54 Spark Plug Heat Range

[Joe R.]

I'm currently using the AC 43-5 plugs in my Blue Flame 6, and the plug is getting black deposits on it. According to the AC picture charts, I'm either running too rich in fuel mixture, have a weak spark, or the plug range is too cold.

I'm in the process of elimination, and knowing that there are no other jets available for the Carter carbs, the idle mixture is good, and the coil is putting out a blue spark, I would like to know what is the next hotter plug for this? My NAPA guy is puzzled. Any help is greatly appreciated.

Joe

[Timothy B.]

Re: 54 Spark Plug Heat Range

Joe,

Just a thought because I don't know anything about C1 corvettes. Does the car see alot of idle time? If so that may explain the black soot like spark plugs. Carburetor idle circuits tend to be richer thus the plug color you describe..

If the car otherwise runs normal I would say it needs to be exercised a bit.

[John T.]

Re: 54 Spark Plug Heat Range

AC43 is a cold plug. Period literature describes as "Heavy duty and high output". AC-44 is "Town and country driving". With AC-46 being "City driving".

Hope this helps.

John

[Willard M.]

Re: 54 Spark Plug Heat Range

In my 1951 with the 216 I had similar trouble with the 43's. Went to 46 and problem solved.

[Dave S.]

Re: 54 Spark Plug Heat Range

I'm currently using the AC 43-5 plugs in my Blue Flame 6, and the plug is getting black deposits on it. According to the AC picture charts, I'm either running too rich in fuel mixture, have a weak spark, or the plug range is too cold.

I'm in the process of elimination, and knowing that there are no other jets available for the Carter carbs, the idle mixture is good, and the coil is putting out a blue spark, I would like to know what is the next hotter plug for this? My NAPA guy is puzzled. Any help is greatly appreciated.

Joe

Joe,
Actually 43-5 is heat range 3.5 which is a heat range in between 3 and 4. That is the period correct original plug for a 6 cylinder Corvette but experience says it is to cold for that engine especially in city driving. If you are looking for a functional plug I would recommend a hotter plug, 44 or 45. (Heat range 4 or 5).

[Jack H.]

Re: 54 Spark Plug Heat Range

If memory serves, there were also 'COM' or Commercial versions of these plugs intended for heavy use applications like trucks, taxi, and police car applications...

[Jim L.]

Re: 54 Spark Plug Heat Range

I would like to know what is the next hotter plug for this? My NAPA guy is puzzled. Any help is greatly appreciated.

Joe

I run an AC R45S in my '54 (well, 6 of them actually) and the plug color is very good.

Jim

[Duke W.]

Re: 54 Spark Plug Heat Range

The "COM" (commercial) AC plugs had larger center and ground electrodes, which theoretically offered greater service life due to more available electode wear material.

Plug longevity is rarely an issue with vintage cars that see low annual mileage accumulation.

Chevrolet recommended plugs were always on the cold side, and experience from owners over the decades indicates that one to two heat ranges higher than OE is best for the light duty service our cars see today.

The best range for most is AC "5" or equivalent in another brand. In the AC number system the first digit (4) means 14 mm thread and the second digit is the heat range, 1-6, and the higher the number the "hotter" the plug. A hotter plug will run at higher average temperture for a given set of operating conditions, and the tip needs to be at least about 700 deg. F to burn off deposts, but a tip temperature above 1500 F can lead to preignition and detonation.

Plug heat range has nothing to do with the engine's rated power in normal road driving. It's dependent on the average load the engine sees, and most vintage Corvettes are run at very low average load. Whether you have a base or the highest output engine, chances are it operates at relatively low load 99 percent of the time and only sees full load for a few seconds at a time.

So for road use you need a relatively hot plug to avoid deposit buildup, but if you were to take your car to a track for a day of hot lapping you should go at least two heat ranges colder than what works best for road use in order to avoid overheating the plug and getting into the preignition and detonation regime.

With high energy ignition systems, you can usually run one heat range colder than with points, because the greater per spark ignition energy will blow a spark through deposits that would cause the low energy point ignition system to missfire. For example, the TI and HEI systems offer double the per spark energy as the point system.

Duke

[Jeffrey S.]

Re: 54 Spark Plug Heat Range

Joe:
The plug that came with the car when new, according to Noland Adams (page 101 Restoration Guide Vol. 1) was either AC44-5 or AC43-5 Commercial. As was said earlier the commercial plug was for heavy duty use such as taxi cabs, trucks etc. The plug was a smooth shank as in the 1st picture posted above with "Corolox" under the size/heat range designation. You can use any 44, R44, 44s, 45, etc plug in that engine and it will work fine. The original AC44-5 (4.5 heat range) would work fine but they are hard to find so if you just want a good plug an R45 would be a good fit. Hope this helps.
Jeff

[Joe R.]

Re: 54 Spark Plug Heat Range

Many thanks to all. One last question; I assume that the "R" designation refers to a resistor type plug. Will that be OK for a 6 volt ignition system?

[Jim L.]

Re: 54 Spark Plug Heat Range

Many thanks to all. One last question; I assume that the "R" designation refers to a resistor type plug. Will that be OK for a 6 volt ignition system?

Your assumption about the "R" prefix is correct and yes, it's fine for our 6 Volt ignitions.

With the R45S plugs I use in my '54, I also use solid core plug wires. (It doesn't make sense to have both resistor plugs and resistor wires.)

No problems, ever.

Jim

[Joe R.]

Re: 54 Spark Plug Heat Range

Got it. Thanks Jim.

[Dave S.]

Re: 54 Spark Plug Heat Range

I'm currently using the AC 43-5 plugs in my Blue Flame 6, and the plug is getting black deposits on it. According to the AC picture charts, I'm either running too rich in fuel mixture, have a weak spark, or the plug range is too cold.

I'm in the process of elimination, and knowing that there are no other jets available for the Carter carbs, the idle mixture is good, and the coil is putting out a blue spark, I would like to know what is the next hotter plug for this? My NAPA guy is puzzled. Any help is greatly appreciated.

Joe

I want to clarify my earlier post where I stated the period correct plug for a 6 cylinder Corvette was 43-5. It is 44-5 as Jeff stated. Sorry for the confusion.

[Troy P.]

Re: 54 Spark Plug Heat Range

Your assumption about the "R" prefix is correct and yes, it's fine for our 6 Volt ignitions.

With the R45S plugs I use in my '54, I also use solid core plug wires. (It doesn't make sense to have both resistor plugs and resistor wires.)

No problems, ever.

Jim

Jim, do you recall where you got the solid core wires. I need a set too.
Troy

[Jim L.]

Re: 54 Spark Plug Heat Range

Jim, do you recall where you got the solid core wires. I need a set too.
Troy

Troy,

I bought a universal set of wires from NAPA, part number 700173 in the Belden line.



These have straight boots for the plugs and your choice of straight or 90* boots for the distributor.

Jim