Main cap walk

[058]

Actually about 34 or so degrees before TDC, you can change the words around if you want, but the fact remains- if the charge ignites too soon, it's gonna put some serious downforce on the rising piston, rod and crank. This will cause the main caps to walk.Not changing any words around at all. Call it what it really is: pre-ignition, the uncontrolled ignition of the fuel prior to the plug firing. detonation is the the collision of 2 or more flame fronts creating excessive pressure spikes. What you are calling detonation is pre-ignition, two different things but both will cause engine damage and not just confined to cap walk.

[Badburn]

Not changing any words around at all. Call it what it really is: pre-ignition, the uncontrolled ignition of the fuel prior to the plug firing. detonation is the the collision of 2 or more flame fronts creating excessive pressure spikes. What you are calling detonation is pre-ignition, two different things but both will cause engine damage and not just confined to cap walk.
I use detonation as a slang term. (knock, ping, pre-ignite, detonate) It pretty much all produces the same result, broken parts. I dont think there are too many people in this world that could differentiate between them-and which occured when and where. So slang works for me. Perhaps you could shed some light.

[396_WAYS_TO_SPIT]

But we are still talking more stroke so I would put a girdle on anything with an arm in it.
That sums it up for me Bob Thanks,Michael

[058]

I use detonation as a slang term. (knock, ping, pre-ignite, detonate) It pretty much all produces the same result, broken parts. I dont think there are too many people in this world that could differentiate between them-and which occured when and where. So slang works for me. Perhaps you could shed some light.
I'd be happy to shed some light on this....Pre-ignition can be identified by the melted piston usually around the edge just above the rings but not limited to that area. Seen pistons melted in the center also. Detonation can be identified by the hole in the piston,usually in or about the center. It will look like someone punched the hole with a hammer leaving jagged raw edges. It also can be i.d.ed when the engine is dissassambled and the rod bearings fall out of the rods. Detonation will hammer on the piston/rod peening the bearing to the point it has no "snap-fit" in the rods. I've seen bearings hammered so bad they are .080-.090" wider at the top than at the parting line, that is if they didn't spin first. Even the best forged/billet cranks can and will crack in the crank pin and other areas when detonation is present. It can crack blocks, main caps, cylinders, kick head gaskets out and generally destroy anything else thats in its path. Pre-ignition as a rule does not have such a devastating effect as detonation. Hope this helps.

[LakesOnly]

pre-ignition, the uncontrolled ignition of the fuel prior to the plug firing.
detonation is the the collision of 2 or more flame fronts creating excessive pressure spikes.Yep, there it is pure and simple.
LO

[Badburn]

I'd be happy to shed some light on this....Pre-ignition can be identified by the melted piston usually around the edge just above the rings but not limited to that area. Seen pistons melted in the center also. Detonation can be identified by the hole in the piston,usually in or about the center. It will look like someone punched the hole with a hammer leaving jagged raw edges. It also can be i.d.ed when the engine is dissassambled and the rod bearings fall out of the rods. Detonation will hammer on the piston/rod peening the bearing to the point it has no "snap-fit" in the rods. I've seen bearings hammered so bad they are .080-.090" wider at the top than at the parting line, that is if they didn't spin first. Even the best forged/billet cranks can and will crack in the crank pin and other areas when detonation is present. It can crack blocks, main caps, cylinders, kick head gaskets out and generally destroy anything else thats in its path. Pre-ignition as a rule does not have such a devastating effect as detonation. Hope this helps.
ok, not to say your wrong, but for the sake of conversation.......I would diagnose a melted piston as maybe a lean mixture, too much heat. How would pre-ignition create enough heat to melt a piston? I would also think that pre-ignition could produce the same jagged broken piston you describe by trying to compress an already ignited charge- this would cause extremely high cylinder pressure as the piston is coming up, which would also hammer the rod and main bearings.
Also, How would you solve a detonation problem as you describe it? What would cause two flame fronts to collide? Again, not saying you are wrong, just hoping to learn something.
.
Sorry to hijack your thread 396 Sounds like you got the answer to your question.

