detonation

[twizted269]

so ive got a good idea of what it is just want to know exactly what causes it. for instance, everyone has concluded that my plugs are backing out from detonation. all i have done is put a cool head on since stock and it never has happend before that. is it from the compression change? went from 110 to 150. also can reeds cause this? it ran rich before but seems lean now. running 93 octane 300 mains, 30 pilots, middle clip, at about 600 elevation. looking for a better explenation. thanks.

[sheerider1026]

yep.. more compression can ause detonation.. throw some more fuel at it, and keep an eye on those plugs.. are you at least using 93 octane fuel?

[swrbansheeboy]

yes, your engine is more than likely backing the plugs out due to detonation and it's more than likely because of your higher compression ratio from your cool head.

 

now for the hard part, an explanation of what and why.....

 

now i'm sure someone just like me who loves to argue will come in and tell me i'm wrong but i'll give it my best shot explaining this so if i miss something forgive me....

 

you know how they can compress coal so hard that it heats up and becomes a diamond? well if your familiar with this then you know that compressive force creates heat. now just as with the coal and diamond thing when you squeeze stuff inside your banshee engine it creates heat also, were not talking about heat from combustion when the fuel and air are ignited. were talking about pre heating your fuel/air mixture by squeezing it inside the cylinder before it gets touched off by the spark plug.

 

now the way this works is that the higher your compression the hotter the fuel/air mixture trapped inside your cylinder gets because it gets "squeezed" harder creating more heat before ignition. now you may ask why does this have anything to do with my plug problem? we'll get to that in a second.

 

now the hotter the fuel/air mixture is thats about to be ignited the easier it is to ignite which would normaly be a great thing if you didnt have this stupid timing thing to deal with.

 

most all of us know about top dead center and what it is, you basicly want your fuel air mixture to go bang when the piston is right after tdc. now if your fuel is pre heated from to much compression and therefore way easier to ignite then it will have a tendency to go boom before it's supposed to. now if you look at your perfect timing as a line that shouldnt be crossed and when the boom happens as something that has to happen on or after it crosses the line this will get realy easy for you.

 

several things can effect if the boom happens before/on or after the line...

 

higher compression moves the boom backward behind the line...

advancing your timing moves the boom backward behind the line....

 

running higher octain fuel moves the boom forward away from the line...

backing your timing off moves the boom forward away from the line...

 

now, you might wonder why everyone always ask's/recomends that you up the octain fuel your running when you are having problems from to much compression. contrary to what many people think the higher the octain rating your fuel has does not equate to how much more it blows up when ignited or that it makes more power because it has that many more octains in it than regular pump gas. it actualy is a measurement of how resistant the fuel is to ignition, the higher the octain the more heat it takes to ignite it.

 

why on earth would you want to use fuel thats harder to ignite you may ask? simple, because you have to much compression thats pre heating your fuel mixture causing it to ignite before it's supposed to. by using the higher octain/harder to ignite fuel your getting your fuel mixture to ignite like it's supposed to.

 

now as to why your plugs are backing out, are you familiar with acoustics? basicly whats happening is that the violent boom caused by the fuel/air ignition is happening before it should and it's reverberating inside your cylinders longer than it usualy does before the exhaust ports open. this causes a resonance that just happens to vibrate your plugs loose.

 

back your timing down or run higher octain fuel to get the boom where it needs to be and this resonance will start occuring late enough that it wont get a chance to shake your plugs loose.

 

now this may not make any sense and as i said before i'm sure i forgot a bunch of stuff (like flame fronts /oil mixtures/ carbon build up and what-not) but hey, i tried dammit!

 

basicly your fuel heats up too easy causing it to ignite to early making a loud bang that comes to early causing your spark plugs to rattle out of there holes, if you want to solve the problem back your timing down or start running higher octain fuel...

[poorboystoys]

yes, your engine is more than likely backing the plugs out due to detonation and it's more than likely because of your higher compression ratio from your cool head.

 

now for the hard part, an explanation of what and why.....

 

now i'm sure someone just like me who loves to argue will come in and tell me i'm wrong but i'll give it my best shot explaining this so if i miss something forgive me....

 

you know how they can compress coal so hard that it heats up and becomes a diamond? well if your familiar with this then you know that compressive force creates heat. now just as with the coal and diamond thing when you squeeze stuff inside your banshee engine it creates heat also, were not talking about heat from combustion when the fuel and air are ignited. were talking about pre heating your fuel/air mixture by squeezing it inside the cylinder before it gets touched off by the spark plug.

 

now the way this works is that the higher your compression the hotter the fuel/air mixture trapped inside your cylinder gets because it gets "squeezed" harder creating more heat before ignition. now you may ask why does this have anything to do with my plug problem? we'll get to that in a second.

 

now the hotter the fuel/air mixture is thats about to be ignited the easier it is to ignite which would normaly be a great thing if you didnt have this stupid timing thing to deal with.

