's going effect our results. So, primarily, when I'm tuning a car on the dyno, I will be looking, if I'm going to be inspecting the plugs, and this is a technique that I predominantly use when I'm tuning higher horse power applications. So, if I'm tuning a factory car or something that's maybe producing 20 or 30% more power, then this isn't something that I'm generally going to be worried about. Provided, first off, that I know that I've got a good set of plugs fitted to the engine, and I know that they gapped correctly, I'm not going to be pulling my spark plugs after every run on the dyno. Once we start getting up into engines where we're producing maybe two or three times more power than factory, then I'm inclined to occasionally pull a set of plugs out after a pull on the dyno, have a look at them, see how they're coloring up, see what the condition of those plugs is, and make sure that they are matching the information that I'm seeing from the rest of my sources.
So, there are a few key points that I'm looking for. Primarily, it's four things. First of all, I'm going to be looking for the heat discoloration on the ground strap of the spark plug. So, this is a good indication that heat range of our spark plug is correct. So, again, we really want to do this on a fresh set of plugs, and what we find is that, if we look at our spark plug, if this was brand new, what we'd find is that the entire spark plug surface has a coating on it, and I think it's a cadmium coating from my recollection, and what we find is that, as we run the car, run the engine on the load, that ground strap will heat up, and what we find is that it burns or discolors the plating on the ground strap, and what we're looking for is whereabouts that ground strap has discolored back to.
So, generally, what we're trying to do is to get the ground strap to discolor approximately 50% along its length. So, this would indicate that we've got a spark plug that is approximately the right heat range for the heat that it's being subjected to. Now, in this condition, in this situation, I'm not quite sure if we'll actually be able to see, but the ground strap is only discolored back to about where my fingernail is here, so that's probably about a third of its length, so in this case, the ground strap hasn't discolored far enough. This means that the spark plug is probably around one heat range colder than we actually needed it to be for this application. Likewise, if our ground strap was discolored, if that plating had been burned off right back down to the body of the spark plug, then that would indicate that we were probably a little bit too hot.
Now, if we take things to extremes as well, the other thing we're going to be looking at, which is the second aspect we're going to be considering, is melting. The actual ground strap will start to round off, and actually be eroded by the heat. We're also likely to see the same situation happening on the center electrodes, so that's the second thing I'm going to be looking for. Any signs of melting or physical damage to the spark plug. So, this is a good sign that there's something wrong with our tune-up or something wrong with our tune-up, or we've dramatically got the wrong spark plug for our application.
And particularly when we're looking at physical damage, another thing that is worth looking for is any sign that the ceramic insulator and the spark plug has cracked. So, that's another thing that's often overlooked, but if that ceramic cracks and actually falls out in operation, it's incredibly hard, and it's going to do a huge amount of damage to your engine before it makes its way out through the exhaust, and in a turbo charge application, you then get the added benefit of that ceramic mess also destroying the turbine wheel of your turbo charger, so definitely not a situation we want to be suffering from. The next aspect is the coloration across our set of plugs. Now, I'm using predominantly my wideband air-fuel ratio sensor to help me tune the air-fuel ratio, and I'm not going to be focusing too much on the actual coloration of the plugs to guide me with my tuning. What I'm looking at is the coloration of the plugs as to whether or not I have a flow distribution problem between cylinders, and whether or not perhaps I've got certain cylinders running richer or certain cylinders running leaner than others.
And what we tend to find is, we'll see that sort of coloration coming around the base of the spark plug and also the ceramic insulator in the center of the spark plug, and what we're generally looking for, if we're on a pump or hydrocarbon-based fuel, we're looking for a light tan color. As we go leaner, we tend to see that sort-of become whiter, and as we go richer, that tends to move towards a darker color. So again, I'm not using it to guide me with a specific air-fuel ratio. That's what the wideband air-fuel ratio sensor is for, but we can use that to help decide whether or not we need to perhaps add a few percent fuel to a certain cylinder, if it's showing signs that potentially that particular cylinder might be running a little bit hotter than another. The last aspect that we want to also look at is the insulator body, so the ceramic in the center of our spark plug, and often this can be worthwhile examining in a little bit more detail with a magnifying glass.
