If you are familiar with, or knowledgeable about, detonation, then you realize that too much ignition timing advance, lean mixture and high compression ratios are contributing factors to the occurrence of detonation. If your engine could suddenly control one or more of these engine operating parameters at the onset of detonation, then your engine would be out of danger. Knock sensors and oxygen sensors work just this way. The Engine Control Module, or ECM, collects data from these sensors and adapts the engine's ignition and fuel systems. When the knock sensor "hears" knocking characteristic of detonation, the ECM will retard the ignition timing, and attempt to control detonation. When the oxygen sensor "smells" too much oxygen in the exhaust, the ECM will richen the mixture and stop the lean condition. These systems work quite will, within their limitations.

High performance modifications can require more fuel than the fuel system can deliver, so the attempt by the ECM to go to "full rich" is just not adequate to prevent detonation. The ECM has limited control over the amount of ignition timing retard that it can provide. With high performance modifications to an engine, an ECM, if present and operational, just may not have enough resources to prevent detonation and engine damage. Aftermarket Nitrous Oxide systems, turbochargers and superchargers increase the likelihood and severity of detonation. More fuel must usually be made available through fuel injectors (bigger and/or additional) or by re-jetting the carburetor(s). Failure to properly supply additional fuel to a forced induction or Nitrous Oxide injected will destroy engines without exception. A properly designed and installed water/methanol injection system is an inexpensive and perfectly suitable method to control detonation as well.

Just like knock and oxygen sensors adapt the mixture and spark advance in response to detonation, PREP (Pressure Relief / Engine Protection) Spark Plugs adapt the "effective" compression ratio of an engine which has suffered detonation. PREP Spark Plugs are commercially available spark plugs with an important modification: a built-in pressure relief valve. They employ a patented structural weakening which vents the cylinder to atmosphere on the occurrence of excessive cylinder pressure.

Suppose a high performance engine was built with a 12:1 compression ratio, this is a lot of compression and detonation will be a concern. Suppose further that, by any of a host of causes, detonation occurs. Cylinder pressure would reflect detonation by jumping to values in excess of 2000psi; this engine should produce no more than about 1200psi in the absence of detonation. The additional cylinder pressure will permanently deform the structural weakening of PREP Spark Plugs, creating a vent port from the confines of the cylinder to the expanse of the atmosphere. Two noteworthy features of PREP Spark Plugs now come into play: (1) the "effective" compression ratio will be reduced, and (2) some of the rapidly expanding gasses of the burning charge may now be vented. Each of these effects will be commented upon in the following paragraphs.

An engine's compression ratio is defined as the volume of the cylinder (plus the combustion chamber) at Bottom Dead Center (BDC) of the crankshaft divided by that volume at Top Dead Center (TDC). At BDC of the idealized compression stroke, both valves are closed and the cylinder is at atmospheric pressure. As the piston ascends, the cylinder volume decreases, causing the pressure and temperature of the charge to increase. At TDC (in the absence of a spark) cylinder pressure and temperature have achieved their maximum values. With a venting PREP Spark Plug installed in the cylinder head, some of the charge will be continuously vented during the compression stroke; there is just not as much of the charge trapped in the cylinder. Consequently, the maximum pressure and temperature attained during the compression stroke will be reduced, just as though the actual compression ratio had been reduced. The "effective" compression ratio of the engine could be reduced, say from 12:1 down to 10:1. This straightforward reduction of compression is enough to prevent the engine from detonating, and destroying itself.

Detonation occurs when the unburned charge reaches autoignition temperature and pressure. In a cylinder with an ordinary spark plug installed, all of the combustion products, both gasses and heat, increase the cylinder pressure and temperature. This is, after all, what forces the piston down to apply torque to the crankshaft. But heating and pressurizing the unburned charge in front of the flame front can cause this charge to reach autoignition temperature and pressure. A venting PREP spark plug installed in the cylinder will allow the venting of some of the products of combustion: gasses and heat. By permitting some of the products of combustion to escape the cylinder and not contribute to heating and pressurizing the unburned charge, the unburned charge remains cooler and at a lower pressure, avoiding autoignition temperature and pressure. By not allowing the charge to attain autoignition temperature and pressure, detonation is prevented.

Not all engines are alike, and likewise, not all cylinders within an engine are alike. Detonation may appear in only one or two cylinders, while the others do not detonate. Unlike oxygen sensors and some knock sensors (which can only retard ignition timing for all cylinders equally), PREP Spark Plugs act on a cylinder-by-cylinder basis. If detonation only occurs in one cylinder, then only one PREP Spark Plug will vent. This makes identification of the detonating cylinder(s) an easy task.

PREP Spark Plugs offers something no other spark plug can provide: Increased Performance Potential. New spark plugs will, almost invariably, provide better engine performance; this is the basis of claims of the "high performance" spark plugs. Actually, any new spark plug will do the same as the new "high performance" ones do. But PREP Spark Plugs provide something no other spark can offer. With PREP Spark Plugs installed, an engine tuner is free to experiment with power adding modifications such as Nitrous Oxide, more boost, more advanced ignition timing, and leaner mixtures. All of these lead to greater power generation, but carry the risk of engine damage from detonation. PREP Spark Plugs provide Increased Performance Potential because the tuner may "dial in" more power, knowing that his risk is a set of spark plugs, not a set of pistons. Consider the ease of installation of each of these.

PREP Spark Plugs are easy and inexpensive to install, unlike the addition or modification of knock or oxygen sensors. High performance "chips" and programmable engine control systems are available… at a price. And the cost of tuning a high performance engine can be the cost of the engine. It has been said a thousand different times and a thousand different ways: Detonation Will Destroy Your Engine. Installation of PREP Spark Plugs is the cheapest, simplest and easiest to install method to prevent detonation.