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How To Eliminate Knock Retard - Feedforward

Eliminating Knock Retard

Ray T. Bohacz Apr 1, 2009

It defines a system that monitors a condition or state and then uses that information via a microprocessor to either maintain, adjust or alter an operation. The most common use of the term was attached to fuel correction and can be viewed through scan data as Block Learn Multiplier (BLM) and Intergrator (INT). The oxygen sensors are used as the auditor of the mixture strength and that data is sent to the engine controller.

A common misbelief in the enthusiast community is that the oxygen sensor is used to control the mixture. That is not its task. Its purpose is to monitor the air/fuel ratio and if something goes astray, let the ECM/PCM know--an over simplification of a complex operation. When writing a calibration, the opposite of a feedback edict is employed that can be identified as feedforward.

A feedforward logic, when it is applied to injector pulse width, means the mathematically correct air/fuel ratio is created by the opening time and no correction is required. This would be represented by a BLM and INT of 128. Since these values can be between 0 and 255 or 256 places, 128 is the mid-point. When at 128 there is no correction to the calculated fuel delivery and the proper mixture strength is being created, confirmed by the oxygen sensor. As the value goes lower, the injector pulse width is being trimmed and as it skews higher, opening time is being added. Now let us examine how this logic is applied to ignition timing control.

Knock, Knock
Most, if not all, high-performance EFI GM engines employ a system called ESC, for Electronic Spark Control, that works in conjunction with EST (Electronic Spark Timing). First seen on older engines that still employed a distributor, ESC has been a standard issue component on later applications. Evolutionary changes in the circuit components and software has been seen but the original premise has not been altered. The desired timing curve is created and the knock sensor listens for detonation. If it occurs, the engine management then responds by pulling out some ignition timing. Sounds simple enough. But there is more going on that needs to be recognized.

The ideal ignition event in a cylinder has a flame originating at the tip of the spark plug and spreading out across the bore from there. Only one flame should exist. If another rouge flame is created it is identified as abnormal combustion. Depending on where in the piston's stroke the abnormal event happens will define preignition or detonation. The enthusiast does not qualify the beginning of the abnormal combustion event, and simply identifies it as knock or ping. Thus, with an ESC system a knock sensor is employed.

The purpose of this primer is to establish methods to eliminate or minimize evoking the knock sensor circuit. Attaching the feedforward logic, if the original timing curve is correct and all is well with the engine, no knock should occur. This is the theory, but is hardly ever the case in the real world.

Anyone that has some experience with fighting knock knows that octane is only one of the areas that needs to be addressed. Often throwing octane at an engine, especially a high compression or boosted one, does little to control knock. By definition, octane is the fuel's ability to resist heat and pressure and wait for a spark to ignite. Low octane fuel is more anxious to ignite when exposed to heat or cylinder pressure. Likewise, higher octane gasoline will withstand more of these elements. Abnormal combustion is the result of extreme cylinder pressure or heat and in most instances a combination of the two.

Deposit Formation
Carbon deposits form in an engine in two distinct areas with each having its own impact on performance. Intake valve deposits (IVD) form on the backside of the valve (the part attaching to the stem) while CCD or combustion chamber deposits collect on the piston crown and the walls of the combustion chamber of the cylinder head.

Contrary to what many believe, deposits in both regions can start to form and build quickly, sometimes in under 1,000 miles, if the conditions are correct. Numerous cold starts and warm-up cycles, excessive idling, short trips and around-town motoring where the stop/start action is amplified are all ideal conditions for deposits to form quickly.

Deposits in the ports and on the backside of the intake valves are particularly detrimental since it will impact the engine's ability to breathe but also absorb fuel from the incoming charge. When this occurs the engine can experience excessively lean cylinders from the wetting of the carbon and induce abnormal combustion. CCD contribute to what engineers call ORI, or octane requirement increase, and will necessitate the engine to consume a fuel with a greater amount of anti-knock quality or have the tune severely compromised to compensate for the engine wanting to ping or knock.

The two components in the fuel identified as C9 and C10 hydrocarbons are held up as liquids by the deposits, the amount increasing with greater levels of valve deposits. During engine acceleration the air/fuel ratio becomes momentarily lean and could be excessive enough to cause a lean misfire or hesitation. The heavy fuel absorbed on the valves is then released during steady state operation and alters the desired air/fuel ratio by introducing additional fuel into the combustion chamber. This is known as the difference in the created and delivered air/fuel ratio and will seriously impact the engine's performance and driveability.

In simpler terms, IVD absorbs the additional fuel provided by the asynchronous fuel pulse and thus starves the engine. Then during steady state operation, as the charge passes over the IVD, the fuel-saturated carbon releases the hydrocarbons and richens the mixture.

IVD can build up to such an extent that it interferes with the closing of the valve, so burning can result. The type of deposit varies with the fuel and valve temperature. Sometimes it is soft and sticky and in other cases hard and brittle. Many researchers have found the heavy aromatic compounds from the reformulating process of modern gasoline are largely responsible for deposit formation. The presence of alcohol appears to increase the deposit rate on intake valves and such blends of fuel may require additional additive treatment to overcome the higher deposit levels.

The engine oil also plays a part in IVD. The valve stems are lubricated by oil flowing down onto the valve underside. Tests have proven that less expensive oils without the necessary additive package increase IVD. Some viscosity improvers are known to raise the level of deposits on the valves.

In regard to CCD, the most important effect of the residue is its impact on the octane requirement of the engine. In addition, the deposits can cause surface ignition, and if it is on the spark plug, engine misfire. The amount and nature of the CCD depends on the fuel, engine oil, engine design, driving style and state of tune.

