Spark plug good or bad idea?

Last year (1st yr i owned it) I ran NGK BP7ES
I believe that is whats in the book and i was
hard to start on cold maine mornings. My
brother (a Harley owner) suggested BP5ES.
I put those in yesterday and it started first try
and never started that easy last year. Is it OK
to run those 5's all year? Or are they to hot.

 

Running a hotter plug may cause premature detonation because the plug reaches its self cleaning temperature sooner which could potentially ignite the air/fuel mixture sooner than expected. If you get pinging or knocking in the engine then change them back. That said, your hard starting may be a symptom of a clogged enrichment circuit. You may want to consider cleaning it instead of changing plug heat ranges. One course of action is a Band-Aid; the other is a solution.

 

There are misconceptions on spark plug heat reanges. The heat range of a spark plug is the heat transfer from the spark plug to the cylinder head. It has nothing to do with the actual spark of the plug. If you want more spark, I suggest an aftermarket ignition system that will increass the spark voltage for improved combustion. Your stock ingnition puts out about 50,000 volts at the coils. This is pretty standard for most vehicles, cars included. Coils are oil filled and get weak with age and exposure to the elememts. You might want to consider changing your coil out for a new one. You woul be surprised how much of a difference that will make. I was reading on the net that MSD and I believe Jacobson make a after market box for XJ's that would increase your coil volatage to 100,000, but this comes at a risk. The increased voltage will wear out an older coil fast and will eventually fail. If you go this route, install anew coil with the box. Dodger 62

 

I live in northern Ontario, and my 650 wouldn't start when it was cold out (<10 degrees C). Replacing them with the Bosch Platinum equivalents solved the issue for me.

 

""A spark plug's heat range has no relationship to the actual voltage transferred through the spark plug. Rather, the heat range is a measure of the spark plug's ability to remove heat from the combustion chamber. 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.

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 500C-850°C. If the tip temperature is lower than 500°C, 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 850°C 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 70°C to 100°C from the combustion chamber. A projected style spark plug firing tip temperature is increased by 10°C to 20°C.

The firing end appearance also depends on the spark plugs tip temperature. There are three basic diagnostic criteria for spark plugs: good, fouled and overheated. The borderline between the fouling and optimum operating regions (500&def;C) is called the spark plug self-cleaning temperature. The temperature at this point is where the accumulated carbon and combustion deposits are burned off.

Keep in mind 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 water jackets. 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 a high rpm for a significant period of time. Colder spark plugs remove heat quicker, reducing the chance of pre-ignition/detonation. Failure to use a cooler heat range in a modified application can lead to spark plug failure and severe engine damage.

Air/Fuel Mixtures seriously affect engine performance and spark plug operating temperatures.

* Rich air/fuel mixtures cause tip temperature to drop, causing fouling and poor driveability
* Lean air/fuel mixtures cause plug tip and cylinder temperature to increase, resulting in pre-ignition, detonation, and possibly serious spark plug and engine damage
* It is important to read spark plugs many times during the tuning process to achieve the optimum air/ fuel mixture

Higher Compression Ratios/Forced Induction will elevate spark plug tip and in-cylinder temperatures

* Compression can be increased by performing any one of the following modifications:

a) reducing combustion chamber volume (i.e.: domed pistons, smaller chamber heads, mill ing heads, etc.)

b) adding forced induction (Nitrous, Turbocharging or Supercharging)

c) camshaft change
* As compression increases, a colder heat range plug, higher fuel octane, and careful attention to ignition timing and air/fuel ratios are necessary. Failure to select a colder spark plug can lead to spark plug/engine damage

Advancing Ignition Timing

* Advancing ignition timing by 10° causes tip temperature to increase by approx. 70°-100° C

Engine Speed and Load

* Increases in firing-end temperature are proportional to engine speed and load. When traveling at a consistent high rate of speed, or carrying/pushing very heavy loads, a colder heat range spark plug should be installed

Ambient Air Temperature

* As air temperature falls, air density/air volume becomes greater, resulting in leaner air/fuel mixtures.
* This creates higher cylinder pressures/temperatures and causes an increase in the spark plug's tip temperature. So, fuel delivery should be increased.
* As temperature increases, air density decreases, as does intake volume, fuel delivery should be decreased

Humidity

* As humidity increases, air intake volume decreases
* Result is lower combustion pressures and temperatures, causing a decrease in the spark plug's temperature and a reduction in available power.
* Air/fuel mixture should be leaner, depending upon ambient temperature.

Barometric Pressure/Altitude

* Also affects the spark plug's tip temperature
* The higher the altitude, the lower cylinder pressure becomes. As the cylinder temperature decreases, so does the plugs tip temperature
* Many mechanics attempt to "chase" tuning by changing spark plug heat ranges
* The real answer is to adjust air/fuel mixtures by rejetting in an effort to put more air back into the engine."" ***

*** NGK Web Site Information Quoted