Eliminate Engine Detonation Without Losing Power and Throttle Response

The Causes of Abnormal Combustion
Eliminating the octane rating of the fuel, one word can describe the cause of detonation: heat. Excessive heating of the combustion chamber, incoming charge, piston crown, and so on, through either external causes or from high pressure, increases the temperature in the bore. As this value escalates, the fuel enters a point where auto-ignition can occur. Many times the fuel may be on the verge of detonation, and an engine may be sensitive to a difference of one octane point.

Once detonation occurs, both the piston crown and combustion chamber become super-heated, which then makes further abnormal combustion events much more likely. For this reason, once an engine starts to ping during a particular driving or load scenario, it usually doesn't stop until the previously mentioned parts cool. Pontiacs equipped with electronic engine-management systems that incorporate a knock sensor acknowledge this and identify it as the hysteresis of knock. Once knock is identified, the engine controller retards the ignition timing an excessive amount, allowing the piston and combustion chamber to cool so the detonation can be controlled. GTP owners who drag race know all about this.

As with any driveability related issue, the problem is often the culmination of many different things, not a single item. A Pontiac that is prone to detonation can be suffering from many different issues that, when put together, result in poor performance or a need for high-octane fuel. A common mistake made during diagnosis, by both the professional and hobbyist, is not acknowledging how the little areas of error can create a bigger problem--a perfect example of the whole being greater than the sum of the parts.

When tackling detonation, always confirm and eliminate each area of concern first. Don't take the mindset that an area is close enough or not out of tune enough to cause the problem. From the author's experience, this mentality often results in failure and then forces drastic steps to cure a problem that could have been repaired easily.

Keep in mind that the goal of this primer is not to get a 500 hp 455 to run on 85-octane fuel. Certain internal design elements drive the required octane of an engine. Factors include the compression ratio, combustion-chamber design, spark-plug location in reference to the cylinder-bore center, cylinder-head material, length of the connecting rod, and camshaft profile, to only name a few. The goal of this article is to show our readers how to possibly either use a lower-octane fuel without a loss in performance or how to make sure your engine is producing the most power and best driveability for the octane you are feeding it.

With premium-grade fuel costing approximately twenty cents more per gallon, savings are quickly realized if you can safely switch to regular without a performance or durability penalty. It's all too easy to add octane to the problem by purchasing more expensive gasoline. It's analogous to keeping the windows open in your house and turning the thermostat up higher to keep warm. None of us would do that, but we think nothing of spending too much at the fuel pump for the results delivered.

Due to the eclectic mix of areas that either create or contribute to detonation in a Pontiac engine, they are listed and explained separately in no particular order, so review all of them before tackling a knocking Pontiac. Some of this information is applicable not only to vintage Pontiacs but corporate V-8 engines and even V-6 and four-cylinder powerplants.

Heat Riser
Older Pontiacs (before early fuel evaporation with CCC feedback carburetor systems) used a heat riser passage that brought hot exhaust gas under the carburetor during cold engine operation. This improved the vaporization rate of the gasoline in a cold engine. If the heat-riser valve fails and remains open, the exhaust gas heats the incoming charge even after the engine has fully warmed. A stuck heat riser creates the need for higher octane fuel.