What we're looking for here is if there's any sign of specks, particularly specks of aluminum, little balls of aluminum starting to become apparent on the insulator. This can be early signs of the onset of knock or detonation, and obviously we want to be very wary of any signs of detonation, so if we've got anything happening there, this can be a sign that we may be slightly over-advanced. You see, we want to also be confirming this with our audio knock detection equipment. If we've got anything like this going on as well, this means that further testing, if we want to remove some timing and test again, this is also going to require us to fit a fresh set of spark plugs. So, this is one of the considerations that I think' is often overlooked when people are talking about reading spark plugs.
If we want to get a true read on the state of tune, we're going to really want to start with fresh plugs in order to get that read. We can't really constantly get data from the same set of plugs time and time again. We're going to be really restricted on what we can read from that. The other aspect there with the coloration on our spark plugs is that the coloration will depend on the fuel we're running on. As I just mentioned, if we're running on a pump-based fuel, gasoline-based fuel, then we can expect to see quite a lot of coloration depending on the air-fuel ratio, particularly if we consider methanol.
Really, we see almost no discoloration on the spark plug at all, which makes it very difficult to really get a read on that in terms of our air-fuel ratio. Now, I just want to also relate this back before we move into questions. I want to relate this back to, I think, where these questions come from, this confusion. I'm remembering again that this was the only technique that was available to tuners back decades ago, when carburetors were available and EFI wasn't yet invented, and the ability to tune the engines was much more limited than what we have now. So, we wouldn't have had back then the ability to tune our fuel delivery at increments of 10 KPI, manifold pressure, and perhaps 250 rpm.
That's much, much broader than that, so this means that getting an overview of the fueling from reading the plug was a valid option. What we can find now, with the ability with the EFI to have dramatic changes in our fueling over just 250 rpm, what we could find is that we might have our fueling too rich or too lean at perhaps three or 4,000 rpm, and then, in the higher rev range, we might have an ideal fuel ratio, and when we cut the engine and look at the spark plugs, what we're really looking at is what the plug was exposed to at the point in the run where we shut the engine down. So, that's a consideration we need to keep in mind. Okay, we'll move into some questions now. Of course, if you do have any further questions, please continue asking those, and I'll get to them when I can.
Our first question comes from Premnath, who's asked, "What's the max gap tuned "on a turbo charged engine?" I can't give you an answer to that. There's so many things that's going to depend on. First of all, it's going to depend on the spark plug. So, are we talking about a projected or retracted gap spark plug? What's the boost pressure we're running? What would be acceptable for 12 psi of boost will probably not work at 35 psi of boost. So, these are the considerations we need to keep in mind.
Also, what about your ignition system? The plug gap that we can get away with is severely affected by the spark energy produced by our ignition system or our ignition coil. So, these are the considerations. So, essentially, I would probably start, if I knew nothing about the particular application, I would probably start with a plug gap somewhere in the region of about 0.8 to one millimeter, and if I start running into ignition misfires, I would tend to reduce the gap from there. I've never, as I mentioned earlier, I've never personally witnessed any real benefit from running a plug gap wider than about one millimeter, although I do know that there are studies that have shown this. It's just not something that I've personally seen with my own testing on the dyno.
And again, as I mentioned in the body of the Webinar, anything that we pick up is likely to disappear really rapidly when we run into ignition misfire problems under wide open throttle and high load. Bathurst Bully’s asked, "Have you had any experience "with modified spark plugs that are drilled out "and used to measure in-cylinder pressure to find MBT? “I believe these are becoming more cost-effective "compared to in the past "and a great alternative to a dyno to find MBT." Personally, no, I haven't, although I am very well aware of what you're talking about. We've done interviews with both AVL and PLEX when we were at PRI a few years back about their in-cylinder pressure monitoring. PLEX offer their instrumentation via a modified spark plug, whereas AVL offer either an in-cylinder pressure sensor that needs to be drilled and taped into the combustion chamber, or also they can supply instrumented spark plugs. Definitely these systems have come down in cost, although I wouldn't quite say they are cost-effective yet.
You're still gonna want a dyno for probably all other aspects of your tuning, and last time I checked, a competent system was still well over 10,000 U.S. dollars, so it's still a pretty pricey effort to go to. TDEChamp has asked, "Will oil separaters let you run a colder plug?" That's a really good question. Potentially, I could see a reason why that would be the case. We live in a pretty great existence here in New Zealand, where we don't have a lot of emissions compliance regulations.