[058]

ok, not to say your wrong, but for the sake of conversation.......I would diagnose a melted piston as maybe a lean mixture, too much heat. How would pre-ignition create enough heat to melt a piston? I would also think that pre-ignition could produce the same jagged broken piston you describe by trying to compress an already ignited charge- this would cause extremely high cylinder pressure as the piston is coming up, which would also hammer the rod and main bearings.
Also, How would you solve a detonation problem as you describe it? What would cause two flame fronts to collide? Again, not saying you are wrong, just hoping to learn something.
.
Sorry to hijack your thread 396 Sounds like you got the answer to your question.Pre-ignition would create excess heat by allowing a longer burn and a shorter time for the heat to dissipate, don't forget with pre-ignition it usually burns the all the fuel in the cylinder from one point just as if the plug ignited it but earlier and pre-ignition does not have the hammering effect of the 2 or more flame fronts colliding. When the flame fronts collide it creates abnormally high pressure spikes in the combustion chamber and that energy has to go somewhere and usually the piston is the weak link to dissapate that energy. In simple terms: an explosion within an explosion. I have seen in a few cases holes punched in the combustion chamber roof or the quench pad and the piston survived but that was on old cast iron heads with thin walls. Material is thin, brittle and survived countless heat cycles prior to the head caving in. With Detonation one flame front will create pressure that will cause another flame front to start in an other part of the combustion chamber, hence the 2 flame fronts that usually meet in the center of the cylinder with damaging effect. It can be avoided by using less total timing, better fuel, richer mixture, keeping cylinder tempatures lower, a more efficent combustion chamber, using a flat top or reverse dome piston.[avoid domed pistons] but if you need domed pistons the domes need to be fully prepaired, and a more centrally located spark plug. Some engines are much more prone to detonation than others....Oldsmobiles come to mind here

[Badburn]

Pre-ignition would create excess heat by allowing a longer burn and a shorter time for the heat to dissipate, don't forget with pre-ignition it usually burns the all the fuel in the cylinder from one point just as if the plug ignited it but earlier and pre-ignition does not have the hammering effect of the 2 or more flame fronts colliding. When the flame fronts collide it creates abnormally high pressure spikes in the combustion chamber and that energy has to go somewhere and usually the piston is the weak link to dissapate that energy. In simple terms: an explosion within an explosion. I have seen in a few cases holes punched in the combustion chamber roof or the quench pad and the piston survived but that was on old cast iron heads with thin walls. Material is thin, brittle and survived countless heat cycles prior to the head caving in. With Detonation one flame front will create pressure that will cause another flame front to start in an other part of the combustion chamber, hence the 2 flame fronts that usually meet in the center of the cylinder with damaging effect. It can be avoided by using less total timing, better fuel, richer mixture, keeping cylinder tempatures lower, a more efficent combustion chamber, using a flat top or reverse dome piston.[avoid domed pistons] but if you need domed pistons the domes need to be fully prepaired, and a more centrally located spark plug. Some engines are much more prone to detonation than others....Oldsmobiles come to mind here
Great information..
By your definition of pre-ignition, it sounds like it would be pretty rare. Can you think of an example?
Tell me your thoughts on this.........
I had a smallblock 400 years ago in which you could blow the head gaskets on command. It was a 12:1 motor that I drove on pump gas. I found that it liked 37 degrees total timing while commuting to and from school on pump gas, however, it would pop a head gasket the instant you opened the throttle up. I could remedy this by backing the timing to 30 degrees total. The other option was to put race gas in it and put the timing back to 37, but I was broke, so I didnt do it very often.
So, without the extreme cases of melted or broken pistons, or walking main caps, hammerd bearings and such as evidence, what would you say was happening there?
Some would call it pre-ignition, because simply firing the ignition 7 degrees later solved the problem. It seems to me that abnormally high cylinder pressure would be a good thing as long as it is after TDC so that it can be used to drive the piston down. This is where the line between the two gets blurry for me. Sorry if I'm rambling...I got a few hours to kill.

[058]

Your 400 used the head gaskets as a fuse. It was the weak link. Could be worse...the gaskets could have held and the engine kicked pistons out, don't need to tell you what that would have cost you on your budget 12 to 1-37deg total on pump gas something has to give. Sounds like a typical case of detonation and the head gaskets were taking the brunt of it. Not knowing how long ago you had this problem but it could have been as simple as cheap gaskets, not torquing the heads properly from a out of calibration torque wrench or not retorquing if required, to a warped head and/or block to things like local hot spots in the block/head. I've heard that SBC 400s had some cooling issues due to the siamesed cylinders unique to the 400 block only or perhaps it was due to the thin head gasket between the cylinders. It could be a mulitude of things, take your pick.:idea: