Spark Plug Heat Range question, Run a hotter plug with dual carbs? To do or not to do? Options

[UberElectricEagle]

Hello,
Should I, or May I, use a hotter plug with dual carbs? I have NGK B6ES in there right now. One range hotter would be a B5ES, Right? I seem (or think) to remember with my 67 BSA that NGK gets hotter the lower the number.

1.7 liter with dual weber 34 ICT. 009 dist with regular old points.

Many thanks to you.

[ericread]

Since no one lese has answered, I'll throw in my $.02.

Please note that plug heat range has nothing to do with the voltage that is sent through the plug. Heat range is a measure of the spark plug's ability to remove heat from the combustion chamber, mainly to avoid pre-ignition, which can be disasterous.

Pre-Ignition can be defined as ignition of the fuel/air mixture before the pre-set ignition timing mark. This is caused by hot spots in the combustion chamber, which results from advanced ignition timing, a spark plug that is too hot, high compression, low octane fuel, lean air/fuel mix, insufficient engine cooling, or carbon in the combustion chamber. Pre-ignition and detonation are separate events, but pre-ignition frequently leads to detonation

Detonation involves rapid, uncontrolled burning of the air/fuel mixture while the piston is still rising. During normal combustion, the flame front propagates across the cylinder at a controlled subsonic speed with the piston near TDC. During detonation, the flame front goes supersonic and/or collides from different directions. This rapid detonation strikes the top of the piston as it is still being pushed upward in the cylinder by the crankshaft. The shock wave resulting when the detonation flame front strikes the top of the piston causes the piston to rattle in the cylinder. The sound of this shock wave and the sound of the rattling piston is what people commonly refer to as ping. Detonation radically increases cylinder pressure and temperature, and can quickly burn the end of the spark plug. Excessive heat is usually what causes engine damage when detonation is ignored.

Back to spark Plugs:

The heat range is determined by the insulator nose length and its ability to absorb and transfer combustion heat, the gas volume around the insulator nose, and the materials/construction of the center electrode and porcelain insulator.

OK, what engine parameters affect the operating temperature of a spark plug?

1. Air/Fuel mixture: Rich mixtures reduce the tip temperature, causing fouling and poor drivability. Lean mixtures increase both tip temperature and combustion chamber pressure, resulting in pre-ignition. It is important to read the spark plugs frequently while tuning after engine modifications, the spark plugs will help you achieve an optimum air/fuel ratio.

2. Compression ratio/forced induction: As effective compression ratio increases through engine modifications or forced induction, a colder plug using a narrower gap and higher ignition voltage is required.

3. Ignition timing: Advancing the ignition timing by 10° increases the plug tip temperature by approx. 70°-100°C.

4. Engine speed and load: Increases in firing-end temperature are proportional to engine speed and load. When consistanly traveling at high engine speeds or hauling, towing, or pushing loads - install a cooler plug.

5. Weather: Temperature, humidity, and barometric pressure all affect air density and intake volume. Higher intake volumes result in leaner mixtures and hotter plug tips.

In identical spark plug types, the difference from one heat range to the next is the ability to remove 70°C to 100°C from the combustion chamber. A longer the nose on a spark plug forces the heat from the tip to travel farther before it is absorbed by the cylinder head, which reatins more of the heat in the plug tip - making the plug "hotter" than a similar plug with a shorter nose. Engine temperature will affect a spark plug's operating temperature, but not the plug's heat range.

Soooo, what have you changed. I assume in going from FI to dual carbs, you installed a new cam? Has your engine work affected ignition timing, compression, air-fuel mixture? Have you installed a cylinder heat temperature (CHT) guage so that you can see at what temp you engine is running?

None of the above really answers your question, but it at least frames some of the issues.

Eric Read

[UberElectricEagle]

Thanks for the reply Eric. Educational and informative.
I usually try not to read that much in one sitting.