Spark plug fouling question ???????

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duster360

duster360

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Back around 1990 my 340 had edelbrock intake, headers, crane cam with 278 duration and still had the factory forged pistons ( advertised 10.5:1 ). But had the thicker felpro blue head gaskets. I am certain because of that and other variables about the factory J heads, compression was most likely down around 9.5ish:1. Now here is my question: At the time I was using 93 octane fuel and timing was just set by ear. I had been using the factory recommended RN9Y spark plugs. At some point one of them fouled. I had a new set of champion RN12Y plugs on hand that were intended for a bone stock 1978 360 engine. So I threw them in the 340 and it was idling as usual. I take it out for a drive and it doing well. I hit the throttle, wind it up over 5000 rpm and it starts missing so bad I barely make it home. I buy some RN9Y plugs and put in it. Never had another problem. What caused the RN12Y plugs to foul. I know the RN12Y plug is hotter, but what happened?
 
It was not fouling the plugs, it was pre-igniting because the plugs you installed were a higher heat range and it was retaining too much heat. The higher compression simply did not like the higher heat range spark plug.
 
278/110 in at 106=ICA of 65*
Static compression ratio of 9.5:1.
Effective stroke is 2.55 inches.
Your dynamic compression ratio is 7.54:1 .
Your dynamic cranking pressure is 148.00 PSI.
V/P (Volume to Pressure Index) is 116

duster360 said:
At the time I was using 93 octane fuel and timing was just set by ear. Never admit to something like that. Say something vague like; I was experimenting with how much punishment my factory forged pistons could really survive, with an undetermined amount of power-timing
Click to expand...

Overheated plugs, my guess too.But if problems continued on the way home at lower rpms,like you allude to, it may have been because one or more spark-plugs actually puked their insulators into the engine,or just cracked them, and were short circuiting, or open circuiting, or in some other way.....not firing at the correct time.

You remember all that from 28years ago? Dayng-it, I wish I could remember last year...lol.
 
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I quit caring what guys do BUT I will tell you something an oldtime Mopar engine builder told me. Around 42-44 years ago I was really into the street racing scene with my 1972 340 (4 speed) duster. we would street race 4-6 times a week and 2 times a week at the track. If I had a race that night there was a hill 4 miles from my house and i would have to go up it and jump on it in 3 rd gear. My car would backfire twice then run great. When I told this oldtimer my story he looked at me and said "Champion plugs right) and I said yes. He told me to pull them out, get a set of Autolite plugs and gap them at .028 and try it. My car NEVER backfired after that and I am still using Autolite plugs.
John's 1972 Duster on a Nu-Be weekend 001.jpg
My old Duster at Keystone 001.jpg
 
AJ/FormS said:
278/110 in at 106=ICA of 65*
Static compression ratio of 9.5:1.
Effective stroke is 2.55 inches.
Your dynamic compression ratio is 7.54:1 .
Your dynamic cranking pressure is 148.00 PSI.
V/P (Volume to Pressure Index) is 116



Overheated plugs, my guess too.But if problems continued on the way home at lower rpms,like you allude to, it may have been because one or more spark-plugs actually puked their insulators into the engine,or just cracked them, and were short circuiting, or open circuiting, or in some other way.....not firing at the correct time.

You remember all that from 28years ago? Dayng-it, I wish I could remember last year...lol.
Click to expand...
AJ/FormS said:
278/110 in at 106=ICA of 65*
Static compression ratio of 9.5:1.
Effective stroke is 2.55 inches.
Your dynamic compression ratio is 7.54:1 .
Your dynamic cranking pressure is 148.00 PSI.
V/P (Volume to Pressure Index) is 116



Overheated plugs, my guess too.But if problems continued on the way home at lower rpms,like you allude to, it may have been because one or more spark-plugs actually puked their insulators into the engine,or just cracked them, and were short circuiting, or open circuiting, or in some other way.....not firing at the correct time.

You remember all that from 28years ago? Dayng-it, I wish I could remember last year...lol.
Click to expand...
Yes, ran like crap at low rpms and idle. Until I swapped the plugs.
 
pittsburghracer said:
I quit caring what guys do BUT I will tell you something an oldtime Mopar engine builder told me. Around 42-44 years ago I was really into the street racing scene with my 1972 340 (4 speed) duster. we would street race 4-6 times a week and 2 times a week at the track. If I had a race that night there was a hill 4 miles from my house and i would have to go up it and jump on it in 3 rd gear. My car would backfire twice then run great. When I told this oldtimer my story he looked at me and said "Champion plugs right) and I said yes. He told me to pull them out, get a set of Autolite plugs and gap them at .028 and try it. My car NEVER backfired after that and I am still using Autolite plugs.View attachment 1715142075 View attachment 1715142076
Click to expand...


I'm taking that first part of your opening line and I'm going to abuse it like a two dolla ***** on coke!!!!!
 
On a '68 cuda BB- had the opposite problem. AC's and Autolite's would crap out at high RPM's after 1000 miles. Champion's would go 3000 miles. That was the old days, wonder what the story is today?
 
I run Champions in stock or almost stock stuff. Anything else gets Autolite or NGK race plugs.
 
70aarcuda said:
Timing by ear is a great way to have lousy running engine
Click to expand...
I use one these days. I didn't even know what a timing light was back then. I was only 19
Bobzilla said:
It was not fouling the plugs, it was pre-igniting because the plugs you installed were a higher heat range and it was retaining too much heat. The higher compression simply did not like the higher heat range spark plug.
Click to expand...

Ran fine when I put them in and was just easing around at highway speed, but once I put my foot in it, things got bad. Misfired all the way home and idled like crap. Swapped plugs back to what I was running before and never had another problem.
 
yellow rose said:
I'm taking that first part of your opening line and I'm going to abuse it like a two dolla ***** on coke!!!!!
Click to expand...
Seems there were a run of bad champions back in the day, we got a hold of some of them for the hemi. A hotter ignition fixed the problem, that and a sponsor giving us a new set every race.
 
i'm gonna go out on a limb here.My old swinger ran fine and one day
it started to miss.It drove me crazy but i finally found the problem...
one spark plug wire had come off enough to cause the miss but it was
impossible to see.Pushed it back on and all was well.
 
It's not the quality of the brand of plug, it's the heat range they convert to or how they are listed in their catalog. I converted a Champion number to a Autolite and it was a heat range colder than the factory Champ number. I rebuilt my 273 and lost a point of compression due to piston availability. I chased a dirty idle problem for 2 summers and a 2 heat range hotter plug fixed the problem.
 
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