Higher
octane can help to prevent
detonation ( '
knocking' or '
pinging' ), as well as
preignition. These two problems are not merely
noises heard from the engine, but are symptoms of something going very wrong, and these problems can literally destroy an engine, breaking
piston rings (or
apex seals in the case of a
rotary engine),
holing pistons, breaking
connecting rods, and otherwise causing major damage.
Tetraethyl lead, in addition to its octane-increasing benefits, also lubricates valve seats and other portions of the engine that are not exposed to crankcase oil. Newer engines have special hardened valve seats that do not require this lubrication. However, running an older engine on unleaded gas can cause damage to the valve seats.
Modern high-compression and highly-tuned engines are more susceptible to detonation, especially on modern lower-octane gasoline. As an engine is leaned out to maximise power output and fuel economy, detonation also becomes more likely.
Tetraethyl lead, though beneficial in several ways to the operation of internal combustion engines, has several drawbacks, notably its toxicity. There are products that can duplicate both the octane-increasing and the lubrication function of tetraethyl lead, and these are being further researched.