Detonation

[Dinner]

I think that is how you spell it :laugh: But im wondering if anyone has some pics of what Detonation looks like on the pistons and/or head? Im just curious to see what it looks like so that when i check my head this winter i can see if anything is going on. I'v never really fully understood Detonation, so can anyone help me out :baseball_original:

 

Thanks

 

Dinner

[Bansh-eman]

i searched for pict but none came up for banshees how ever i can tell you it looks like rocks bounce around inside when it happens and if its doing it long enough it will burn a hole through the top of your piston.

[bansheeseat$$]

Here you go Dinner.From my Powervalve 'shee.

[trex banshee]

Here you go Dinner.From my Powervalve 'shee.

 

 

this was caused from more of a lean condition but like stated above detonation can do the same.

 

[BigRed350x]

You will notice it on your domes before you do on your piston. Your domes will look like a handful of little tiny rocks have been riding around in your cylinders. The domes will show little indentations, dents, etc. Almost like someone tried to sandblast your domes with rocks.

[trex banshee]

You will notice it on your domes before you do on your piston. Your domes will look like a handful of little tiny rocks have been riding around in your cylinders. The domes will show little indentations, dents, etc. Almost like someone tried to sandblast your domes with rocks.

 

yeah that is a good explination of it. lets just hope he catches it before it gets the piston.

[easttexasbanshee]

Also look for burnt spots on the domes, and you had a question about what detonation is, it usually happens from not running enough octane, or sometimes being very lean. I've replaced many a piston from detonation, also domes, and a crank or two.

[Dinner]

When i first bought my banshee it needed a new top end. So once i got it home, looked it over i got a new top end done on it. Got the pistons back and they looked like shit. Rings were stuck to the pistons, had some chips out of the pistons, etc. So i got it all bored out, etc. In the spring i took the cylinders and head off to re-paint them. I looked at my domes(stock head) and they had like nicks in them, almost looked like someone took a screw driver and would hammer it into the domes to make little nicks. I'm pretty sure that it is just from when the old pistons where in there, cause they are missing some pieces. This was in the spring, and it has been running strong all year so far on 91 octane. So i dont think im having detonation issues.

[bigscary]

Also look for burnt spots on the domes, and you had a question about what detonation is, it usually happens from not running enough octane, or sometimes being very lean. I've replaced many a piston from detonation, also domes, and a crank or two.

or too much timing or compression.....too hot of a spark plug.....hot spots on domes or pistons.....CDI malfunction.....

there are TONS of causes....

[sredish]

Detonation - the spontaneous combustion of the end-gas (remaining fuel/air mixture) in the chamber. It always occurs after normal combustion is initiated by the spark plug. The initial combustion at the spark plug is followed by a normal combustion burn. For some reason, likely heat and pressure, the end gas in the chamber spontaneously combusts. The key point here is that detonation occurs after you have initiated the normal combustion with the spark plug.

 

Unburned end gas, under increasing pressure and heat (from the normal progressive burning process and hot combustion chamber metals) spontaneously combusts, ignited solely by the intense heat and pressure. The remaining fuel in the end gas simply lacks sufficient octane rating to withstand this combination of heat and pressure.

 

Detonation causes a very high, very sharp pressure spike in the combustion chamber but it is of a very short duration. If you look at a pressure trace of the combustion chamber process, you would see the normal burn as a normal pressure rise, then all of a sudden you would see a very sharp spike when the detonation occurred. That spike always occurs after the spark plug fires. The sharp spike in pressure creates a force in the combustion chamber. It causes the structure of the engine to ring, or resonate, much as if it were hit by a hammer. Incidentally, the knocking or pinging sound is not the result of "two flame fronts meeting" as is often stated. Although this clash does generate a spike the noise you sense comes from the vibration of the engine structure reacting to the pressure spike.

 

The high impact nature of the spike can cause fractures; it can break the spark plug electrodes, the porcelain around the plug, cause a clean fracture of the ring land and can actually cause fracture of valves-intake or exhaust. The piston ring land, either top or second depending on the piston design, is susceptible to fracture type failures. If I were to look at a piston with a second broken ring land, my immediate suspicion would be detonation.

 

Another thing detonation can cause is a sandblasted appearance to the top of the piston. The piston near the perimeter will typically have that kind of look if detonation occurs. It is a swiss-cheesy look on a microscopic basis. The detonation, the mechanical pounding, actually mechanically erodes or fatigues material out of the piston. You can typically expect to see that sanded look in the part of the chamber most distant from the spark plug, because if you think about it, you would ignite the flame front at the plug, it would travel across the chamber before it got to the farthest reaches of the chamber where the end gas spontaneously combusted. That's where you will see the effects of the detonation; you might see it at the hottest part of the chamber in some engines

 

What typically happens is that when detonation occurs the piston expands excessively, scurfs in the bore along those four spots and wipes material into the ring grooves. The rings seize so that they can't conform to the cylinder walls. Engine compression is lost and the engine either stops running, or you start getting blow-by past the rings. That torches out an area. Then the engine quits.

