Pump Gas And Detonation
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Eliminate Engine Detonation Without Losing Power and Throttle Response
Knock, Knock, Go Away
Understanding the Causes of Detonation to Better Help You Eliminate It In Your Engine.
The demon of engine knock is something an owner of a traditional high-performance engine knows all too well. An engine designed when 102-octane high test was at almost every fuel station in the country being forced to run on today's gas means that ping has become an all too common companion, and what if your engine was rebuilt by a previous owner? You may not even know the compression ratio. If octane boost or race gas is not in the budget, the typical frustrated hobbyist's response is a twist of the distributor to retard the ignition timing.
With today's pump-gasoline prices, the thought of filling up a big car's fuel tank is enough to wreck the budget, even without the expense of high-octane supplements. The problem with retarding the ignition timing to eliminate detonation is power, throttle response, and fuel economy also go away with the knock. Less ignition lead, though necessary in some circumstances, isn't the cure for every engine with an appetite for octane--, corporate, or what have you.
When fighting ping, retarding the timing should be the last resort, not the first approach. There are, of course, instances when even retarding the ignition timing and adding octane doesn't eradicate pinging. This is not often the case with a properly built, tuned, and maintained engine unless you're trying to run a very high compression ratio on the street. It proves there is more going on to create detonation than just fuel quality and ignition lead. The battle must be fought systematically as opposed to randomly.
What is Octane?
Tuning an engine begins with an understanding of gas octane ratings. The calibration of the fuel and ignition system should be set for the octane level of the car. Thus, if you want to burn 87-octane fuel, the tune-up is calculated for that grade of gasoline.
By definition, octane is a fuel's ability to resist auto ignition from pressure and heat. Thus, the fuel doesn't ignite, instead waiting for the arcing of the spark plug. The higher the octane rating of the gasoline, the more pressure and heat it can withstand before self-igniting. Conversely, lower octane fuel ignites independently of the arcing of the spark plug through either pressure or heat or a combination.
The chemical isooctane is considered a hydrocarbon 2, 2, 4 (a trimethylpentane), which has eight carbon atoms. It's used as a primary reference fuel with an assigned octane rating of 100 for both RON and MON, and n-heptane has an assigned value of zero. The octane number of a fuel is the percentage of isooctane in a blend with n-heptane that gives the same knock intensity as the fuel when evaluated under test conditions in a standard engine. Oil companies use what is called a CFR (Cooperative Fuel Research) engine to determine octane. It's a single-cylinder, overhead valve, variable compression-ratio engine designed for the purpose of testing gasoline.
During the past few years, the octane rating of street gasoline that is posted on the pump is an average of the RON and MON. That is why you see the octane listed as R+M/2. Many years back, there was only one octane rating on the pump, and it was not an average of the two test methods.
RON stands for Research Octane Number and measures the antiknock quality of a gasoline as determined by the ASTM D 2699 method (ASTM stands for the American Society of Test Methods). It is a reference to the antiknock performance of a fuel when the vehicle is operated under mild conditions such as low speeds and light loads.
The Motor Octane Number (MON) measures the antiknock quality of a fuel as determined by the ASTM D 2700 method. It is a guide to the antiknock performance of a fuel under relatively severe driving conditions such as at wide-open throttle when the incoming air temperature and engine speed are both high.
Due to the way fuel is rated today, many experts in the petroleum industry are quick to note the octane rating is not as low as one would think when making a comparison to the old days. Many believe that today's 93 octane fuel calculated as R+M/2 is equal in the antiknock quality of the 97-98 octane-fuel years ago.
But keep in mind that during the 60s, fuel of this antiknock rating was considered regular grade. Most, if not all, gas pumps list the octane rating but don't list the RON or MON independently, only the average. This information would be helpful when fighting a pinging problem. For example, if your engine only knocked at light load, then a fuel with a higher MON is needed. A higher RON value most likely wouldn't do anything to help the problem. But since gasoline is what it is and the retailer will not list the octane ratings separately unless mandated by law, we need to accept the facts and learn to work around them.
Knock, Ping, and Other Noises
In a spark ignition (SI) engine, the combustion event should be initiated by the arcing of the plug electrode, and the air/fuel mixture should burn smoothly and evenly across the bore. When this occurs, it is known as normal combustion. Anything else is identified as abnormal combustion. On the hobbyist level, we refer to any abnormal combustion event as ping, knock, or detonation. Within the engineering community, where the abnormal ignition event occurs in relation to the piston's position in the bore identifies whether it is pre-ignition, post-ignition, knock, or ping.
Some abnormal combustion is the result of two colliding flame fronts: the first started by the arcing of the spark plug and a separate, rouge flame that was the result of auto-ignition. For this reason, before making any tuning changes to your engine, you should experiment with different brands of gasoline. The RON and MON values can be manipulated to achieve the same average and will impact the way the engine runs. You may find your car runs perfectly on a certain brand of fuel and may require no further tuning.
In an engine, auto-ignition of the end gas in the combustion chamber causes a knocking or pinging sound. When this occurs, engine damage is created by the sharp pressure rise and rough combustion taking place. Localized pressure peaks and the explosion, rather than the burn of the fuel, causes the piston, rings, and rod bearings to shake and make a sound similar to a ball-peen hammer hitting a metal surface. Over time, an engine that constantly pings will show excessive wear in the rings, cylinder walls, and rod bearings. If the detonation becomes severe, the possibility of burning a hole in the piston or lifting a ring land away is very high. This usually occurs when the abnormal ignition event occurs during high engine loads.
For our discussion, we aren't concerned with when the pressure spike and uneven burn occurs, so we're quantifying all abnormal combustion under detonation.
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. Engines 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. 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. An engine 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. 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 an engine, they are listed and explained separately in no particular order, so review all of them before tackling a knocking engine. Some of this information is applicable not only to vintage cars, but corporate V-8 engines and even V-6 and four-cylinder powerplants.
Older cars (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.
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.
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.
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
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 cars produced from the mid 70's 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!
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 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 an 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
Keep in mind that a glowing spark-plug electrode is a sure way of starting and maintaining abnormal combustion.
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. 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 an engine'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. 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 their engine 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, get out in the garage, and eliminate the knock from your engine!
When The Flag Drops,,,
The Bull Chit Stops,,,
P. Engineer, Engine Builder