Effects of using bent electrode plugs.

[Frank Acampora]

These photos show the neat hole melted in the piston crown at the sparkplug. Note that there is no other damage to the piston other than a bit of melting in the general area of the hole. All four pistons were damaged like this yet the engine did not have any other problems. Timing was correct and carbs were set properly. Thus, the damage must have been caused by the plugs. Typical detonation damage destroys the rings and occurs at the exhaust side of the piston so this is atypical damage not due to lean running or advanced timing.

I bought this particular extra long shaft Chrysler 140 because it was a 25 inch shaft. When I got it home, the first thing I noticed was the incorrect OMC plugs, QL77---. A compression test revealed no compression on all four cylinders. Removing the head showed no cylinder damage and crosshatching was still visible.



A set of Wiseco pistons installed through the reed opening corrected the damage.

 

[TwoFish]

Re: Effects of using bent electrode plugs.

Thanks for posting the photos Frank.

I?ve learnt so much from this forum and posts like this.

Nothing like a photo to get the message over.

It never ceases to amaze me how many ways there are to destroy an engine.

 

[Robert D]

Re: Effects of using bent electrode plugs.

Are you thinking an incorrect heat range on those plugs? That's an expensive error to make!

 

[fucawi]

Re: Effects of using bent electrode plugs.

Because and enging runs normal plugs rather than surface discharge does not make the combustion any hotter....indeed the piston will be a long way down the bore when the plug fires and not be sparking when the piston passes it ....the hole is not where it is because of plug position but is the weakest part of the piston ..the pre ignition has overheated the piston and the centre has melted. Pre ignition caused by glowing particles on the cylinder head ie in this case the plug centre electrode.. which often also melts ...igniton is not started by the spark but by the glowing centre electrode. I dont know this QL 77 plug ( being in the UK) but the std style plug from a similar Chrysler with the same or higher compression/carburation should not cause pre ignition.. Avoid extended nose as these move the initiation point of combustion nearer the centre of the chamber and put the centre electrode of the plug out in the "wind" of the incomming charge making the pre igniton easier.
Research on the champion web site shows that the surface discharge plug does not have a heat range so the plug stays at the cylinder head temp..so the coldest you can get is a good starting point.
Just had another thought....

A standard plug keep cool by loosing heat through its seat onto the cylinder head and hence into the water ...I dont have a head to examine but do the plugs seat properly and is the surrounding area cooled? Could be the head on a surface discharge is a different design because the plug does not need cooling...??

 

[pnwboat]

Re: Effects of using bent electrode plugs.

There is a water jacket in the head that runs very close to each of the spark plug holes. Attached is the cylinder head cover that has been removed exposing the water passage in the head.

 

[Frank Acampora]

Re: Effects of using bent electrode plugs.

QL 77---I don't remember the rest ---is a standard bent electrode OMC plug. Runs fine in "Rudes.

Fucawi: We are not talking hotter combustion temperatures. I did not say that! Please do not misquote me. Burning gas is burning gas and under NORMAL conditions burns with the same temperature. However, the surface gap electrode plugs specified for the larger electronic ignition engines are dead cold! They have no center electrode porcelain insulator exposed to combustion heat and the entire center electrode except for the firing point is sheathed in the plug jacket, thus cooled by the head. The bent electrode plug, no matter what type will always be much hotter.

Now, since for maximum horsepower generation the air/fuel mix must burn equally before and after TDC, and since it takes a finite time for the mix to burn, we start at idle by firing the plug at or near TDC. As engine speed and power is increased we fire the plug earlier, to compensate for increased piston speed, up to (on Chrysler) 32 BTDC. This corresponds to (due to crankshaft stroke length) around .028-.032 inches before top dead center. You can see that the piston is not way down in the bore at the time of firing. It is at this point at or near WOT, or somewhere within the beginning of the burn cycle that the Bent electrode plug, running hotter will LOCALLY pre-ignite the mix, causing local overheating and melting of the piston right under the plug--AT its thickest and strongest point: The baffle.

AND: You probably will not hear the pre-ignition audible "ping" under these conditions. The ping occurs when glowing deposits ignite the fuel mix in a different area than the plug. When the plug fires, the two colliding flame fronts produce the sound. Here, we have the plug prematurely igniting the mix then firing in an already burned portion--there are no flame fronts to collide and make noise.

