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Spark plug length


Mertz
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The plugs that were in my truck don’t extend into the combustion chamber. I know that plugs work best when they can spread the flame evenly into the chamber. With plugs recessed like mine are, the flame goes straight down with very little spread. So I’m looking for a long reach plug that isn’t to long.  Do you have any suggestions for a longer plug?

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The first question is what current plug are you using?  So you state your 1940 Plymouth truck. Are you using a cold or hot plug. Why do you feel the need to spread out the spak for the combustion? I have pasted a Spark Pug Table so you can see what the regular plugs that were used on your 1940 Plymouth. A hotter plug will go down a little into the chamber but you have to make sure that they do not extend to far and hit the top of the piston.

 

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Rich Hartung

Desoto1939@aol.com

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I don’t know what plugs were in the engine. I left them in when I painted the head. I could clean off the paint to find out but I have never heard the engine run. The reason I’m concerned about flame spread is that this low hp engine needs all the help it can get. If you watch any of the horsepower shows they spend time on pointing the spark toward the chamber and getting the best flame distribution in the chamber. 
 

Thanks for the plug chart. 

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lots of stuff on this subject, dyno test and this practice, negligible increase except if misfire data....the operation range of the dyno test and this practice shows rpms so over the top any of these flatheads will ever see...the data again concludes that lower misfires the main benefit from indexing.  Temp range, depends on many other variables of your build and if stock, odds are your driving habits with this car may show that your stock plug is already been factory proven as best inasmuch as heat range goes.  If you have altered your CR/Induction/Exhaust....then you will have to drive your car and read your plugs and adjust up/down for your parameters.  This is part of YOUR tuning of the car if you want to squeeze another chirp from a cricket.

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Metz:  I run the Autolite A9 Stock plugs in my 39 Desoto.  I just drove my car to the National Desoto car Show from Philadelphia Pa to Altoona, PA.  I took the Pa Turnpike because it had service plaza's approx every 40-50 miles apart and the grading over the mountains is setup as a max 5% grade incline.  I drove a total of 550 mile round trip and got around 16-18 mpg.  Not bad for an 83 year old car. I drove the car at 50-53 MPH.

 

These older cars are not speed cars they are good reliable cars and you are not going to win a battle at the drag strip.  If you treat them with respect they will give it back to you.  So, use the chart I sent you and your car will run jsut fine with any of the recommended plugs.

 

I have had my car for 35 years.

 

Rich Hartung

Desoto1939@aol.com

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  • 2 weeks later...

I ordered autolite 303 plugs but got one box of 303 and another 4092. 303 is resistor plug while 4092 is not. The 4092 has a reach 1/8” longer than the 303. These are both copper core. I checked the reach on my old cylinder head and the electrode is just inside the combustion chamber. I think I’ll use the 4092 and see how they work. I feel better about the flame spread with these plugs. I know this is not a high performance engine but I guess I’m trying to get the most out of it.  These have the same heat range as the A9 in the chart. 

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