CAPTION: Besides obvious notches for valve clearance, a similar notch in a large piston dome is necessary to clear the spark plug ground strap. If the notch is not in the correct place or the ground strap is position at the bottom, the piston can make contact with the plug and close up the plug gap causing a dead cylinder.

In most cases, the higher the compression ratio, the more consistent an engine might be in addition to allowing it to closely follow weather changes, a fact that most racers can appreciate. An engine with a high compression ratio can extract more power from a given load of air/fuel mixture. As such, achieving that high number has been the goal of engine builders everywhere.

Arriving at a high compression number requires a generally small combustion chamber in the cylinder head combined with a piston dome that adequately fills that chamber at Top Dead Center. Noticeably, you have to make provisions for valve pockets along with one other thing… the spark plug.

For obvious reasons, spark plugs are available in a variety of heat ranges and physical sizes. However, there are really only two different basic variations; standard and extended reach. A standard plug would have the ground strap located just above the end of the threads while an extended reach plug has the electrode protruding out of the body of the plug. Because the spark emanates from the tip of the electrode and the ground strap, an extended reach plug would place that said point closer to the inside of the chamber and the air/fuel mixture, which can add to a source of power.

CAPTION - While there are several different spark plugs, there are really only two basic variations, standard (right) and extended reach (left), of which the latter is the preferred plug but requires clearance in the combustion chamber.

However, getting back to a large piston dome, it can create a clearance problem with the spark plug ground strap. This is where spark plug indexing comes into play. In earlier days, astute racers would index the plug in a way that would have it face away from the incoming air/fuel mixture, pointing the ground strap more toward the exhaust valve than anything. All of that was in an effort to maybe increase horsepower. Today, the thought that you could increase horsepower by relocating where the ground strap of your spark plug is located in the combustion chamber is a debatable fact. Almost every engine builder we spoke with agrees the results vary from maybe one horsepower to absolutely none, with the majority leaning towards the none. It’s just not possible to quantify any increase.

However, engine builders are still indexing spark plugs to clear piston domes. The reasoning is simple. When you’re trying to build as much compression ratio as possible, one has a tendency to have a rather large piston dome which ultimately can create a clearance issue with the spark plug ground strap. Henceforth we have plug indexing whereby the spark plug ground strap is conveniently indexed to position the ground strap at a point in the chamber where it is above the piston dome. The downside is the fact that not all spark plugs have the ground strap located in the exact location in reference to the threads. This means that you could have one plug with the ground strap in the ideal location and the next one not even close.

CAPTION - Indexing spark plugs requires marking the location of the ground strap and then screwing the plug into each individual cylinder until you find a set that fits, oftentimes requiring 10 or more plugs for a V-8 engine.

Even when there is adequate clearance for the spark plug ground strap, indexing your plugs can be one of those things which lends itself to taking care in every step of your racing program.

The task might seem simple but it is a very time-consuming project which requires first marking the position of the ground strap on the plug insulator and then screwing one into each individual cylinder noting its location. It’s not necessary to place the electrode exactly at the 12:00 position. Clearance is usually achieved as long as the ground strap in located anywhere between 10 and 2. It’s not uncommon to require ten or more plugs to accomplish the task on your particular cylinder heads. A set of spark plug washers available from Moroso Performance in a variety of thicknesses can help to achieve the correct result. One complete turn of a spark plug is roughly .028” due to its metric thread and the plug washers are .040”, .050” and .064” in thickness. Most race cylinder heads utilize a flat seat gasket, but available from Moroso are also washers for tapered seat plugs.

Once you’ve determined a set of plugs which fit your engine, Moroso offers a tool which you can screw each plug into and mark the tool for the cylinder it fits. From DRC Race Products is the same type of tool but has provisions for eight spark plugs that can mimic your particular cylinder heads. In addition, the DRC product can also be used to store a complete set of plugs.

While most engine builders and racers today will notch their piston domes to clear an extended reach spark plug, you will be giving up a certain amount of compression ratio. Exactly how much is dependent on a number of factors, but if you’re looking for every last bit of power, plug indexing should be in your future.    

CAPTION - Once a given plug is placed in a cylinder head and found to have the correct ground wire position, you can screw it into this tool from DRC Race Products and mark it for future reference when changing to a new set of spark plugs.


DRC Race Products

Moroso Performance