Note that this is not a given or absolute occurence: A few on the forum have changed to bent electrode plugs and are very happy with no apparent problems. The hotter plugs do improve idle and starting by reducing fouling. However, my thought is: Why take a chance?

 

[fucawi]

Re: Effects of using bent electrode plugs.

1/ Frank I have not quoted you ..what I said was part of my argument.
2/ I fully understand igniton having spent my life lecturing on the subject to thousand of students.If I dont know what I am on about there are thousands of invalid Phd s out there !!!
3/ do not confuse "ping" which is detonation with pre igniton which is silent. pinging or pinking in europe is the explosion of the end gasses due to excessive pressure ..nothing to do with glowing particles thats pre ignition . Pinging is caused by a small amount of over advanced ignition...and yes continuous pinging can lead to pre ingnition ...complex and interesting isnt it .....All of us must be very specific in the terms we use when talking detonation and pre igniton not to confuse the two or the readers.

All the surface discharge plugs I have seen have been taper seat .originally devised for Rotary engines ..I wondered if some outboard owners were using standard seat plugs in taper fittings hence the overheat....

 

[tater76]

Re: Effects of using bent electrode plugs.

Ok, so now I am confused here folks. I know that both Frank A and fucawi have read my threads on surface gap and electrode plugs, and the issues I am dealing with. With that said, I am now feaking out with this commentary! I am running electrode plugs on my 1985 Force electronic ignition, which I to my understanding is a hotter system than the mag ignition it had. I was under the impression I had to swap the electrode plugs for surface gap or face issues like the ones in the pics Frank posted. I am the guinea pig here fellas, I have run bent electrode Champion L82C, NGK BR6HS, and Autolite 4093 with no ill effects! I have also run surface gap L20V, UL18V, and Merc Champion L76V. The engine did not like the Merc Champion L76v, or UL18V, however the L20V will idle and run although rough. I have 20 plus hours running the NGK BR6HS plugs with NO issues. Timing and carb are correctly adjusted. SO.... this is obviously not a cut and dry situation, could I melt a piston like the one in Frank A's pics? Absolutely! Is there any damage? not yet. Does it run well with no pinging/knocking/"pinking"? Yes it does. I think what we need to take away from this thread is that running a plug that is not designed for your motor can, does, and will cause damage. All plugs I mentioned were designed to fit my motor in both its ignition configurations, Mag/Electronic. I have melted pistons before in this motor, although it was from running lean. I am not willing to rebuild it again, so I am playing it easy when it comes to plugs and such. Anyway, hope that helps.

 

[fucawi]

Re: Effects of using bent electrode plugs.

this is not a confusing issue ....
have you got the coldest plug possible with the correct seating ?
Is the timing correct or certainly not over advanced?
Is the mixture right .?
If so then its your decision if you use the plugs ..your wallet not mine ..
If it blows a hole in the piston get the tig welder out .........
there is no such thing as a hot igniton that is american techo speak crap ...a spark is a spark ..you can light a cigarette with a match or oxyacetaiine still gets lit ...( and you still get cancer )

 

[tater76]

Re: Effects of using bent electrode plugs.

I mean no disrespect as I am still learning about ignition on these old motors, so bear with me. When you say cold plug, where do you find this rating or info? As far as seating, how would you know "for sure" how far the plug is seating within the head without pulling the head and making measurements? I mean the measurement of how much the plug protruding into the chamber is pretty important stuff right? My timing and carb have been tuned and checked "a dozen times now to be exact :facepalm:" Now, I am confused as to hotter ignition comment you made. From what you said I gathered that the plug is what controls the amount of electricity that makes the arc? I was under the impression that the coil or magneto was responsible for the amount of energy flowing to to plug? When I think of hot ignition, I think of automobile engines with an MSD box that jumps the power going to the ignition two or three fold! This supposedly creates a cleaner, hotter, and more efficient spark. As for going from magneto to electronic in my engine, I would figure the output of electricity would increase? I understand that I don't fully understand ignition systems yet, so as before, bear with me. I am not trying to start a war here folks, I want to learn from what sounds like ALOT of years of experience from you all.

