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Old 4 Days Ago   #1
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Hotter spark plug question

I'm thinking of using a slightly hotter spark plug in my 1986 Fiat 126. Stock 650 motor except for exhaust. Currently using NGK B7HS but thinking of NGK BPR6HIX. Is there any risk to the engine with the hotter plug?
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Old 4 Days Ago   #2
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Re: Hotter spark plug question

Quote Originally Posted by USA126 View Post
I'm thinking of using a slightly hotter spark plug in my 1986 Fiat 126. Stock 650 motor except for exhaust. Currently using NGK B7HS but thinking of NGK BPR6HIX. Is there any risk to the engine with the hotter plug?
You should be OK in theory if you drive normally, but if you are a pedal to the metal driver, over time pre ignition could occur. Why do you want to change?
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Old 4 Days Ago   #3
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Re: Hotter spark plug question

Jimboy, I have a engine hesitation issue right off of idle. A mechanic friend of mine took my spark plug wire and held it near the end of the spark plug and the car ran better. He said I should go to the next heat range plug. How does that sound to you?
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Old 4 Days Ago   #4
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Re: Hotter spark plug question

Quote Originally Posted by USA126 View Post
Jimboy, I have a engine hesitation issue right off of idle. A mechanic friend of mine took my spark plug wire and held it near the end of the spark plug and the car ran better. He said I should go to the next heat range plug. How does that sound to you?
Actually running a hotter spark plug will not give a bigger spark or more power, it just runs hotter due to thicker/longer insulation, this in turn burns off more carbon. Thatís the nuts a bolts of it. Not sure what you are saying about the plug leads.
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Re: Hotter spark plug question

Thanks for your expertise.
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Old 4 Days Ago   #6
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Re: Hotter spark plug question

From the description of holding the leads near the plug, I'm thinking he introduced a gap between the plug lead and the plug. That gap will be significantly larger than the one at the plug electrodes. The voltage necessary to jump the larger gap will be much higher than normal, and will of course transfer to the plug tip, giving a fatter spark.

This suggests that the contact breakers, coil, condenser, rotor arm, distributor cap or leads are between them not supplying a normal spark. One or more of those items needs servicing or replacing to regain normal ignition performance.

Changing plugs is trying to address an effect instead of a cause, and is a poor way to go. And it brings a risk.

Most engines burn at a similar temperature. Plugs have different heat paths due to different lengths of insulator inside the body, before they contact the body. This is chosen when the engine is designed to ensure the plug runs at its optimum temp. Heat path will be dependent on the cooling of the cylinder head, which is why we need different plugs to achieve the same internal temp for different engines.

When tuning engines for motorsport, different characteristics may be needed. This is the only time a different plug should be fitted. A plug with a longer heat path, to run the tip hotter, will burn off deposits more easily, but the hotter tip can, and does, melt a hole in the top of the piston. As those engines tend to be flat out or idling, the tisk is greater than with a more powerful engine running lighter loads.

My advice would be to stick with standard, and fix the rest.
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Re: Hotter spark plug question

This is an interesting question I wish I could answer, my interest is the fact I have a 1972 500l with a lot of engine mod work i.e Weber 40dco fast cam etc, one problem I have is every time I switch the engine off after a 15/30 minute run I get a loud backfire due to a build up of waste unburnt fuel. One suggestion from a very helpful forum member was to fit hotter plugs(you can follow my thread on classicfiat500) on the classic Fiat forum. at the moment the car is at an Abarth specialist trying to sort her out, so I will watch your thread with interest. Best of luck Barry
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Old 1 Day Ago   #8
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Re: Hotter spark plug question

Quote Originally Posted by ClassicFiat500 View Post
This is an interesting question I wish I could answer, my interest is the fact I have a 1972 500l with a lot of engine mod work i.e Weber 40dco fast cam etc, one problem I have is every time I switch the engine off after a 15/30 minute run I get a loud backfire due to a build up of waste unburnt fuel. One suggestion from a very helpful forum member was to fit hotter plugs(you can follow my thread on classicfiat500) on the classic Fiat forum. at the moment the car is at an Abarth specialist trying to sort her out, so I will watch your thread with interest. Best of luck Barry
There's a risk here that we may now create two threads within one, but here goes anyway.

If the engine internals have been modified, it is possible that the combustion chamber may run hotter. (Or cooler) This may then need a plug that will dissipate the heat more quickly, a cooler plug, or more slowly, a hotter plug. Changing from standard would normally be triggered by evidence on the plug tip of runnign too hot or too cold. (Cold is a relative term here, petrol generally burns at around 850deg celsius. So quite warm.)

A backfire, is when fuel in the carburettor intake ignites, which is rare, and usually caused by ignition timing issues.
More likely what you are experiencing is unburnt fuel passing into the exhaust, and exploding there. For either issue, the spark plug is irrelevant.

The exhaust manifold receives the exhaust gases, which are only slightly cooler than the combustion temperature, so normally at least 800deg C. Any unburnt fuel passing through the combustion chamber is likely to burn or explode in the hot manifold. It is unlikely that it will sit as a puddle, or a cloud of vapour, waiting until the engine is turned off before it combusts. What is happening is that when the engine is turned off, the ignition stops, but as the engine turns for a moment or two, more fuel is sucked into the engine, and passes through into the hot exhaust. Pop!
This has been a potential, and occasional, problem with many engines over the last 100 years. In the 70s, and until cars all went injection, carburettors were often fitted with anti-run-on valves to alleviate this. Sometimes the fuel would burn in the cylinder, so the engine continued to run without an electric spark. Sometimes a fix (band-aid) for this was a cooler plug, not hotter. An anti-run-on valve can work in either of two ways. One type shuts off the air supply to the idle passages in the carb, whilst the other type shuts off atmnospheric pressure to the float chamber, discouraging fuel from being drawn through the jet. Fixed jet carbs tended to go with the former, whilst the latter was more for variable venturi carbs, such as SU or Stromberg.

Do you blip the throttle just before you turn off the engine? This is a poor practice, with a flawed reasoning, and serves no purpose other than to waste fuel and wash the oil from the bores. But with an engine spinning faster, the opportunity to draw fuel through is greater. With any engine, once stopped, allow the engine to idle for a few moments before turning off. This reduces the risk of drawing fuel through.
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Old 1 Day Ago   #9
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Re: Hotter spark plug question

Hi Portland_bill, really good explanation & thanks very much, I hope Middle Barton Garage reads this thread, they are not far from you have you ever used them or do you do all your own maintenance? unlike me I have to rely on this brilliant forum where there is a wealth of great knowledge.
Barry
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Re: Hotter spark plug question

Quote Originally Posted by ClassicFiat500 View Post
Hi Portland_bill, really good explanation & thanks very much, I hope Middle Barton Garage reads this thread, they are not far from you have you ever used them or do you do all your own maintenance? unlike me I have to rely on this brilliant forum where there is a wealth of great knowledge.
Barry
Never had reason to use them. Been past the door a couple of times, only when they were shut.

From what you say, Middle Barton were not the ones who did the mods. And I'd not expect them to suggest a different plug, unless their diagnosis led them that way, but not for a pop on turning off. Different cam could be affecting the issue, so fixing that may be difficult. They've been specialising in classic 500s for a very long time, so it is in good hands. Let us know if/how they fix it.
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No M.B.G didnít do or over modify the engine, I am confident they are going to sort it though. Fingers Crossed
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