Unleaded Gasoline
Prior to 1975, lead was added to gasoline during the refining process to enhance octane and provide some wear protection for internal engine components. Contrary to popular opinion, higher octane fuels do not burn more readily than lower octane fuels. In fact, just the opposite is true. Octane actually measures a fuel's resistance to burning.

Octane enhancement allows higher compression ratios, which in turn yield higher power output from a given engine. The lubricating properties of gasoline are not like those associated with oil, rather they perform as chemical hardening agents which form lead oxides and halides on the seats and valve faces. These hardening properties protect the wear faces from localized welding and pitting.

The introduction of unleaded gasoline has required the installation of hardened valve seats in all 1975 and later cylinder heads. Unleaded gasoline has also prompted the retrofit of hardened seats in pre-1975 heads when a valve job is being performed. Normally, if a pre-1975 engine has been run with leaded fuel early in its life cycle, the valves and seats have had the benefit of the face hardening prior to the introduction of unleaded fuel.

The need to install hard seats occurs after the original seats have been cut or ground during a valve job. Servicing the seats removes the hardness developed during the use of leaded fuel. The resurfaced seats, now soft, will rapidly deteriorate with the continued use of unleaded fuel. It should be noted that this generalization may not apply to pre-1975 aluminum cylinder heads, where cast iron seats were factory installed.

Detonation
One of the biggest problems faced by engine rebuilders is detonation -- a problem that is sometimes difficult to explain and even harder for someone outside the automotive industry to understand.

In the simplest of terms, that's what detonation is: self-ignition or spontaneous combustion.

The difference between normal combustion and detonation is like the difference between a fuse and the firecracker it's attached to. A fuse (normal combustion) has a certain combination of air and fuel and burns slowly and steadily from one end to the other. A firecracker (detonation) may have the same type of fuel but in a different combination of air and fuel; therefore, there's a very rapid explosion. During normal combustion the spark plug ignites the air-fuel mixture and the flame burns from that point out across the chamber, building pressure and temperature as it goes. This pressure is what makes the engine run.

An engine starts out with approximately 150 PSI of static compression. When the spark goes off, the mixture expands and pressures rise to 900 PSI or more. Temperatures in the chamber rise from approximately 100 degrees F to more than 800 degrees F. All this drives the piston down and causes the wheels to turn. If the mixture has too low a spontaneous ignition threshold there would still be the nice, steady burn at the beginning, but as the flame burned across and the pressure increased, the remaining fuel mixture would self-ignite and explode (like a firecracker). Not only does the pressure increase quickly (to as much as 2,000 PSI), but temperature rises quickly as well (up to 2,000 degrees F). It's this increase in pressure and temperature that causes the piston, rings, gaskets and other items to break and fail -- they aren't designed to handle those abnormal conditions.

Detonation, therefore, is the uncontrolled explosion caused by the air/fuel mixture not being stable under the temperature and pressure conditions that it's exposed to. This is liable to happen anytime that the air/fuel mixture is put into an unstable condition.

Ignition timing is another cause-effect item. If the spark plug fires too early, the normal burn will be occuring and increasing pressure while the piston is still going up. This continues to increase pressure from normal compression and the expansion of the already ignited mixture. The resulting pressures may be too much for the unburned portion of the mixture, causing self-ignition, an explosion and detonation.

Pre-ignition creates problems by igniting the air/fuel mixture too early, and again, the mixture is expanding while the piston is still compressing the mixture. The resulting increase in temperature and pressure can burn and break things. Most of the time, pre-ignition will lead to detonation because the pressure rises quickly and causes the unburned portion to explode.

Back in the 1930s and 1940s, most automotive engines had a compression ratio of 6 or 7 to 1. When anyone tried to make the compression higher, detonation would occur. When an additive called tetraethyl lead came along ("ethyl"), it raised the octane rating of gasoline. Another way of defining octane is the fuel's stability. The higher the octane, the more stable the mixture and the more resistance to self-ignition. With the addition of TEL, automobile engine designers were able to increase compression ratios to 10 or 11 to 1 without causing self-ignition or detonation. When the lead was removed from the fuel (1970s and 1980s) to keep from harming emission control devices and the environment, we should have gone back to 6 to 1 compression ratios. Instead, we ended up with too much compression for the fuel, EGR valves, computers and sensors to keep the engine from getting into a situation where self-ignition could occur. The auto makers did reduce the compression of the newer engines somewhat but not enough, so today virtually every engine has the ability to create too much pressure and heat for the stability of the fuel that's available.

But engines have detonation sensors, oxygen sensors, throttle sensors and more. All of these sensors relay information to the computer, and the computer adjusts the air/fuel mixture and the timing to keep the engines from detonating. However, when one sensor goes bad, we are back to an engine with too much squeeze for the quality of the fuel that's available. Even worse, the shut-down sensor may send the wrong signal by default and the computer's adjustments may cause more problems. Today's engines haven't been "fixed" to avoid the detonation problem, they just have a bunch of band-aids (sensors and computers) to keep the problem to a minimum.