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Understanding Spark Plugs

Spark plugs are one of the biggest "Gimmicks" on the market. What I mean by that is there are dozens of bullshit marketing reasons to get you to spend more money on something you don't need.

Spark plugs are one of the most misunderstood components of an engine. Numerous questions have surfaced over the years, leaving many folks confused.
 
Spark plugs are the "window" into your engine (your only eyewitness to the combustion chamber), and can be used as a valuable diagnostic tool. The spark plug displays symptoms and conditions of the engine's performance. The experienced tuner can analyze these symptoms to track down the root cause of many problems, or to determine air/fuel ratios (to a degree).

SPARK PLUG BASICS:
The spark plug has two primary functions:

  • To ignite the air/fuel mixture
  • To remove heat from the combustion chamber

Spark plugs transmit electrical energy that turns fuel into working energy. A sufficient amount of voltage must be supplied by the ignition system to cause it to spark across the spark plug's gap. This is called "Electrical Performance."
The temperature of the spark plug's firing end must be kept low enough to prevent pre-ignition, but high enough to prevent fouling. This is called "Thermal Performance", and is determined by the heat range selected.
It is important to remember that spark plugs do not create heat, they can only remove heat. The spark plug works as a heat exchanger by pulling unwanted thermal energy away from the combustion chamber, and transferring the heat to the engine's cooling system. The heat range is defined as a plug's ability to dissipate heat.
The rate of heat transfer is determined by:
  • The insulator nose length
  • Gas volume around the insulator nose
  • The materials/construction of the center electrode and porcelain insulator
Understanding what a heat range is:
A spark plug's heat range has no relationship to the actual voltage transferred though the spark plug. It will not increase engine torque or horsepower. The heat range is a measure of the spark plug's ability to remove heat from the combustion chamber in order to burn carbon deposits off of it's electrode (clean itself). The heat range measurement is determined by several factors; the length of the ceramic center insulator nose and its' ability to absorb and transfer combustion heat, the material composition of the insulator and center electrode material.

HOT  TYPE
It has a larger surface exposed to the combustion gasses.
 It dissipates heat slowly.
 It's firing end heats up quickly

COLD  TYPE
It has a smaller surface exposed to combustion gasses.
It dissipates heat quickly.
It's firing end heats up slowly.

The insulator nose length is the distance from the firing tip of the insulator to the point where insulator meets the metal shell. Since the insulator tip is the hottest part of the spark plug, the tip temperature is a primary factor in pre-ignition and fouling. Whether the spark plugs are fitted in a lawnmower, boat, or a race car, the spark plug tip temperature must remain between 932°F-1562°F. If the tip temperature is lower than 932°F, the insulator area surrounding the center electrode will not be hot enough to burn off carbon and combustion chamber deposits. These accumulated deposits can result in spark plug fouling leading to misfire. If the tip temperature is higher than 1562°F the spark plug will overheat which may cause the ceramic around the center electrode to blister and the electrodes to melt. This may lead to pre-ignition/detonation and expensive engine damage. In identical spark plug types, the difference from one heat range to the next is the ability to remove approximately 158°F to 212°F from the combustion chamber.

Normal: Grey to Light Golden-Brown Color

  • This condition is ideal, the spark plug and engine air/fuel mixture are operating properly.

Dry: Black Soot Buildup

  • Air/fuel mixture is too rich, the carburetor settings are incorrect, or choke was left out too long.

  • Spark plug heat range is too cold for the operating conditions.

  • Ignition system problems causing a weak or intermittent spark.

 

Oil Fouling: Shiny,  Black Appearance

  • Excessive wear or damage to valves or valve guides and seals.

  • Excessive wear or damage to piston and/or piston rings.

  • Excessive wear or damage to cylinder walls.

Overheated: White, Blistered, Melted Electrode

  • Lean air/fuel mixture due to incorrect carburetor settings or intake leak.

  • Spark plug heat range is too hot for operating condition of the engine.

  • Plug is not properly gapped and/or torqued onto head.

  • Overly advanced timing.

Lead Fouling

  • Lead fouling usually appears as yellowish brown deposits on the insulator nose.

  • Caused by using incorrect mixture of lead additive to fuel (lead is rarely found in modern pump fuel).

Excess Deposits: Bumpy, Chalky Buildup

  • Poor fuel quality.

  • Poor oil quality.

  • Contamination to fuel or oil.

