Quest For The Ultimate Flathead 6 Spark Plug?

[Powerhouse]

Ok...so I've gotten many different opinions on which spark plug is correct for a Plymouth flathead 6. So far I've been told:

Champion J8, AC R44, AC R45, AC R46, Autolite A7, Autolite 303, Autolite 306, Autolite 85…

I run a T53 motor in my P7…which has a 39 distributor. The Plugs I’m using are R45 at .35. Seems to be running fine, despite a sticky#5 Intake.

My Question is WHICH IS BEST? And also what is the difference between the R44,R45, and R46?

I’m going to ask a Mopar magician to-nite and get some more opinions as well. We’ll see…

Please feel free to let me know which plugs you’re running on and your opinion of them. Thanks and Happy Motoring!

[captden29]

the difference is heat range. the hotter plug, #46, will help keep the plug cleaner if you are burning oil. i have a 54 windsor that was using j12, but i just switched to r45. no difference.the auto parts store was more confused than i was about which is the correct plug. dennis

[Don Coatney]

The Plugs I’m using are R45 at .35. Seems to be running fine

Are you trying to fix something that is not broken?

[Powerhouse]

Are you trying to fix something that is not broken?

HAHAHAAHA...No, that's never a good idea.

My friend's P15 needs a tune up badly and I want to make sure he buys the best plugs available since it's his Daily Driver. Also, I'd like to know for my next tune-up.

[Normspeed]

I use Autolite 306 gapped at .028-.030. No issues at all, and they burn pretty clean, even with the dual carbs. Tried one set of Champions, forgot the number though. Not as easy to start with those.

[Suddensix]

I use Autolite 303 in my 218 with dual carbs. I recently read an article in a magazine which quoted an NGK spark plug rep. as saying in part "today's fuels are blended with 5% to 10% ethanol. The addition of ethanol to gasoline results in an increase in combustion heat and a decrease in the amount of energy a litre of fuel will provide. It is important to remember that spark plugs do not create heat, they can only remove heat from the combustion chamber to the engines cooling system. Changing to a colder plug should resolve the problems that ethanol blending brings us. In addition to using colder plugs you should adjust your plug gap to .010 tighter than the factory specified gap. Do not reduce the gap below .035." I think the plug gap is probably for newer cars with electronic ignition.

[greg g]

Been running AC's found them to be better than the Champions for starting and running. might be part of improved fuel economy also. Another article I read say todays fuels have slower flame front this gives the power stroke a push rather than the bang of the old formuleas. The article sugested a slightly advanced intitial timing setting to compensate. I run about 5 BTDC.

[De Soto Frank]

I've been using Autolite 303's or 306's in my '41 De Soto since I started driving it ten years ago...

My preference these days is for Auto-Lite plugs...

If I'm working on a Stovebolt Chevy, then I'll use AC plugs. (The "purist" in me:D )

I think I've replaced the plugs in the De Soto once in 20,000 miles of driving.

The Auto-Lites start fine and are pretty resistant to fouling, even in a 102,000 mile oil-pumper like my wheezy 228 "big six".

I haven't used Champion plugs in over ten years...

For what that's worth...

De Soto Frank

[Plymouthy Adams]

I have not bought a Champion plug since about 1967/68 I try to get Autolite..do not care for Bosch either. I have the Autolite 306 in my vehicle.

[martybose]

How many of you are running these same plugs in your aluminum heads? If your aluminum heads are like my Edmunds head, you've got the wrong plugs!

The stock head uses a 3/8" reach plug; my Edmunds is designed for a 1/2" reach plug. I used an Autolite 306 in mine for some time, and it seemed to be okay, until one day that I had the head off and actually looked at the installed plugs, and saw unused threads in the head. I now run an Autolite AP 425, which is an relatively expensive 1/2" reach plug. I ran it at .032" gap with the stock ignition, and .060" with my Langdon HEI. According to the Autolite engineers, it is a slightly colder iridium plug than a 306.

Highly recommended!

Marty

[Powerhouse]

OK...so 3522 is the NGK brand plug equiv to A303. Most of the guys in my club run on Autolite's 303. So...I guess I'll go with the Autolite 303 next time round. I ownder if Autolite 85 is the same as 3303 or 306?????

[Don Coatney]

OK...so 3522 is the NGK brand plug equiv to A303. Most of the guys in my club run on Autolite's 303. So...I guess I'll go with the Autolite 303 next time round. I ownder if Autolite 85 is the same as 3303 or 306?????

Once again I must ask if you are fixing something that is not broken? Why do you want to change brands when what you are using works?

