DAVE05GT

Hey guys I noticed a knocking sound the other day when I started my car. After about 45 sec. to 1 minute it went away. This happened one other time that I noticed but I didn't think anything of it. Any input?

Herknav

no notice for me...

jay750092001

if yoru timing is off on the engine it will knock like that at startup, kinda sounds like rocks in the engine, try tweaking your timing and upping it just a smidge and it should fix that problem

DAVE05GT

Ok, by upping it you mean advancing it, or backing it off? I have the timing advanced 1 point past Diablo's tune from 2-4k and backed off 1 btw 4-6k (because of pinging in high rpms). Also, could you explain how the timing causes the noise.

SpazHairlip

Preignition

Preignition is defined as ignition of the fuel prior to normal ignition, or ignition before the electrical arcing occurs at the spark plugs.

Preignition may be caused by excessively hot exhaust valves, carbon particles or spark plug electrodes heated to an incandescent or glowing state. In most cases these local "hot spots" are caused by the high temperatures encountered during detonation.

This form of abnormal combustion has the same effect on the engine as an early or advanced timing of the ignition system, and is so harmful in its effects that an engine will continue to operate normally only for a short period of time. This holds especially true if detonation and preignition are in progress simultaneously. During preignition conditions, cylinder pressures are in excess of the normal limits of the cylinder and the engine structure.

One significant difference between preignition and detonation lies in the fact that if the conditions for detonation exists in one cylinder, they may exist in all cylinders; but, preignition may exist in only one or two cylinders. This can make preignition rather difficult to detect, because of the possibility of preignition occurring in a cylinder which is not the location of the thermocouple which measures cylinder head temperature. Probably the most reliable indication is a loss of power, but this also may be difficult to determine unless the engine has a torquemeter. Another indication of preignition may be the observation of glowing carbon particles being discharged from the exhaust system.

Corrective actions for preignition include any type of engine operation which would promote cooling, such as enriching the fuel/air mixtures, reducing cylinder/manifold pressures, and properly controlling engine cowl flaps when available.

Detonation

Detonation is the result of an amplification of pressure waves, such as sound waves, occurring during the combustion process when the piston is near top dead center (TDC). The actual "knocking" or "ringing" sound of detonation is due to these pressure waves pounding against the insides of the combustion chamber and the piston top, and is not due to 'colliding flame fronts' or 'flame fronts hitting the piston or combustion chamber walls.

Detonation usually happens first at the pressure wave's points of amplification, such as at the edges of the piston crown where reflecting pressure waves from the piston or combustion chamber walls can constructively recombine - this is called constructive interference to yield a very high local pressure. If the speed at which this pressure build-up to detonation occurs is greater than the speed at which the mixture burns, the pressure waves from both the initial ignition at the plug and the pressure waves coming from the problem spots (e.g. the edges of the piston crown, etc.) will set off immediate explosions, rather than combustion, of the mixture across the combustion chamber, leading to further pressure waves and even more havoc. Whenever these colliding pressure fronts meet, their destructive power is unleashed on the engine parts, often leading to a mechanical destruction of the motor. The pinging sound of detonation is just these pressure waves pounding against the insides of the combustion chamber and piston top. Piston tops, ring lands and rod bearings are especially exposed to damage from detonation. In addition, these pressure fronts (or shock waves) can sweep away the unburned boundary layer of air-fuel mix near the metal surfaces in the combustion chamber.

Additional Info

Abnormal combustion, more commonly known as knock or detonation, has been the limiting factor in internal combustion engine power generation since the discovery of the otto cycle (4 stroke) itself. To tune an engine for maximum power, you need to understand this undesirable yet ever-present problem.

Abnormal combustion manifests in many different ways. To the typical enthusiast, all abnormalities are referred to as knock or detonation. In practice, though, when the undesired flame front is initiated and how it is propagated define the cause and, in turn, the appropriate cure.

Abnormal combustion got the nickname knock from the noise that is transmitted from the colliding of the multiple flame fronts and the increased cylinder pressure that causes the piston, connecting rod and bearings to resonate. Any sort of abnormality in the combustion process has serious consequences in the power output, longevity and emissions generation of an engine.


Causes of Detonation

Trying to isolate a detonation problem in a highly modified or even slightly modified engine requires you to determine if the combustion problem is being caused by a change you made to the engine, is inherent to a design flaw, or is derived from an external cause such as carbon deposits or poor fuel. Obviously the area with the largest variable is the fuel quality. Octane determines a fuel's ability to resist auto-ignition, or in lay terms, its ability to wait for a spark to initiate a flame. For our purposes we will explore knock independently of fuel octane rating.

When knock occurs, there is uncontrolled combustion with multiple flame fronts, with only one of them being initiated by the spark plug. During knock, cylinder pressures experience an extremely rapid rise and spontaneous burning and release of the end gases' energy. It is not uncommon to see a pressure rise of up to 8 bars per degree of crankshaft angle, or up to 50,000 bars per second. Anything that will either increase heat or retain heat in the combustion chamber will promote auto-ignition.

Common problem areas are sharp edges of metal either on the piston or in the combustion chamber. For instance, if the piston has a valve relief cut into it, there is usually a very defined edge that the cutting tool leaves. This sharp edge is greatly prone to super heating and will actually retain enough heat that it will start to glow. If the fuel should hit this glowing edge either prior to the lighting of the spark plug or even after ignition, it is very likely that another flame front will initiate. If this unintended ignition occurs very early in the compression stroke, then the piston will be forced up against the increased pressure of the burning gas and will result in a form of abnormal combustion referred to as preignition. When this happens, the end result, if severe enough, is that the connecting rods bend. Turbocharged engines are specifically prone this. Carbon deposits that build up in the combustion chamber or on the piston top will have the same effect, along with the decreased volume at TDC, which in turn raises the compression ratio.

As stated earlier, anything that will raise temperatures in the cylinder will help promote detonation. This rise in temperature could occur many different ways: very lean air/fuel ratios, heating of the charge air prior to entering the combustion chamber, compression of the air molecules from forced induction, or poor heat dissipation through the cooling system. Increase of cylinder pressure will also prompt detonation. Camshaft profile, compression ratio and quench area all come into play. Design criteria as to combustion chamber shape and mixture motion, spark plug location and piston design can all make an engine more or less inherently prone to abnormal combustion. Naturally, spark timing and fuel quality round off the list.

Effects of Detonation

Other than running an engine without oil, there is probably nothing that you can do that is more detrimental to an engine

hawgman

How old is your vehicle? There are some known issues involving the variable cam timing solenoids.

jay750092001

well, i mean making it go higher, if you put in a mod that uped your revs at all that would make the idle lower, you need to increase your idle , when your car idles too low the engine is struggleing to stay on, wich makes it knock like thier are rocks in the engine becuase its working hard as **** to not stall out, so if you bump it up and make your ilde higher it should make the knocking stop, had the same problem on a civic i had haha

DAVE05GT

Thanks, I'll be checking on that as soon as I get home in the AM.

DAVE05GT

Thanks for that info Spaz, I have adjusted my tune a little more conservatively now and the car is still pulling hard as hell through the gears, and my mind is more at ease. Also, I haven't heard the knock again since I started this thread but it's best to be on the safe side.