So, it's not essential for us to have crank case breathers, etc., feeding back into the intake system, so our natural tendency is always to run an external catch can or oil separator, and obviously, hence, that's my experience. It's not something I'm generally really worried about. However, just for those who are following, to sort-of go into what Tyler's talking about there, one of the reasons we need to run a hotter spark plug is to burn off those deposits and contaminants off the spark plug. A lot of these come from the oil vapor that's being ingested though the engine as part of the crank case breather system. So, potentially removing these vapors via an oil separator or a breathe to atmosphere catch can will remove those contaminants and allow us to potentially run colder without the risk of fouling the spark plug.
So, yes, that's a possibility. However, in reality, I've never found on a healthy engine that re-introducing those crank case vapours into the intake system will limit what we can run in terms of our spark plug and prevent us running a spark plug that is well-matched to the power levels of the engine. So, I figure that sort-of explains myself there a little bit of a long winded answer, I'm sorry. EvoAuto has asked, "Do you have any experience "with indexing spark plugs?" Yes, I do. So, again, for those of you who are unaware of what indexing spark plugs means, that is where we would take our plug, and we would draw a line or a mark on the porcelain of the spark plug, in line with the plug gap, and what we're trying to do here is align the spark plug when it's installed in the cylinder head, so that that plug gap faces towards the incoming fuel air charge on the intake valves, and this is done via small packing washers that go underneath the head of the plug.
I've tried it on a couple of our drag engines, and quite honestly, couldn't see any benefit for doing so. It is a bit of effort, so I quickly stopped doing that. Can't say that it's useless. I know that a lot of people do do it, but again, in my own circumstances, I haven't seen any benefits from doing it. Barry G’s asked pretty much the same question, so that'll answer that.
"Does the concept of indexing your plugs "really result in gains or better results?" Dave has asked, "Can a plug which is too hot "in the heat range run the engine hotter "but show no signs of misfires?" Just trying to comprehend that question there. Okay, so, first of all, the heat range of the plug isn't going to have any effect on how hot the engine runs. It's a sort of a cause and effect. We choose the spark plug to suit the heat being produced by the engine, so it needs to handle that combustion chamber heat. To answer the other part of that question, though, "show no signs of misfires," it would be possible to run a plug, potentially, that is too hot that wouldn't misfire, but of course, we all of the nasty potential side effects of that spark plug being too hot, involving the potential for maybe pre-ignition, or at a minimum, that spark plug will end up melting and becoming eroded from the excessive heat.
Second question from Dave is, "Are there signs on the plug "if they haven't been torqued down tight enough?" I guess I'm pretty careful with the installation of my spark plugs, so I don't know if it's something I've really noticed. Potentially what we may notice is there is a washer that seals the spark plug to the cylinder head, and this is what we're going to crush down when we're tightening it. I would guess that we may be likely to see some discolouration up the hex and the side of the body of the spark plug from combustion gases escaping past the spark plug. I'm not quite sure if that's a valid concern, given that it would still have to negotiate the threads as well. Either way, probably not something we really want to go and investigate too deeply.
Bathurst Bully’s asked, "How heavy would detonation need to be "before it's seen as pitting marks on the porcelain "of the plug?" When we're talking about those first signs, what we may see is initially some black specks, and then probably more significant detonation, which I would certainly expect would be audible through audio knock detection equipment. That's when we're talking about the potential to start seeing small balls of aluminium. So, reasonably significant, and certainly while it is an indicator, I would like to think that if we're doing our job correctly, and we're using the equipment that we promote and talk about in the courses, we're going to be picking that up audibly before we see any signs of that on our spark plug. TDEChamps asked, "Is fouling and build up the only real issue "you'll come across with a plug that is too cold?" Probably the other aspect that I didn't touch on is you may find that your cold starting ability suffers, and again, with a colder plug, we're removing that spark plug out of the centre of the combustion chamber. We're retracting it back out of the combustion chamber, where it's a little bit harder to ignite the fuel-air charge so hence, we can see our cold starting performance is maybe going to diminish.
