picture by
Robert Bosch CompanySpark plugs:
Most of us do not think much about our spark plugs, in fact I have reached the conclusion after working on cars for the last 35 years that most of you think they should last forever and one plug is pretty much the same as another!
In some ways you are correct and others, well let's just say NO! In today's cars with modern technology and fuel management plugs do seem to last forever, 35,000, 60,000 and even 100,000 miles between changes. But for most of you out there with older vintage cars 15,000 to 25,000 is more like it. There is not a great deal of difference between one top line plug and another beyond your own personal preference ( yes I am ducking now as each of you swears your plug brand is the best) What is different and very important is heat range and tip type - all of those numbers and letters before or after the main number on a plug. Take for instance Champion plug # N12 now this is much different than an N12Y though they will both screw into the same hole. A RBL12Y will not even screw into the same hole. But on the other hand a N12c is totally interchangeable with a N12 but not with a N11. Getting the wrong plug can cause no more problems than fouling out to burning a hole into your piston.
So perhaps we need to revisit that lowly little plug;
picture by
Robert Bosch Company
1: Connector - 2: Shell - 3: Iron/Glass seal - 4: Spark Plug Seat - 5: Insulator Seat - 6: Firing Electrode Pin
Cold plugs - Hot Plugs? What does it mean? The ceramic
insulator seats on a gasket in the shell, and that is where heat flows
out of the insulator into the shell and then into the head of the engine. With
more ceramic between the insulator seat and the firing tip heat travels more
slowly away from the tip and the tip stays hotter
The basic spark plug is much the same as it was 80 years ago. A steel outer shell threaded into the engine provides a ground path, and an insulator with an imbedded center electrode seats in the shell.
The ceramic insulator used in plugs made today is extremely hard, as it needs to withstand very high temperatures, vibrations, insulate high electrical voltages but still conduct heat. Once the insulator is seated in the steel shell, the shell is swaged at the top and then heat shrunk. This is what holds the insulator in place and forms the gas-tight seal that can withstand the pressures created in the combustion chamber. The steel shell is nickel plated to prevent rust and corrosion. You basically have two types of plug seats, tapered or flat. Flat have a compressible gasket and tapered do not. The small peanut plugs used in many late model cars are an example of a tapered seat plug.
Besides just sealing the plug into the combustion chamber the thread and seat transfer heat from the plug. One of the reasons you need clean threads and seat surface in your head.
The ground electrode is a separate piece that is welded to the shell. The size, shape and location influence the heat dissipation and spark formation. It is usually made of a nickel-chrome alloy or at least plated with it. Some plugs with special center electrodes may even have a pad of the same material.
The center electrode is where it gets to very interesting. It is actually two pieces. The ignition wire connector and the firing electrode itself. There is actually a slight gap between these two. The connector piece (the part you hook your plug wire to) Is usually copper or nickel-plated copper and extends about two thirds of the way down the insulator. And now is why you buy top line plugs. The most critical part of the plug is where this connector is sealed with a mixture of powdered iron and glass that melts when the shell is heat shrunk. This is the seal that prevent all of the combustion pressure from leaking back up the plug. It also acts as a high resistance conductor between the connector and the firing electrode. This actually increases your firing voltage and is referred to as the iron/glass seal. If you have a radio suppressor plug this is where the suppressor is. The seal also holds the firing electrode in place. The diameter of the firing electrode is smaller than the insulator hole so it can expand without cracking the insulator.
Older plugs center electrodes were made of a nickel-chrome alloy. Latter it was a copper core covered with a nickel-chrome alloy. Remember we need to withstand very high temperatures yet still be conductive. A hardened silver is used in some applications. Though its temperature resistance is less it excels at transferring heat to the insulator and really handles unleaded fuels well. Platinum pads may be mounted at the tip and a matching pad on the ground electrode. This reduces the firing voltage demand greatly. Building a platinum plug gets quite exotic sintered platinum is used and a tiny hole is drilled into the ceramics and then filled with the sintered platinum ( sintered means the powdered metal is heated to become one lump but not heated enough to melt) Platinum will with stand very high temps and higher temps reduce plug fouling and extend the life of the plug and keep good spark formulation.
To keep the tip from developing deposits the plug needs to be above 750 degrees F and it has to get to that temp very quickly after startup. But then you have to be able to shed heat almost as quickly as you build it as the tip can not go over 1650 degrees F even under full load!
Since combustion chamber design and material will play a large role in temperatures and heat transference many types of plugs have been developed to handle this. Extended tips, hot plugs, cold plugs, shielded tips. For most of us the plug recommended by the factory is the correct plug. But, change combustion pressures, increase or decrees fuel flow, change exhaust systems, change cooling systems, all will have a direct effect on the type and temperature of the plug used.
Spark plug gaps are mostly controlled by the capability of the ignition system used. Rule of thumb is a point system can produce or handle no more than 20KV at peak load. And no putting a coil rated at 40KV will not let you get anymore out of the system. Simple electronic systems, such as many after market bolt on. Will produce around 35KV at peak. Hei systems, Chevy is a prime example will produce 50KV at peak. Now plugging a Chevy Hei system into your Triumph and not addressing plug gaps and the like will just give you a system that can produce 50K but will only produce around 19K. The ignition system will only produce enough power to jump the distance between the two electrodes. If I leave my electrodes closed down to the .025 gap that was originally speced the most power it will take to jump that gap is 20KV no matter how much I have in reserve.
In a future article we will go into greater detail on what actually happens when the plug fires and why the correct gap is so critical
