Whenever the subject of spark plug indexing comes up, those familiar with the subject almost immediately divide into two camps. One side thinks it’s all hogwash, while the other can be rather fanatical about the merits. As with most things in life, the truth falls somewhere in the middle. Because we wanted to hear the opinions of a spark plug manufacturer on the topic, we enlisted the help of Brisk USA to walk us through their perspective on the topic.
The theory of indexing your spark plugs is simple. You want to ensure that when fully seated into the cylinder head, each plug is oriented in the optimal position inside of the combustion chamber. Seems simple, right? Well, the debate really comes in when deciding on what is considered “optimal.” “This has been a highly debatable topic throughout the years as some claim that it is extremely important and some claim that there are at best marginal differences, if any at all. Generally speaking, we see spark plug indexing being most prevalent for ground strap clearance of the piston and or valves,” says Bret Lednicky of Brisk USA.
The idea is to expose the unshrouded side of the spark plug firing tip toward the intake valve so that the spark discharge has direct access to the air/fuel mixture coming into the cylinder when the intake valve opens. – Bret Lednicky, Brisk USA
“With high compression engines or a combination which has a very high dome on the piston, you can run into issues of the ground strap making contact with the piston if the ground strap is not orientated properly toward the relief cut out on the piston surface. For your average street vehicle, indexing spark plug ground straps is not something that is commonly practiced, as the compression ratio’s very low compared to a purpose-built race engine.”
Seems relatively cut and dry: orient the ground strap so that it doesn’t hit the piston. Done. Well, that’s not really the end of it, because if you don’t have any piston-interference concerns, there is still reason to consider indexing your plugs. “When a spark plug ground strap is indexed, the idea is to reduce the shrouding of the spark discharge by the ground electrode,” Lednicky says.
“Conventional ground electrodes are welded on to the shell and have a 90-degree angle in the middle of the electrode, so you have a vertical plane and a horizontal plane which shrouds the center electrode and spark discharge at certain angles. The idea is to expose the unshrouded side of the spark plug firing tip toward the intake valve so that the spark discharge has direct access to the air/fuel mixture coming into the cylinder when the intake valve opens.”
Orienting Your Center
Having established that the ideal ground strap location is 180-degrees away from the intake valve, in order to offer the most unshrouded area possible to the incoming air-fuel charge, the next issue becomes how to properly orient the spark plugs. Part of the issue here is that threads — even perfectly formed, in-spec threads — don’t always line up the way you want them to when they are fully seated and torqued.
There are two popular methods to get your plugs to point in the direction in which you want them to point. The first one takes a large supply of available spark plugs as it is essentially a volume-based approach. You sort plugs by their reading in a spark plug indexing gauge and then match those numbers to the individual combustion chambers in your cylinder heads.
This method requires you to purchase and sort a large number of plugs to make sure you have a decent supply of the orientation you need. Or, if buying plugs by the pallet isn’t your idea of a good time, you will be spending hours with the folks at the parts counter, testing their entire stock of plugs. If you have a great relationship with your parts supplier and a lot of free time, this can work.
However, the second method is much more palatable for most people, and that’s to use shims in order to clock each spark plug to its ideal position in each cylinder. This only requires that you have an ample supply of indexing shims on hand, which are significantly less expensive than an equivalent number of spark plugs. This method is the one endorsed by Brisk.
However, the easier, more convenient method isn’t without its potential drawbacks. “When using shims to adjust the orientation of the ground electrode in the cylinder you are also changing the depth of the spark plug in the cylinder,” Lednicky explains. “Every cylinder head is different and there is no set specification of how far is too far. Something that should be kept in consideration though is combustion deposits. If you back a spark plug out too far, you could run the risk of combustion deposits building up in the threads, which can be problematic and can lead to the spark plug potentially not seating properly in the head.”
While that sounds intimidating, Lednicky points out that the altered depth of the plug is likely not an issue, especially when considering something like the taper-seat shims we’re using range from .010- to .032-inch thick. “Most spark plug indexing does not require a significant amount of shims where you run the risk of pulling the spark plug completely out of the cylinder.”
Moving Beyond Indexing
You might be asking yourself at this point, “It’s 2021 — shouldn’t we be past indexing the spark plugs?” That’s a fair question, and one that Brisk has addressed with the new LGS design. At first, it might look like the multiple-ground-strap designs of decades past, but upon closer inspection, we see that it’s closer to a surface-discharge design, without protruding ground straps. With the center electrode actually being the largest protrusion on the nose of the plug.
“The center electrode and ceramic insulator nose project just a bit more than the ground electrodes,” explains Lednicky. “This forces the spark to travel across the ceramic insulator nose and to jump the spark plug gap, leading to a spark that has much more surface area. This ignites more fuel molecules per capita with the larger surface area spark discharge than a conventional style spark plug, which greatly increases combustion efficiency.”
Because of the way the LGS plugs are manufactured, the plug gap is not adjustable. This is why they are made in two variations: The LGS for naturally aspirated combinations and the LGS-T with a smaller gap for forced-induction applications. “The LGS design utilizes the ceramic insulator nose to bridge the air gap between the ground electrodes and the center electrode, this is referred to as “surface gap” in the spark plug world,” Lednicky explains of the apparent large gap in the LGS lineup.
“The LGS design has a very large gap if you were to measure it as an air gap, but with the design actually being a surface gap, it greatly reduces the voltage requirement as opposed to what it would be with a static air gap. Any time you have an increase in the distance between the ground electrode and center electrode, you are effectively requiring more energy to fire the gap.”
“To help mitigate the demand for energy supply, we have designed the grounds to be slightly retracted, which forces the spark to travel on the ceramic insulator nose. That is how we are able to achieve the ‘effective gap’ range of 0.035-inch to 0.060-inch-plus with something that looks significantly larger.”
In addition to providing higher spark energy, the LGS design also removes the need for spark plug indexing, since there is no ground strap to inhibit flame kernel propagation. Also, the multiple ground-electrode positions allow the spark to follow the path of least resistance, which is usually the direction of the highest fuel concentration within the chamber.
So, is indexing a traditional spark plug worth the effort? Obviously, if there is a piston clearance issue, absolutely. However, Lednicky is less resolute in the case of just unshrouding the flame front. “There is always a potential for a gain with indexing spark plugs, especially with a conventional ground electrode configuration. However, that gain may not always be a significant increase in power output because at the end of the day, the ground electrode is still inhibiting an even expansion of the flame kernel.”
Ultimately, the decision to index your spark plugs or not is up to you. For us, the minimal cost and effort is worth the peace of mind to know we’re taking every possible step to make the engine as efficient as possible. Along those lines, we’ll also be testing the LGS design in future dyno testing once our current engine projects are together, so make sure to stay tuned for that.