In the old days when leaded gas was the norm spark plugs required cleaning and/or replacement every few thousand miles. With the clean burn unleaded gasoline available today modern engines can and do run well over 100,000 miles with no spart plug service required. If a flathead engine has healthy internals there is no longer a reason to replace spark plugs every few thousand miles.

[Powerhouse]

well...I was just curious to see how the Autolites work...since everyone I know here likes 'em and swears by them. Besides...they are not so costly to worry about. And I don't have a spare set in case anything should go wrong on the road...so the r45 plugs will go in the trunk as spares and the Autolites will go in the motor. I don't throw anything out...so these will not go to waste. I don't think I'll be buying the NGK...

Edited December 9, 2010 by Powerhouse

[De Soto Frank]

I think the Auto-Lite 85 might be an 18mm plug...

The 303 and 306 have done well enough for me over the last ten years that I'm not looking for a different type plug.

De Soto Frank

[Normspeed]

OK, now ya gotta tell us...what dirty word?

I don't think I'll be buying the NGK...I saw a dirty word on the plug. I don't like that stuff.

[jd52cranbrook]

How many of you are running these same plugs in your aluminum heads? If your aluminum heads are like my Edmunds head, you've got the wrong plugs!

Marty

Earl Edgerton Heads use short reach plugs.

[Guest rockabillybassman]

Do you eat peanut butter?...... China.

Got a TV?...... most all components, even in a domestic brand... Japanese, Mexican, Chinese.

Etc,etc. It's a fact of life.

[Mr. Belvedere]

Whats the difference between a 303 and 306? Is one hotter than the other?

[RobertKB]

I am running NGK B-4 plugs in my flatheads with nothing but good results. I also have NGK plugs in a Ford 260 CID I have in my '63 Fairlane - again with great results. If I wasn't running NGK, I would likely try to use Autolite as I always had good luck with them also. Local jobber (mom and pop) place I usually deal at sells the NGK and highly recommends them. Just kinda strange to be running Japanese plugs in an old flattie but if works what the hey.

[old woolie]

Went to a one night course on spark plugs a couple of years ago that was put on by one of our local parts stores. The guy giving the course said that NGK makes the plugs for all the major brands. I don't know, thats just what he told us.

[De Soto Frank]

Autolite 303 is the "recommended" standard plug for a 1940's flathead MoPar, I believe it replaces the original Autolite A-7.

The 306 is one heat-range higher; I use it in my high-mileage De Soto to try to delay the inevitable carbon-fouling due to oil burning. I've been averaging 10-12,000 miles on a set of plugs.

Once upon a time, I used nothing but Champion plugs, but they seemed more prone to fouling in a tired engine.

So, for me, either Autolite or AC plugs.

De Soto Frank

[p10plymouth]

I've been waiting to add this.. I have been using these plugs sold in the 60's and 70's by JC WHINTEY.. I will have my 1940 Plymouth for 2 years this November... I did a complete "tune-up" new everything .. I installed these plugs . checked them after 6-8 mouths and perfect .. great idle and top end! I used these same plugs in the 70's for my Dads nw 73 chevy 350 .. never never bought a new plug they stayed in till we traded it in the 80's.. the ones I use have a number on them S.A. 14-165 finding them on ebay is easy.. I would recomend them ..

[RHDSP15C]

A spark plug is a spark plug, all the same if the reach and heat range are correct. Unless of course you believe in fairies.

[Rodney Bullock]

I was using the Champion plug J12 or something around that, I have a 6volt pos. system with an FM/AM converter. I could not for the life of me hear the radio clearly in my 1940 Plymouth however I could hear it in my 1941 Studebaker. One day for kicks I looked at the plugs I had in the Studebaker and they were AC45 What was the difference....one plug was resister and the other was not. I switched to the AC45 in the Plymouth and all that interference went away and the hp increased. There is a reason you are looking into the different plugs. My car runs very clean and you don't smell any unburned fuel. This should be our biggest concern.

[steveplym]

I was using the Champion plug J12 or something around that, I have a 6volt pos. system with an FM/AM converter. I could not for the life of me hear the radio clearly in my 1940 Plymouth however I could hear it in my 1941 Studebaker. One day for kicks I looked at the plugs I had in the Studebaker and they were AC45 What was the difference....one plug was resister and the other was not. I switched to the AC45 in the Plymouth and all that interference went away and the hp increased. There is a reason you are looking into the different plugs. My car runs very clean and you don't smell any unburned fuel. This should be our biggest concern.

Rodney, AC45's will work in our cars? I've got champions in mine and just never liked them. Was wanting to go with an AC plug as I've had good luck with them in all my vehicles. I went with the splitfire plug one time, that was a mistake. Truck missed like crazy. Not sure of others experience, but that was mine.