Spark Plugs - heat range?

[Mike1956]

I am just about to purchase spark plugs for a Yamaha 2B outboard (NGK BR5HS) and a Yamaho 5C outboard (NGK BR7HS). I am wondering what heat range I need to order. I have looked in the 5C manual but no mention of heat range. The plugs I took out are 7 and 10 but this does not mean they are correct, may have been what was available at the time.

Look forward to replies

Thanks

Mike

[GordMay]

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.
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.
Spark plug tip temperature must remain between 500C-850°C.
If the tip temperature is lower than 500°C, the insulator area surrounding the centre 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 centre 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.

NGK Outboard Motor Spark Plug Guides ➥ NGK Outboard Motor Spark Plug Guide

Yamaha Outboard NGK Spark Plug Guide ➥ Yamaha - NGK Outboard Motor Spark Plug Guide

[SVHarmonie]

Originally Posted by Mike1956

I am just about to purchase spark plugs for a Yamaha 2B outboard (NGK BR5HS) and a Yamaho 5C outboard (NGK BR7HS). I am wondering what heat range I need to order. I have looked in the 5C manual but no mention of heat range. The plugs I took out are 7 and 10 but this does not mean they are correct, may have been what was available at the time.

Look forward to replies

Thanks

Mike

Mike,

If you dive in to the stuff Gord has posted, you’ll see that the heat range is already coded into the plug part number the Yamaha provided.

You are over thinking it!

[a64pilot]

I believe for NGK the the lower the number the hotter the plug, counter intuitive but I believe that’s correct.
So if you mostly only idle and low power, run one heat range hotter, if you get in and take it to full throttle then go one colder, otherwise run what is specified. 90% of the users need what’s specified.
Going cold won’t hurt anything, just foul the plug, too hot can hurt the motor through pre-ignition.
If you go hotter listen for a rattle, if you ever hear it, back off.

Oh, and if there are any electronics on the boat, like a radio, get an (R) plug

[Mike1956]

Originally Posted by GordMay

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.
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.
Spark plug tip temperature must remain between 500C-850°C.
If the tip temperature is lower than 500°C, the insulator area surrounding the centre 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 centre 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.

NGK Outboard Motor Spark Plug Guides ➥ NGK Outboard Motor Spark Plug Guide

Yamaha Outboard NGK Spark Plug Guide ➥ Yamaha - NGK Outboard Motor Spark Plug Guide

Thanks, did not know any of the above.

Mike

[Mike1956]

Originally Posted by SVHarmonie

Mike,

If you dive in to the stuff Gord has posted, you’ll see that the heat range is already coded into the plug part number the Yamaha provided.

You are over thinking it!

Thanks. The reason for my post is there is no heat range specified in the Yamaha manual so I assume the NGK BR5HS is only available in heat range 7 and the NGK BR7HS is only available in heat range 10. This is why the heat range is not mentioned in the manual? Makes sense now.

Providing I buy NGK BR5HS and NGK BR7HS it cannot be wrong.

Mike

[Mike1956]

Originally Posted by a64pilot

I believe for NGK the the lower the number the hotter the plug, counter intuitive but I believe that’s correct.
So if you mostly only idle and low power, run one heat range hotter, if you get in and take it to full throttle then go one colder, otherwise run what is specified. 90% of the users need what’s specified.
Going cold won’t hurt anything, just foul the plug, too hot can hurt the motor through pre-ignition.
If you go hotter listen for a rattle, if you ever hear it, back off.

Oh, and if there are any electronics on the boat, like a radio, get an (R) plug

So the lower number refers to less heat transfer the higher number more heat transfer. Less heat transfer = hotter. More heat transfer = cooler.

Got it!

Mike

[GordMay]

Originally Posted by a64pilot

I believe for NGK the the lower the number the hotter the plug, counter intuitive but I believe that’s correct...

Indeed. According to NGK, where 2 is hot, and 11 is cold.
However, some manufacturers numbering systems are opposite the other. For Champion, Autolite and Bosch, the higher the number, the hotter the plug. However, for NGK, the higher the number, the colder the plug.


Tech Info » Spark Plugs ➥ https://www.ngksparkplugs.ca/tech-info-spark-plugs.cfm

“... Bearing 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 water journals. 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 plug's heat range)....”

[Mike1956]

Thanks for all your advice,

Its a pity I noticed the heat range as it made me curious and led to me posting on here, had it not caught my attention I would have just bought a couple off ebay as usual. The plugs are just for engines on dinghys tootling about in the harbour accessing the boat, I guess they do not have the chance to get to full operating temperature after 3 or 4 minutes running.

As well as posting on here I contacted a NGK spark plug supplier on ebay who has replied. I noticed he has a plug for sale NGK BR7HS and another BH7HS - 10

He advises the number in the middle is the heat rating (7 in this case) but this begs the question what is the 10.

Any ideas

Mike

[GordMay]

Originally Posted by Mike1956

... He advises the number in the middle is the heat rating (7 in this case) but this begs the question what is the 10.

See the chart #2
The '- 10', at the end, refers to wide gap, 1.0 mm, or 0.040"

[a64pilot]

If you are unsure, start cold, always run the coldest plug you can without excessive fouling, too cold only means a fouled plug, too hot often means serious engine damage.

Most often you can clean a plug, as an old Hare Scrambles dirt bike rider we would often pull the plug and holding it upside down fill the cavity with fuel and set it on fire. This would often clean it enough to run well, but carrying a spare was far smarter.
That was back when we ran Castor bean oil which didn’t burn clean and fouled everything, but did keep an engine from seizing better than anything else.
A two stroke runs best lean, it makes it’s best power just before it seizes, so run a little rich and a little colder plug for safety and you’ll be happy.

Plug gap used to be extremely critical and often you could close up the gap on a fouled plug and get home, but that was back in the days of points and condensers, modern ignition systems are very, very much more powerful and I swear wold fire a rusty nail, but still take a Women’s finger nail file AKA Emory board and clean the electrodes and close the gap up a little and you can almost certainly get home.

[Mike1956]

Thanks again,

Everything much clearer now.

Mike