Most passenger cars use a design referred to as an extended "reach" design. Reach is defined as the dimension from the base of the metal seating surface (generally the threaded portion of the plug) to the top of the shell intruding into the cylinder head. Using the engine manufacturer's design specification for reach is critical. Less than optimal reach obscures the plug allowing deposits to form and missing to occur. Threads exposed from a plug with greater than optimal reach will overheat the ground strap causing misfiring and detonation. Additionally, extended reach plugs risk being struck by the piston as well as accumulating carbon deposits on the plug threads such that removal becomes impossible or damages the engine block threads.  

The dimension known as the gap location is the measurement of the electrode and its insulating ceramic beyond the rim of the metal shell. The design intention of the gap location is to position the spark at the optimum position for detonating the fuel/air mixture. Adjusting this dimension is an additional method of spark plug manufacturers manipulating the heat range of the plugs.

Gap spacing or plug gap is the distance between the electrodes. Plug reach is designed based upon the physical engine dimensions. Gap location and spacing are designed based upon the type of service set for that engine. Engines with equal dimensions are built for many different types of service as in truck applications compared to a racing application. Some spark plug designs include multiple electrodes to increase gap growth by multiplying gap paths or changing resistive elements in the center electrode.  

Spark plugs are rated in "heat ranges". Plugs designated and designed as "cold" are designed to channel heat away from the electrode, leading to longer life of the electrode, less gap erosion producing more consistent sparking across the life of the plug. "Hotter" plugs with higher heat ratings keep a higher temperature at the spark plug tip. The design difference is based upon the type of load the plug will be running under. Cold plugs last longer under high-load conditions like racing. A hot plug is most often recommended for engines that idle frequently and for long periods or operate in low-load conditions. Running a hot plug in a high-load race engine, would not last long and could contribute to opportunities for pre-ignition and sever damage as the spark plug begins to glow. Most street performance engines spend time in both areas where the manufacturer's recommended heat rating is an excellent choice.  

Operating temperatures within the combustion chamber produces temperatures at the plug tip between 900º F and 1600º F. Those extreme temperatures reduce the voltage required for the first electrons to bridge the spark plug gap.  

Spark plugs on most cars use resistor plugs. Resistor plugs help reduce radio interference and gap erosion. The resistive element (typically 10,000 ohms) changes the high-frequency, high-current portion characteristics of the ignition circuit reducing the arc discharge "noise" and as a byproduct helps to reduce high frequency radio noise and spark plug gap erosion.  

To continue to provide an environment for the best spark, spark plugs need sharp edges on the center electrode and ground strap. Certain manufacturers try to provide and prolong the optimum surface by developing plugs with, V-grooves (example - NGK), split ground straps (example - Splitfire), U-grooves (example - Nippondenso), multi-ground strap (example - Bosch), etcetera. Each design is an attempt to increase the number of sharp edges available to the spark.  

The other major area of advancement in spark plug design is in the manufacturing materials that change behavioral aspects of parts of the spark plug function. Those advancements are, to the greatest extent, found in the electrodes.  

Platinum plugs
Platinum is a very dense metal. Although not as conductive as copper its extreme density is the property that makes it attractive to engine designers and therefore spark plug designers. Platinum, due to its hard and dense nature is resistant, as an electrode, to erosion caused by the spark arcing in the spark plug function. The main purpose of platinum as a component is to extend the life of the spark plug to maintain the integrity of the electrode and gap, delivering consistent performance. Many types of spark plug wear can be readily differentiated to different engine environments.  

Other aspects of spark plug manufacturing include uses of different alloys and variances in the physical design to deliver a better spark. Some of those design ideas include ribbed ground straps, multiple side straps, and tapered side straps. Each design has its own best performance results depending upon the individual application. Some of the better examples of differing designs can be found in offerings by A/C Delco, Accel, Nippondenso, Autolite, Bosch, Champion, NGK, and Splitfire.   As consumers readily accept that better spark plugs can make a difference in performance, the importance of the delivery system in the spark plug wire is often overlooked. A complete ignition system must have integrity in each component or the system fails at that weakest point - coil, wire, plug. The two system components designed as serviceable, replaceable items, are the wires and plugs. If great attention is paid to finding the best spark plug the same attention must be given to the delivery system - the wire. The best wire in the industry is offered by Vitek Performance products.