 

[Frank Acampora]

Re: Effects of using bent electrode plugs.

You may have lectured a lot BUT--- Your statement that ignition takes place when the piston is well down in the cylinder is blatantly wrong AND your statement that the damage in the center of the piston is at the weakest point is also incorrect. To my thinking, this puts the rest of your statements as suspect too. However, not wanting to argue, I will defer to your expertise.

Tater: Obviously we are using colloqialisms when we talk about a "hotter" spark. what we really mean is that the ignition system delivers more voltage to the plug. NOW: If the plug has high resistance for any reason, the spark will have trouble jumping the gap. It may appear to be a bit weak and rather red/orange, but any spark will ignite the air/fuel (within limits--I am giving generalities.) With a higher voltage ignition system the spark will be more consistent and may appear blue-white. As with flames, blue IS hotter than red or orange but again, anything that lights the mix is enough. The hotter spark will jump a bigger gap and will feel less effects from fouling, so the higher voltage ignition is desirable.

As for your plugs, remember that others have used bent electrode plugs with reported good results. It is not an absolute that damage will occur, just a strong possibility.

To those on the forum: Do not take as gospel anything written here by me or anyone else. Use it as a starting point for your own thought process. And remember, ultimately, you must learn, judge the information, and make your own decisions.

 

[tater76]

Re: Effects of using bent electrode plugs.

Well put Frank, so with that said, say I can get my motor to run surface gap plugs consistently. Is there any added gain between the two styles that you or anyone knows of?

 

[fucawi]

Re: Effects of using bent electrode plugs.

first Tater76 ...no advantage in either plug regarding power/ running but a surface discharge plug is not so prone to foul up when used with a high energy ignition ( I did not say high voltage). If your engine normally takes either a taper seat plug or conventional then if you change to wire electrode then it must be the same taper or conventional or it will overheat and pre ignite the fuel.The heat in the centre electrode passes to the cooling via the seat and by the way the plug reaches operating temp in 3 firing of the engine..
Hi Frank ..I have had these conversations a thousand times ....
AT WOT the spark will occur about 36 deg BTDC and will be gone so ignition will start at this point. I dont see how you can say that is wrong. There is a big difference between American English and European English and this often causes misunderstanding to those not aware of this .

You also stated combustion equally each side of TDC...I think you mean peak cylinder pressure just after TDC .I dont think you can give percentage volumes to the combustion required for this ..PV diagrams dont support your assumption.

The hole in the piston is where the temperature/thickness/pressure causes the piston crown to collapse.. The piston is the worst cooled part of the combustion chamber and hence under pre ignition becomes the hottest first ..of course it is an alloy with a low melting point.

This spark colour being important is a load of rubbish ...the spark colour is determined by the metal its jumping from as its the glowing particles of metal that help colour the spark as well as the gas in the gap ( neon signs ??) .... A spark is a current in air ..air is a gas

Spark voltage is not as high as most people think ..about 5KV to start the spark dropping to 2kv once its initiated ....at WOT raise that to 12kv and 4 kv....A plug which is physically cold ..ie at 15 deg C will take 40% more volts to fire it hence your ignition system needs extra capacity for the starting phase. Do not confuse with a cold /hot plug that equates to the centre electrode temp when the engine is running. Too hot it pre ignites too cold it fouls.

The term HOT spark is a sort of american hot rod expression not technical. This is generally related to the voltage an igniton can produce ..maybe 40kv BUT the plug only uses the voltage it requires to initiate the spark and then the energy available is dischaged into the spark ....Hence the term high energy igniton is a better term than HOT.

Your last paragraph is interesting ...due to your highly respected positon in this forum anything you state will be taken as true and in 99% of cases this will be OK .
I take another view ...I only give information on things I am sure about..not gearboxes.bearings etc and if someone has been given 5 bull**** suggestions by different members in a post I keep away even if I know the problem as they are usually so confused that I would be wasting my time ...

Now lets all go have fun with those Chryslers ,,bought a small block 30 hp electric start 1978 yesterday only $ 35........must go sort its problems.....may need your advice soon ...