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I think it means your timing is out and the spark is igniting the mixture way before the piston has reached the top of its stroke. Not good. My first mini did this and ended up with lots of indentations in the tops of the pistons and a hole the size of a 50p piece blown through the top of one piston!
 

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it does leave pin pr1ck like marks and can lead to holes in pistons yes
edited as you cant say *****lmao
 

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Basically as you have an explosion going on in the chamber and the piston is still rising up against it, the force of the explosion stresses the crown and blows bits out of it. Just like throwing a grenade really. Add to this that the mixture has probably not been fully fueled yet so you get a series of explosions as more fuel/air is fed in - hence the pock markings.

As usually the piston is about to go down at the time of the explosion, the force exerted on the piston is less so it doesn't break.
 

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richie00boy said:
Basically as you have an explosion going on in the chamber and the piston is still rising up against it, the force of the explosion stresses the crown and blows bits out of it. Just like throwing a grenade really. Add to this that the mixture has probably not been fully fueled yet so you get a series of explosions as more fuel/air is fed in - hence the pock markings.

As usually the piston is about to go down at the time of the explosion, the force exerted on the piston is less so it doesn't break.
I see what you mean, i take it this also can happen if there are hotspots in the chambers to ignite the mixture before the spark?
 

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TurboCavGSI said:
I see what you mean, i take it this also can happen if there are hotspots in the chambers to ignite the mixture before the spark?
thats pre-ignition which is different again

rodgerq
 

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rodgerq said:
thats pre-ignition which is different again

rodgerq
So if the sparks fires to early that is DET but if the mixture ignites before the spark thats pre ignition?

Can the effects be the same as if it ignites on the up stroke cant that cause damage?
 

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a simple answer (but likely its not totally correct but easy to understand..)


there is no "explosion" tha happens in the cylinder...its a controled burn!

the piston is pushed down with a layer of gas before the flamefront

this protects the piston from heat as the flamefront does not actually come into contact with the piston

detonation is when the charge ignites by itself before the spark hits the charge

its like a lightening bolt (explosion) that kicks the gas out of the way and damages the piston in a pinprick type mark

im sure someone has a better description of detonation than i, wheres chip!:confused:
 
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detonation is where the mixture ignites before the spark happens. As then the piston is still travelling up the barrel, the explosion is at exactly the wrong time and tries to push the piston etc back down the barrel. Imagine the forces on piston, rod and crank with a explosion trying to stop it dead and force it backwards. Not good..
It can also mean the flame is started in more than one place at a time, and you can have lots of flame fronts which collide and cause explosions that are more powerfull (and more destructive) than a normal burn.. If you know diesels, they rely on detonation to ignite the mixture full stop, which is why they have to have thick smelly detonation resistant fuel to stop them firing too early and why petrol in a diesel motor is a Bad Thing...
If your motor starts to detonate, you can usually hear a rattling sound, this as you can imagine isnt a good thing for the engine, and if you hear the rattle then damage is already occuring.
Also a motor detonates depending on how much you squash the air fuel mixture, as highly squashed mixture is more likely to explode without the aid of a spark, so a motor with eg a CR of 6.5:1 will run on fuel thats only fit for a stationary engine as its rated below 90, yet if you ran 12:1, it would probally self destruct at the mere smell of that fuel. Other factors affect it too, like the shape of the combustion chamber (if its a odd shape, bits of it get squashed more than others first effectively locally making the CR higher in that spot), heat retension in the chamber (a hot spot warms up the mixture making it more likely to self ignite, think air cooled 2stroke near the exaust port), air intake temperatures (warm mixture burns easier, and a pain for turbo guys, since the turbo itself heats the intake air). And lots and lot of other factors..


Fuels resistance to detonate is called its anti knock rating, thats what 95 or 98 denotes, its rating in RON (in europe). 98 is more resistant than 95. They change the knock resistance of fuel by blending in different stuff (used be lead but thats all but gone) that isnt as explosive. The little cans of octane booster is usually mostly toulene for e.g.
In a test lab they (used to) measure the rating of a fuel with a dirty big single cylinder test engine, that has a adjustable compression ratio (part of the head moves in and out on a wheel) and when the fuel just starts to detonate on the rig, this is its rating as calibrated. Theres probally some fancy chemical test now...
Its also worth noting that higher octane fuel is actually less explosive, so if you can get away with 98, and your running 105, your actually putting in less explosive fuel than you need to be and slightly reducing your power
 

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its a shame most companies dont release the MON or motor octane number and stick with the RON(research octane number) as the MON value is taken under conditions much more akin to a high performance petrol engine
 
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