 

most all of us know about top dead center and what it is, you basicly want your fuel air mixture to go bang when the piston is right after tdc. now if your fuel is pre heated from to much compression and therefore way easier to ignite then it will have a tendency to go boom before it's supposed to. now if you look at your perfect timing as a line that shouldnt be crossed and when the boom happens as something that has to happen on or after it crosses the line this will get realy easy for you.

 

several things can effect if the boom happens before/on or after the line...

 

higher compression moves the boom backward behind the line...

advancing your timing moves the boom backward behind the line....

 

running higher octain fuel moves the boom forward away from the line...

backing your timing off moves the boom forward away from the line...

 

now, you might wonder why everyone always ask's/recomends that you up the octain fuel your running when you are having problems from to much compression. contrary to what many people think the higher the octain rating your fuel has does not equate to how much more it blows up when ignited or that it makes more power because it has that many more octains in it than regular pump gas. it actualy is a measurement of how resistant the fuel is to ignition, the higher the octain the more heat it takes to ignite it.

 

why on earth would you want to use fuel thats harder to ignite you may ask? simple, because you have to much compression thats pre heating your fuel mixture causing it to ignite before it's supposed to. by using the higher octain/harder to ignite fuel your getting your fuel mixture to ignite like it's supposed to.

 

now as to why your plugs are backing out, are you familiar with acoustics? basicly whats happening is that the violent boom caused by the fuel/air ignition is happening before it should and it's reverberating inside your cylinders longer than it usualy does before the exhaust ports open. this causes a resonance that just happens to vibrate your plugs loose.

 

back your timing down or run higher octain fuel to get the boom where it needs to be and this resonance will start occuring late enough that it wont get a chance to shake your plugs loose.

 

now this may not make any sense and as i said before i'm sure i forgot a bunch of stuff (like flame fronts /oil mixtures/ carbon build up and what-not) but hey, i tried dammit!

 

basicly your fuel heats up too easy causing it to ignite to early making a loud bang that comes to early causing your spark plugs to rattle out of there holes, if you want to solve the problem back your timing down or start running higher octain fuel...

pretty well said :thumbsup:

 

[sheerider1026]

:thumbsup: a lean condition can cause detonation also, 150 psi should be good on 93 octane.. i am not disagreeing with the above ^^^^ at all.. i would richen up the main jet and go from there.. i am always cheating, so i try to fight the deto demons by throwing more fuel to it.. just my .02

[AKheathen]

if your static compression was 110 before, and it's 150 now, then you raised your dynamic compression a little higher than you may think. now the burn is more on the chamber than it should. ideally, you want the burn more in the chamber, allowing the incoming charge to cool the surfaces more between cubustion cycles. detonation is actually a result of surface temperatures being too high. surface burning really only happens tward the end of the burn, so burning too soon from timing, and too quick from compression, or low grade fuel can cause more heat on the surfaces. high surface heat will ignite the fuel as the compression increases. this is the detonation process which begains the burn from the surface, and accumilates heat higher with every stroke, causing the physical damage. you can combat this by igniting later in the stroke, using fuel that burns longer in th chamber, lowering the compression for a less energetic burn spread, or adding more fuel to cool faster and short circut the burn via lack of oxygen. mid life compression should be a little bit higher on static, which is ok, but when you jumpped to 150, your compression ratio jumped higher, but is giving a respectively lower reading on the test.

[dajogejr]

AK...if I didn't know better, sometimes I'd swear you actually know what you're talking about....

 

Less Energetic Burn Spread? Remind me not to play you in scrabble.

 

Bottom line is Cranking compression (what you measure with the gauge by kicking it over) does not determine octane need.

Compression ratio (in short, total chamber volume vs. trapped chamber volume) does.

Compression ratio more so than a timing advance will require more octane.

I think you're fat on your pilots, lean on the mains...and you need a little more octane.

 

Have you checked the deck surfaces for flatness. You changed the head out but have you checked the squish?

Are you the same guy that said your mic only went to .035 and yours was "a bit more"???

 

Does the compression gauge you're using have the same length of threads as the actual spark plug, in other words...does it displace the same amount of volume as the spark plug while in the head?

 

If not...your stock reading could very well be 125PSI and with the head/domes it could be in the low 170s.

Combine higher compression (yeah, I know I preached about compression ratio...this is an EXAMPLE...), lack of fuel AND too tight of a squish band...you're asking for deto...

[twizted269]

well i definatly have a better insight of what going on now. thanks guys. ill get to it when im back from vacation and post up the results of everything mentioned above. first off ive got some 22cc domes from a buddy to borrow just for testing purposes. more or less shits and giggles. then moving one to tuning in my domes. thanks again. great help here.