Engine condition and its use also play a large part in deposit formation. It has been found that coolant temperature has the greatest effect and that air/fuel ratio is also important. Compression ratio and intake air temperature have such a small influence on CCD formation that they are considered non-offensive. The engine oil formulation contributes to combustion chamber deposits, and the amount depends on the oil consumption rate, particularly on the piston crown, and the volatility of the sulfated ash content of the lubricant.

Engines with a very small squish region, such as those with a wedge-style cylinder head, are prone to a knocking sound, especially when the coolant is at ambient temperature. The high level of CCD along with the minimal piston crown to cylinder head clearance at TDC in the squish or quench region causes mechanical contact between the two. This results in a knocking noise, which has come to be called carbon knock or carbon rap. It has nothing to do with detonation. The sound usually disappears after the engine has been warmed-up. Excessive carbon build-up causing this problem may often be mistaken for a failed rod bearing since it is rhythmic and may not occur in every bore.

The Cooling System
In an engine the radiator is used to remove heat from the coolant but the liquid has the job of cooling the engine and especially the cylinder head and combustion chamber. This fact is often not recognized since a temperature gauge reads the amount of heat in the liquid and not the surface temperature of the combustion chamber. The metal surface temperature is where detonation begins and needs to be addressed.

There are three reasons for the existence of a cooling system on an engine: to promote a high volumetric efficiency by limiting the amount of heat transfer into the charge air; to ensure normal combustion takes places in lieu of abnormal combustion known as detonation; for mechanical operation and reliability of the components and the complete engine.

High cylinder head metal surface temperatures, be it either local or general, can affect the performance of an engine. Excessive heating can lead to a loss of strength. As an example, aluminum alloys soften at temperatures over approximately 400 degrees F and the piston ring grooves can then deform by a phenomenon known as creep. Furthermore, if detonation occurs and is severe the piston can either melt or erode the top land region. Where the damage occurs on the piston usually is the hottest region and coincides with the area that the end-gas spontaneously ignited. The second concern is the top piston ring groove temperature must be limited to 400 F degrees if the oil is to do its job. Above this temperature the oil can degrade, leading to a loss of lubrication and clogging the rings and groove with decomposed oil. Finally, failure can result from thermal strain. This phenomenon is directly proportional to the temperature experienced over time. Failure is not likely from a single occurrence of overheating, but from repeated exposures. The regions most likely to suffer from thermal fatigue are those within the combustion chamber that have both a high temperature and a high gradient. A good example of this would be the valve bridge region, which is the area between the intake and exhaust valve seat.

Heat is transferred from the cylinder bore and the cylinder head walls to the liquid coolant in a number of convection or semi-convection phases. These phases are dependent on the rate of heat flow through the metal-per-unit area along with the temperature difference between the metal surface and the liquid coolant. As the coolant reaches the hottest part of the cylinder head, which is usually around the combustion chamber and exhaust valve, the coolant will actually start to boil. This phase change is identified as the nucleate boiling point and allows the efficient transfer of heat from the metal surface to the liquid coolant following Boyle's gas law. The coolant media's chemical and thermal reaction is responsible for how efficient this process becomes. When the coolant first comes in contact with the hot metal it will boil, changing phase, and then due to the pressure in the cooling system, the gas bubbles will be pushed from the localized boiling spot and carry with them the heat and recondense into a liquid again.

Once you grasp the job of a coolant you recognize that the traditional anti-freeze mix with water is very poor at the task of cooling the cylinder head. Straight water is even worse, but many still falsely think that it is a good choice. If you want to limit detonation then a better coolant needs to be used.

The Hysterisis of Knock
When an engine knocks, even when no damage occurs, the piston crown and combustion chamber become super-heated. The calibration of the ESC system recognizes this and when evoked pulls out a large amount of ignition lead so these components can cool down. After a programmed amount of time it then adds the ignition timing back. Thus, if the engine experiences abnormal combustion the performance will be severely compromised to cool the piston and combustion chamber. This kills horsepower and ET. The purpose of this exercise between GMHTP, IDA Automotive and the owner of our subject 1987 Buick GN, Anthony Petridis, is to not evoke the hysterisis of knock, or in simpler terms, the ESC circuit.

The car we used was a completely stock Buick with only 71,000 miles. The plan was to chassis dyno the engine at an elevated coolant temperature of 193 degrees F and record the power and knock retard being issued, using a scan tool. The coolant temperature was not the ideal for a dyno pull but would represent a real driving scenario on a summer day. We did not want a false test protocol. Most dyno sessions are performed with the engine cool, hood open and a huge fan in front of the radiator. That only tells you the power the engine will produce under ideal conditions not what it will do in actual street driving.

Our approach would be to properly tune the engine, chemically remove the IVD and CCD and upgrade the cooling system with the advanced Evans NPG+ coolant.

Evans Cooling Systems, Inc. has developed a coolant that has a lower surface tension than water and traditional anti-freeze, boils at 369 degrees F at atmospheric pressure allowing the coolant to remove more heat from the cylinder head and eliminates all water and the possibility of corrosion.

C.A.T. Products, Inc. has created an ingenious tool that accurately and safely administers its proprietary carbon removal chemical into any spark ignition engine. The tool drips the chemical into a vacuum hose as the engine is run at a fast idle of approximately 1,400 rpm.

After performing the services, we would then bring the coolant up to temperature and dyno test again. So follow along and see what we find.


Evans Cooling Systems, Inc.
Sharon, CT 06069
C.A.T. Products, Inc.
Stroudsburg, PA 18360
IDA Automotive, Inc.
Morganville, NE 07751



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