Street car octane creep is the need for better gas to prevent spark knock due to deposit buildup on piston tops and combustion-chamber surfaces that causes spark knock.
Does your street-driven car ping now when it didn’t use to? It may be suffering from octane creep caused by deposit buildup inside the combustion chambers.
New engines on the dyno and regularly refreshed race engines run OK with higher static compression ratios compared to otherwise similar but regularly driven, identically fueled street engines. As miles on a street engine pile up, combustion chambers accumulate deposits that increase the engine’s need for higher-octane gas by raising both the effective compression ratio as well as inner cylinder-wall temperatures. These deposits cause audible spark “pinging” or “knocking” to increase. Engineers refer to this syndrome as “octane creep.” Severe knock can shorten engine life. To a point, slower-burning, higher-octane gas reduces audible spark-knock.
Octane creep is more readily apparent on an old engine with a distributor and carburetor, but it also affects late-model, electronically managed, computer-controlled EFI engines. You just aren’t as aware of it because the engine computer crutches the problem for a while by retarding ignition advance when the knock sensor “hears” the engine pinging. But retarded timing means the engine won’t make as much power as it used to.
Causes of spark “knock” may be due to detonation, preignition, or a combination of both. Preignition is the spontaneous ignition of the air/fuel charge before ignition spark occurs. Preignition results from localized hot spots, including very hot carbon particles adhering to the piston top, combustion chamber surfaces, or valve heads, as well as due to overheated spark plug electrodes. Preignition is technically not the same as detonation, which is autoignition of the end gas that hasn’t yet been consumed by the normal spark-plug-initiated flame travel after the spark plug fires. Under detonation, compression of the residual end gas by expansion of the burned part of the charge raises its temperature and pressure to the point that it spontaneously autoignites.
Although technically different, pre-spark preignition and post-spark detonation do sound audibly similar. In severe cases, preignition and detonation can feed off each other, creating an ever-increasing spiral that leads to engine destruction. Ways to minimize this problem include polishing the combustion chamber and piston top, thermal barrier coatings, and periodically cleaning out the deposit buildup with upper engine and fuel injector cleaners like GM’s Top Engine Cleaner (GM PN 88861803 or ACDelco X66P).
On average, a new engine’s octane requirement jumps as much as four numbers in the first 10,000 miles and then tapers off between 15,000 and 20,000 miles. Assuming normal maintenance, it remains relatively constant until the piston rings and valve guides wear enough to let oil into the combustion cavity.
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How Octane Creep Affects Engine Performance
- Deposit buildup inside higher-mileage engines increases an engine’s need for higher-octane gas by raising both the effective compression ratio as well as cylinder and combustion chamber temperatures.
- All engines suffer from octane creep, but modern electronic engine management systems can crutch this tendency for a while.
- Preignition is the spontaneous ignition of the air/fuel charge before ignition spark occurs.
- Detonation is autoignition of the end gas that hasn’t yet been consumed by normal spark-plug-initiated flame travel after the spark plug fires.
- Detonation and preignition causes audible spark “knock” or “ping. “
- Left unchecked, engine knock or pinging can cause major engine damage.
- Fight octane creep, preignition, and detonation tendencies by polishing the combustion chamber and piston top, thermal barrier coatings, and cleaning out deposit buildups with an upper-engine and fuel-injector cleaner.
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