Again, in reality, particularly if we're talking about street cars that are modified from sort of mild through to even moderate, it's really not, in my consideration, not in my experience, a huge consideration. Although I will also temper that with, here in New Zealand, we don't see really, truly, sort of sub-zero temperatures that frequently. Here in Queenstown it's a lot colder than where my old workshop was located, but the coldest we're likely to see is maybe minus 10, and that's very, very rare. So, for those in very cold climates, where we're much below that, or spending a significant part of the year at sub-zero temperatures, then that may be a bigger consideration. Barry G’s asked, "Do you recommend removing spark plugs "from the motor while the engine is hot, "and does that really damage the threads and the head?" I've never had problems with removing spark plugs from an engine hot or cold.
One little trick that I do use when I'm installing spark plugs into the engine is I will use a very light smear of our high temp anti-seize on the threads. I find that this just prevents any chance of the spark plug catching in the threads or galling up and causing us an expensive and big headache as we have to remove the head to install a thread repair kit or something. ADVAuto’s asked, "I've stated that the heat range is dictated "by length of the ground electrode. "I'm old school and learnt that the heat range is dependent "on the insulator length." Okay, well, the two really go hand-in-hand, and this is what I was talking about with, again, we'll just reference our two spark plugs here. And what we're looking at here is a hot plug versus a cold plug, and what we can see is that both the ground electrode and the insulator body are both retracted down into the spark plug and are both reduced, so the two sort of work hand-in hand.
It's essentially what I was talking about during the Webinar, is getting the heat out of the ground strap, but of course as we shorten that ground strap, it needs, in order to retain a spark plug gap, the rest of the spark plug needs to move down into the spark plug as well, so both really go hand-in-hand. Jordan has asked, "Do you want to touch "on boosted rotary 13B REW plugs?" "Plug selection," sorry, "Some guys get awesome plug life "but foul every other day." Okay, I would say, in my experience with rotaries that the problem with fouling spark plugs is less a problem with the spark plug and probably more a problem with the tune. I think, if I was to put rotary tuning in a little nutshell, what I find is that most of the rotary tuners out there are probably less educated than piston engine tuners. That's not to say there aren't some great rotary tuners out there in the market, but certainly there's a lot more people out there in the rotary tuning market that are getting by on the trial and error method, I guess would be putting it politely. The other aspect that can affect the plug life for a rotary engine is how heavily modified the engine is.
Now, in a stock engine, it should be able to get reasonably good plug life, but of course, once we start talking about heavily modified rotaries, perhaps bridgeport or peripheral port engines, where we've got a lot of, essentially a lot of overlap occurring, these engines do end up being more prone to the spark plugs being fouled. This is made worse by the fact that with these sorts of engines, quite often we're going to be running a much colder plug, sort of the 10.5 heat range is a pretty common one that was used in a lot of the drag rotary engines that I've tuned. So, it's not necessarily a case of the plug. There's a little bit more to it as to why, I think, a lot of people suffer from really poor plug life in rotary engines. Tuned by Mr.
Rogers has asked, "Larger spark plug gap in theory requires more voltage "for the spark to cross the gap. "Would the higher voltage from the larger gap reduce plug "or electrical parts life, or if the gap's too small, "typically it doesn't make any real difference?" Look, I think there's probably some potential life expectancy issue there on your ignition system components, but in the real world terms, no. I don't think it's something that you really need to be too worried about, and in fact, if the ignition system is suffering from misfires, this in itself can actually potentially damage the coil because when the spark, the coil discharges, that energy has to go somewhere. If it can't jump across the plug gap, then quite often what it will do is bridge or short internally inside the coil, and this can end up actually damaging the coil. So, in terms of reducing the spark plug electrical parts life, I don't really see that as something as a huge concern, provided we are not actually suffering from ignition misfires, which is obviously the key aspect to all of this anyway.
All right, that's brought us to the end of our questions, so I'll assume at this point that everyone now knows a lot more about spark plugs than they did when they came in. Hopefully that information is going to be helpful to you in your day-to-day tuning. And as I said at the start, really this is the knowledge that I've sort of developed or needed to know through 15 years of tuning professionally, so we don't need to know every single aspect of spark plug construction in order to actually go about EFI tuning, nor do we need to be an expert in reading spark plugs to do an incredibly good job of tuning using modern EFI. As usual, if you do have any more questions, please ask those in the forum, and I'll be happy to answer them there. And thanks for joining us.
I look forward to seeing you all next week.