Eliminate Engine Detonation Without Losing Power and Throttle Response

Air Filter Snorkle
From the late '60s on up, carburetor-equipped Pontiacs have a hot-air stove that connects to the air-cleaner snorkle along with a vacuum motor. It's task is to close a flap in the snorkle and allow the engine to breathe heated air from the exhaust-manifold stove. It was designed to aid vaporization and hasten choke engagement. As with the heat riser, if the vacuum motor doesn't open the snorkle to feed cool air, the engine is prone to ping. This can be confirmed visually or by removing the air-filter top so fresh air is inducted. If the detonation is reduced, your engine is probably breathing preheated air.

Vacuum Leaks
Lean mixtures create higher combustion temperatures and increase the propensity toward detonation. But often neglected is vacuum leaks that are unique to a single cylinder, as with a poor or loose intake-manifold to cylinder-head connection. Often detonation occurs on only one cylinder and can be traced to a leaky intake gasket. Always check for individual cylinder vacuum leaks, even small ones that cause a light ping, usually at part throttle.

Fuel Distribution
Poor fuel distribution from a misadjusted/mistuned carburetor or injector flow variation with port EFI are a sure way of creating detonation. Again, lean cylinders want to detonate while the richer bores are happy since the fuel is acting as a coolant. If your engine is dyno tuned before it's bolted into the car, monitor exhaust-gas temperature in each header pipe to determine fuel distribution. Don't check only EGT at full throttle; be concerned with light load operating conditions when pinging on the street may occur.

Exhaust Gas Recirculation
From 1973 forward, most Pontiacs were equipped with EGR valves. The purpose of this emissions-control device is to introduce inert exhaust gas at part throttle and light load to take the place of combustable mixture. Its effect is lower cylinder-operating temperatures and reduced oxides of nitrogen emissions. Various Pontiacs produced from 1977 to the late '80s are prone to severe detonation, especially at light load, if the EGR system isn't functioning properly. It's common for either the valve to fail or the passage in the manifold to plug up. Since there are many styles of valves, always reference the proper shop manual for the correct test protocol. Regardless of the valve design, a quick test is to lift the diaphragm with your fingers from underneath the valve while the engine is idling. This should cause rough running or a complete stall. If there is little or no change, the passage is carbon blocked and not feeding exhaust gas. Just make sure you wear insulated gloves before doing this, or you'll have toasted fingers!

Cooling System
The radiator cools the liquid while the coolant removes heat from the cylinder heads. Metal-surface temperatures of the combustion chamber have a huge impact on the engine's ability to detonate. Advanced, waterless coolants, such as the Evans NPG+, create lower metal-surface temperatures and limit detonation in Pontiac engines. If traditional anti-freeze is used, employ a pressure cap of at least 15 psi, and use a proper 50/50 mix of coolant with water. Higher concentrations of EG-based antifreeze increase combustion chamber metal temperature.

Cooler thermostat opening ratings, such as a 160 or 180 degrees F, help but not as much as one would think. It may make the temperature gauge reading look better, but it may have minimal impact on the metal-surface temperature of the cylinder head.

Rate Of Ignition Advance
The amount of ignition lead at a given load and engine speed are crucial in eliminating detonation. This is in contrast to twisting back the distributor. The rate of ignition advance in a Pontiac engine is a two-dimensional graph. Too much advance at a given load and speed creates excessive heat and a rouge flame front. Too little advance allows potential energy from the end gas to exit the exhaust port unburned. An adjustable vacuum advance and distributor recurve is the proper approach for an efficient advance curve while maintaining octane tolerance.

Spark Plug Heat Range
The Aug. '06 issue of HPP had a complete primer on spark plugs and should be referenced before attacking a detonation problem. Keep in mind that a glowing spark-plug electrode is a sure way of starting and maintaining abnormal combustion.

Carbon
Gasoline is a hydrocarbon-based fuel, and the end result of combustion is carbon deposits. An increase in required octane occurs in an engine over the first several thousand miles of its life due to the build up of carbon and other deposits in the combustion chamber and piston crown. These deposits not only retain heat, but slightly increase the static compression ratio by decreasing the volume at TDC. "Performance with Efficiency," Part 1 in the June '06 issue provides a full explanation on how to keep your engine carbon free. The rate at which deposits form is influenced by driving mode, gasoline composition, and the presence of other additives such as those used in the engine oil.

Determine What Is Happening
The first step in solving a Pontiac's appetite for octane is to get a handle on how and when the detonation occurs. Does it happen only at part throttle or all the time? Is the ping only there at the top of the gears at full power? Does air temperature and humidity affect the condition ("Weather or not you go faster ..." HPP April '06). Do different brands and grades of gasoline help any?

As stated in the beginning, there does come a time when the distributor needs to be moved and the total timing retarded, but only after all of the areas that impact abnormal combustion are addressed. Many hobbyists believe Pontiacs will always ping on today's fuel. There is nothing further from the truth. They ran great years ago, and with a little attention to detail, they can run perfectly on today's modern fuel. So put this magazine down, get out in the garage, and eliminate the knock from your engine!