[sheerider1026]

here is my view on this.. i assume the bike was in perfect tune before installing the head?? and you bolted the head on and started to get deto?? i know this will start an arguement, but compression and timing DO affect jetting.. compression and timing promote more complete burning of all the fuel mixture in the cylinder, so the higher the timing and compression, the more fuel you can squeeze in and burn.. so your condition sounds like you are lean, 150 psi should be fine with 93 octane.. its not like you are running a drag ported, stroked, and extremely high compression bike.. bolting a coolhead on a stock banshee motor and getting 150 psi is a pretty normal thing.. it happens all the time.. how many people do this and never check squish?? A LOT.. and they never have a problem.. they just rejet the bike and all is well.. i suggest rejetting and going richer on the mains.. i bet that your deto problem goes away, and the bike will run like a raped ape.. now if you did a lot of things at once, such as a stroker crank, porting, and extremely high compression, i would suggest checking squish, run some higher octane fuel, and jet accordingly.. just my .02

[dajogejr]

Compression doesn't burn more fuel. It creates more heat...more fuel "cools" the motor.

Advancing the timing doesn't burn more fuel. It starts to burn the fuel that you have earlier.

 

In extreme cases, and I'm talking high compression, highly advanced timing drag motors, you tune by more or less fuel for overrev, and timing as well...

 

The reason I check squish is because you never know who cut what dome. Domes really aren't one size fits all.

If someone cuts a dome with a squish range in mind for a user with a .012 base gasket, then they throw a .020 under there....it's incorrect. And vice versa.

 

It takes minutes to check squish... The default answer isn't always throw more fuel at it....

[sheerider1026]

so dave, are you saying that a fuel/air mixture that is under more compression is not any more explosive than with less compression?? i have to disagree.. heres an example for you.. lets take a firecracker for example,, it has gun powder that is tightly wrapped and compressed inside, light it and it goes BOOM! unravel that firecracker and spread the powder on the ground, now light it. no boom, and it burns slower.. but of course like said before, not having the bike here for us to see, we can only give little help and pointers as advice. and throwing more fuel at a motor is not a "default" answer that i always have. when you have a fuel mixture under more psi, and the spark is advanced, that mixture is going to burn more complete, and faster.. let say for anothe example. dave, lean your bike out until it detonates, now start lowering the compresion and timing!! i bet that the detonation will start to disapear as yo lower your timing and compression..

[blowit]

so dave, are you saying that a fuel/air mixture that is under more compression is not any more explosive than with less compression?? i have to disagree.. heres an example for you.. lets take a firecracker for example,, it has gun powder that is tightly wrapped and compressed inside, light it and it goes BOOM! unravel that firecracker and spread the powder on the ground, now light it. no boom, and it burns slower.. but of course like said before, not having the bike here for us to see, we can only give little help and pointers as advice. and throwing more fuel at a motor is not a "default" answer that i always have. when you have a fuel mixture under more psi, and the spark is advanced, that mixture is going to burn more complete, and faster.. let say for anothe example. dave, lean your bike out until it detonates, now start lowering the compresion and timing!! i bet that the detonation will start to disapear as yo lower your timing and compression..

 

 

 

I don't think we can compare fire crackers to IC engine combustion. You refer to combustion as an "explosion" and that is what is called detonation in an IC engine. In a fire cracker, gun powder is not the propellant The oxidizer is a 600-800 mesh indian black head Al powder in which confinement is required for burn because it already has the oxidizer in the chemical compound.

 

I think the terms "squish action" and "compression" are getting confused. Squish action determines vaporization of fuel and turbulence of charge BTDC. A rich mixture offers a cooling effect for a couple reasons. A, not all of the fuel will burn and dilute the burn. B, heat of vaporization kicks in and cools the chamber more as it is vaporized.

 

 

 

Brandon

Edited March 21, 2009 by blowit

[sheerider1026]

i used the firecracker as an example.. lol.. and yes i agree, more fuel will let the engine run cooler, but less fuel casues more heat (lean condition). im not saying load the engine up with fuel to the point that you are throwing unburnt waisted fuel out of the pipes. just TRY jetting a little ritcher in this situation, and yes, checking squish IS important, but it is done more on stroker engines etc. a stock motor with bolt on's hardly ever gets checked by your average joe.. less fuel, more timing, and compression all create more heat.. every engine setup will require tuning of its own. yes there is a close "ballpark figure" to tuning on most engines.. there are always different ISSUES that can occur also.. such as improper squish, poor grade fuel, and so on. as for an explosion, i never refereed the IC of an engine being an explosion, i was referring to the firecracker EXAMPLE.. there is a difference between an explosion and combustion. not enough fuel can cause detonation. can somone argue otherwise?

[blowit]

so dave, are you saying that a fuel/air mixture that is under more compression is not any more explosive than with less compression??

 

 

Volatile = correct word.

 

I get where you are coming from. I suck at explaining things myself.

 

 

By the way, if you do have a large enough pile of black cat propellant ( flash powder), it will certainly go boom. Now you put that stuff in a shot gun shell, you are taking a trip to the ER, that is a guarantee. JFYI I think our pyrotechnics experience has really helped in our understanding of burn speeds and how it affects work done or parts failure.

 

 

 

Brandon

Edited March 21, 2009 by blowit

[sheerider1026]

Volatile = correct word.

 

I get where you are coming from. I suck at explaining things myself.

 

 

 

 

Brandon

 

 

yea. i suck at explanations also.. at least you get where i am coming from.. lol..