 

Detonation is influenced by chamber design (shape, size, geometry, plug location), compression ratio, engine timing, mixture temperature, cylinder pressure and fuel octane rating. Too much spark advance ignites the burn too soon so that it increases the pressure too greatly and the end gas spontaneously combusts. Backing off the spark timing will stop the detonation. The octane rating of the fuel is really nothing magic. Octane is the ability to resist detonation. It is determined empirically in a special running test engine where you run the fuel, determine the compression ratio that it detonates at and compare that to a standard fuel, That's the octane rating of the fuel. A fuel can have a variety of additives or have higher octane quality. For instance, alcohol as fuel has a much better octane rating just because it cools the mixture significantly due to the extra amount of liquid being used. If the fuel you got was of a lower octane rating than that demanded by the engine's compression ratio and spark advance detonation could result and cause the types of failures previously discussed.

 

Detonation causes three types of failure:

 

1. Mechanical damage (broken ring lands)

2. Abrasion (pitting of the piston crown)

3. Overheating (scuffed piston skirts due to excess heat input or high coolant temperatures)

 

----------------------------------------------

 

Pre-ignition - defined as the ignition of the mixture prior to the spark plug firing. Anytime something causes the mixture in the chamber to ignite prior to the spark plug event it is classified as pre-ignition. The two are completely different and abnormal phenomenon.

[J-Madd]

Detonation - the spontaneous combustion of the end-gas (remaining fuel/air mixture) in the chamber. It always occurs after normal combustion is initiated by the spark plug. The initial combustion at the spark plug is followed by a normal combustion burn. For some reason, likely heat and pressure, the end gas in the chamber spontaneously combusts. The key point here is that detonation occurs after you have initiated the normal combustion with the spark plug.

 

Unburned end gas, under increasing pressure and heat (from the normal progressive burning process and hot combustion chamber metals) spontaneously combusts, ignited solely by the intense heat and pressure. The remaining fuel in the end gas simply lacks sufficient octane rating to withstand this combination of heat and pressure.

 

Detonation causes a very high, very sharp pressure spike in the combustion chamber but it is of a very short duration. If you look at a pressure trace of the combustion chamber process, you would see the normal burn as a normal pressure rise, then all of a sudden you would see a very sharp spike when the detonation occurred. That spike always occurs after the spark plug fires. The sharp spike in pressure creates a force in the combustion chamber. It causes the structure of the engine to ring, or resonate, much as if it were hit by a hammer. Incidentally, the knocking or pinging sound is not the result of "two flame fronts meeting" as is often stated. Although this clash does generate a spike the noise you sense comes from the vibration of the engine structure reacting to the pressure spike.

 

The high impact nature of the spike can cause fractures; it can break the spark plug electrodes, the porcelain around the plug, cause a clean fracture of the ring land and can actually cause fracture of valves-intake or exhaust. The piston ring land, either top or second depending on the piston design, is susceptible to fracture type failures. If I were to look at a piston with a second broken ring land, my immediate suspicion would be detonation.

 

Another thing detonation can cause is a sandblasted appearance to the top of the piston. The piston near the perimeter will typically have that kind of look if detonation occurs. It is a swiss-cheesy look on a microscopic basis. The detonation, the mechanical pounding, actually mechanically erodes or fatigues material out of the piston. You can typically expect to see that sanded look in the part of the chamber most distant from the spark plug, because if you think about it, you would ignite the flame front at the plug, it would travel across the chamber before it got to the farthest reaches of the chamber where the end gas spontaneously combusted. That's where you will see the effects of the detonation; you might see it at the hottest part of the chamber in some engines

 

What typically happens is that when detonation occurs the piston expands excessively, scurfs in the bore along those four spots and wipes material into the ring grooves. The rings seize so that they can't conform to the cylinder walls. Engine compression is lost and the engine either stops running, or you start getting blow-by past the rings. That torches out an area. Then the engine quits.

 

Detonation is influenced by chamber design (shape, size, geometry, plug location), compression ratio, engine timing, mixture temperature, cylinder pressure and fuel octane rating. Too much spark advance ignites the burn too soon so that it increases the pressure too greatly and the end gas spontaneously combusts. Backing off the spark timing will stop the detonation. The octane rating of the fuel is really nothing magic. Octane is the ability to resist detonation. It is determined empirically in a special running test engine where you run the fuel, determine the compression ratio that it detonates at and compare that to a standard fuel, That's the octane rating of the fuel. A fuel can have a variety of additives or have higher octane quality. For instance, alcohol as fuel has a much better octane rating just because it cools the mixture significantly due to the extra amount of liquid being used. If the fuel you got was of a lower octane rating than that demanded by the engine's compression ratio and spark advance detonation could result and cause the types of failures previously discussed.

 

Detonation causes three types of failure:

 

1. Mechanical damage (broken ring lands)

2. Abrasion (pitting of the piston crown)

3. Overheating (scuffed piston skirts due to excess heat input or high coolant temperatures)

 

----------------------------------------------

 

Pre-ignition - defined as the ignition of the mixture prior to the spark plug firing. Anytime something causes the mixture in the chamber to ignite prior to the spark plug event it is classified as pre-ignition. The two are completely different and abnormal phenomenon.

 

Excellent info. I thought I fully understood detonation already, but I know I do now.