 



The firing end appearance also depends on the spark plug tip temperature. There are three basic diagnostic criteria for spark plugs: good, fouled and overheated. The borderline between the fouling and optimum operating regions is called the spark plug self-cleaning temperature. The temperature at this point is where the accumulated carbon and combustion deposits are burned off.
Keeping in mind that the insulator nose length is a determining factor in the heat range of a spark plug, the longer the insulator nose, the less heat is absorbed, and the further the heat must travel into the cylinder head and to the cooling fins. This means the plug has a higher internal temperature, and is said to be a hot plug. A hot spark plug maintains a higher internal operating temperature to burn off oil and carbon deposits, and has no relationship to spark quality or intensity.
Conversely, a cold spark plug has a shorter insulator nose and absorbs more combustion chamber heat. This heat travels a shorter distance, and allows the plug to operate at a lower internal temperature. A colder heat range is necessary when the engine is modified for performance, subjected to heavy loads, or is run at high rpms for a significant period of time. The colder type removes heat more quickly, and will reduce the chance of pre-ignition/detonation and melting or damage to the firing end. (Engine temperature can affect the spark plug's operating temperature, but not the spark plugs heat range).

Harley
Spark Plug
Cross-reference

Twin Cam
88/96 ci

VRSC
V-Rod

EVO 1340
80 ci

Shovelhead
1982-1984

Shovelhead
1975-1981

Shovel
Panhead
1947-74 

H-D Standard
(Champion)

6R12

10R12A

5R6A

5R6A

5R6A

4R or 4R5 

H-D Gold

6R12G

10R12AG

5R6AG

5R6AG

5R6AG

4R5G 

H-D Platinum

6R12PP

10R12APP

5R6APP

5R6APP

5R6APP

- 

Screamin' Eagle

EX12S

-

EVS13S

SH13S

EVS13S

SH12S 

ACCEL

Y2418P

-

2410A

2417

2410A

-

AC Delco

-

121XLS

R44NS

44NS/44XLS

44NS/44XLS

- 

Autolite

4164

4133

4265

2585

4265

- 

Autolite Platinum

AP4164DP2

-

AP4265DP2

-

AP4265DP2

- 

Bosch / Platinum

YR6LDE

X4CS

WR8DP

WR8DC4

WR8DC4

- 

Denso Iridium

IXU22

IXU24

IW16

IW16

IW16

 IWF22 

NGK

DCPR7E

-

BPR5ES-11

ZGR5C-11

BPR5ES-11

- 

NGK Platinum

-

-

BPR5EVX-11

-

BPR5EVX-11

-

Splitfire

SF416D

SF416B

SF6D

SF98E

SF426E

- 



Sportster
Spark Plug
Cross-reference

XL 883/1200
1986- *

XR 1000cc
1983-1984

XL 1100cc
1979-85

XL 1000cc
1972-1978

H-D Standard
(Champion)

6R12

6R7

4R5

4R5

H-D Gold

6R12G

-

4R5G

4R5G

H-D Platinum

6R12PP

-

XL10S

XL10S

Screamin' Eagle

EX12S

-

2402

2413

ACCEL

Y2418P

-

-

-

AC Delco

-

-

42F/42-6FS

42F/42-6FS

Autolite

4164

-

996

4123

Autolite Platinum

AP4164DP2

-

-

-

Bosch / Platinum

YR6LDE

WR8DP

W5AC

W5AC

Denso Iridium

IXU22

IW22

IWF22

IWF22

NGK

DCPR7E

B7EB

B6HS

B6HS  

NGK Platinum

-

-

-

-

Splitfire

SF416D

SF6B

SF21F

SF412C

 

Harley-Davidson brand Spark Plugs
 Harley-Davidson Spark plugs are manufactured by 'Champion Spark Plugs'.  H-D has a contract through Champion for the optimum heat range for H-D street motors, this heat range is only available on H-D plugs and is not sold in stores. Many other brands of spark plugs will work in your Harley, some of them will work very well, but I don't believe any of them will measure up to stock  H-D plugs.

Harley-Davidson Gold and Platinum Spark Plugs?
More money for the same stock plug

Screamin' Eagle Split-fire Spark Plugs?
Impossible to accurately gap
Electrodes break off causing serious engine damage

 

Little known fact:
2005 and later Harley-Davidson EFI systems incorporate an "Anti-knock" feature that operates by reading the current resistance through the spark plugs.
The use of any other spark plug other than 'Stock' (6R12) disables the anti-knock feature.
(Yes, that means the Gold, Platinum and SE plugs will also disable this function)