Flatchat raised the detonation possibility on Howards oil pressure thread and asked for a " hopefully" clear description of this. As it now seems that howards problems have been due to an out of balance flywheel I will attack this detonation subject. Pre-ignition will also have to be addressed as many seem to have trouble which condition they are dealing with.
Detonation: This is an uncontrolled burn of the fuel .A smooth even burn thru the compressed mixture does not occur. The spark plug ignites the mixture and very shortly after a second or even third etc self ignition takes place in other areas within the combustion chamber and cylinder.When the flame fronts of these seperate ignitions collide the pressure rise in the cylinder is extremely rapid. As the piston is still approaching TDC you should be able to envisage that it is not going to like being told to go back down the cylinder while it has a crankshaft and conrod telling it that it has to go up approx another 1/4" before coming back down!! Every area that is exposed and connected while this takes place can suffer damage. Valves can become tuliped and will have the lash reduced as a result, Spark plug insulaters may crack,Cylinder Head studs may be stretched enough to allow the head gasket to move, Pistons may have the crown fail or ring lands break. If the explosion occurs to one side of the cyl it may rock the piston in the cyl enough to the point where the piston skirt will crack and or the cyl wall will fail. As you can now imagine these pressures are now transfered thru the wristpin ,rod, brgs,crank,brgs, and caps into the block. If the condition is allowed to continue component failure is rapid and Terminal.
Pre-Ignition: This occur's when any exposed part in the combustion chamber/cyl becomes incandesant(red hot) and ignites the fuel mixture at a very early point in the compression stroke. This could be an exposed plug thread, a sharp edge of the combustion chamber or valve pocket/piston dome,a piece of exposed head gasket or a buildup of carbon. If the engine has been run in a state of undetected detonation previously there may be damage to the valves or pistons which will now create a sharp edge to cause this.As this occurs relatively early in the compression cycle the explosion is not as violent as the Detonation ,but the heat that is generated does the damage.
As pre-ignition quite often follows detonation it can become difficult to tell which you have. In Detonation any broken pistons will be quite jagged at the fracture point. Pre-ignition damage will usually have a fused or melted appearance in the failed area.
Causes; I know I wont remember them all but here are a few with a couple of head scratchers thrown in. Over advanced ign timing both total and to fast a curve,Incorrect Fuel mixture and distribution,Poor quality fuel,Excessive oil clearances, Oil from valve guides, High or low water temp ,and poor coolant flow along with oil temp,Bad driving habits ie; lugging the motor,running in the revlimiter zone. Crossfire and phasing issues with ignition.
Now here are a couple of oddballs that drove me nut,s for a while. Pleasure boat ,SBC Electronic Ign With Knock Sensor. Both heads cracked along above spark plugs on outside. Owner reckoned Frost damage and cringed at Mercruiser replacement price, I suggest and procure Dart equivalent @ 1/3 price. Fit to motor & run in shop. Owner fits to boat, Icheck ign timing(total& Initial) Owner rings on Saturday ,V happy all well. 2 pm Sunday rings, boat runs terrible fellow boatee reset ign timeing which he said was 15deg retarded. Tells me Im a D****!! Boat in driveway with angry owner monday morning. Check Ign Timing- now 45 deg total!! Owner suggests my timeing light stuffed leaves in huff. Pull engine down ,Detonation City!! Problem -never was any frost damage, Cause-Faulty Knock Sensor that thought it had a part time job along with Fuel that had Gone OFF.
The other one concerns MSD and two crossed wires, get a copy of Circle Track june 2005 and read the article.(not MSD's problem but the human factor coming into play)
Well Flatchat ,thats it in a BIG nutshell. Any wiser or totally confused??
Cheers Jack.
Detonation: This is an uncontrolled burn of the fuel .A smooth even burn thru the compressed mixture does not occur. The spark plug ignites the mixture and very shortly after a second or even third etc self ignition takes place in other areas within the combustion chamber and cylinder.When the flame fronts of these seperate ignitions collide the pressure rise in the cylinder is extremely rapid. As the piston is still approaching TDC you should be able to envisage that it is not going to like being told to go back down the cylinder while it has a crankshaft and conrod telling it that it has to go up approx another 1/4" before coming back down!! Every area that is exposed and connected while this takes place can suffer damage. Valves can become tuliped and will have the lash reduced as a result, Spark plug insulaters may crack,Cylinder Head studs may be stretched enough to allow the head gasket to move, Pistons may have the crown fail or ring lands break. If the explosion occurs to one side of the cyl it may rock the piston in the cyl enough to the point where the piston skirt will crack and or the cyl wall will fail. As you can now imagine these pressures are now transfered thru the wristpin ,rod, brgs,crank,brgs, and caps into the block. If the condition is allowed to continue component failure is rapid and Terminal.
Pre-Ignition: This occur's when any exposed part in the combustion chamber/cyl becomes incandesant(red hot) and ignites the fuel mixture at a very early point in the compression stroke. This could be an exposed plug thread, a sharp edge of the combustion chamber or valve pocket/piston dome,a piece of exposed head gasket or a buildup of carbon. If the engine has been run in a state of undetected detonation previously there may be damage to the valves or pistons which will now create a sharp edge to cause this.As this occurs relatively early in the compression cycle the explosion is not as violent as the Detonation ,but the heat that is generated does the damage.
As pre-ignition quite often follows detonation it can become difficult to tell which you have. In Detonation any broken pistons will be quite jagged at the fracture point. Pre-ignition damage will usually have a fused or melted appearance in the failed area.
Causes; I know I wont remember them all but here are a few with a couple of head scratchers thrown in. Over advanced ign timing both total and to fast a curve,Incorrect Fuel mixture and distribution,Poor quality fuel,Excessive oil clearances, Oil from valve guides, High or low water temp ,and poor coolant flow along with oil temp,Bad driving habits ie; lugging the motor,running in the revlimiter zone. Crossfire and phasing issues with ignition.
Now here are a couple of oddballs that drove me nut,s for a while. Pleasure boat ,SBC Electronic Ign With Knock Sensor. Both heads cracked along above spark plugs on outside. Owner reckoned Frost damage and cringed at Mercruiser replacement price, I suggest and procure Dart equivalent @ 1/3 price. Fit to motor & run in shop. Owner fits to boat, Icheck ign timing(total& Initial) Owner rings on Saturday ,V happy all well. 2 pm Sunday rings, boat runs terrible fellow boatee reset ign timeing which he said was 15deg retarded. Tells me Im a D****!! Boat in driveway with angry owner monday morning. Check Ign Timing- now 45 deg total!! Owner suggests my timeing light stuffed leaves in huff. Pull engine down ,Detonation City!! Problem -never was any frost damage, Cause-Faulty Knock Sensor that thought it had a part time job along with Fuel that had Gone OFF.
The other one concerns MSD and two crossed wires, get a copy of Circle Track june 2005 and read the article.(not MSD's problem but the human factor coming into play)
Well Flatchat ,thats it in a BIG nutshell. Any wiser or totally confused??
Cheers Jack.
