1750 degrees. That’s where all the trouble starts. Detonation, pre-ignition, too much compression, too much timing are some of the buzz phrases surrounding how we got there. Sure your E.G.T. gauge only showed 1500 degrees but most of the time you can add 200 degrees to that to get an actual number on what is going on in the chamber.
O.K. back up a minute. Lets relate this to what our customer base is calling us and asking. Most of the time when we get a customer selecting a carburetor he already has an engine program in his mind. However, he almost always questions spark plug selection. Selecting spark plugs is one of the most difficult tasks in a non-nitrous application and only slightly less of a struggle in a nitrous design.
We all know that nitrous engines make a lot of heat while they’re being sprayed and we need to be sure not to burn the tips off during the run. But there are other factors involved in order to maintain consistency in a program.
Keep it clean. A spark plug that is too cold will try to foul as you idle around or stage after a burnout then the plugs clean up as you travel down the track. A plug that is not firing properly puts an extra load on the rest of the engine (an unbalanced load) and is one of the leading causes of engine and even transmission failures. An erratic running engine stresses everything from the valve train to the chassis. So be sure to select a heat range that’s warm enough to burn the fuel when it’s introduced, yet cold enough to stave off pre-ignition as the vehicle travels down the track. Many racers use a hotter plug for cruises and light nitrous runs and then switch to a colder plug for serious nitrous or heavy WOT use.
Remember as you increase the RPM and load on the engine the plugs tip temperature really climbs fast. Its only a thin piece of metal but it harbors a deep secret. The tip is responsible for handing out the kernel of spark that initiates the combustion process. The larger that kernel of spark is, the more fuel that will burn “in the cylinder” at a USEABLE crankshaft angle during the combustion process. Think of it as a forest fire. If I drop a lit match in the forest, in about an hour I may burn an acre of woods. But if I throw a Molatov Cocktail into the forest that same acre will burn in just a few minutes. Now you see why a strong ignition system can increase an engines performance.
The fuel is burning in a tighter range of time and at a more usable crankshaft angle. That is why some engines will really respond to more gap spacing, provided you have a strong enough ignition system to blast the gap. Think of it this way, a fluorescent light tube can be 3 foot or 6 foot long. Both designs will use the same amount of voltage to ionize the gases but the 6 footer will require a larger ballast to jump the gap. Sure the 3 foot ballast may eventually get the 6 footer lit up, but until then you’re in the dark. So the idea here is to have a LARGE spark kernel. If you have to close the gap up to get the kernel larger then close it up. No one likes being in the dark.
Although an IMEP rating is a great way to compare sparkplugs, it rarely works in the real world for initial selection. Combustion chamber designs, compression ratios, camshaft duration, etc…offer up too many variables to use this pre-ignition rating system as a rule.
The best unit of measurement for selecting a plugs heat range is by mapping the “temperature curve”. However, not many of us have access to amp driven resistors in the chamber to measure the temperatures. So once again you’re still back to reading the plugs keeping in mind that the combustion chambers temperatures change with fuel type, air temp and vehicle load. E.G.T. sensors can’t be used reliably for initial setup as you may be mislead by residual gases.
So it comes down to the “human factor”. One machine that can look and listen. Listen to the engine as its idling. Is it crisp and crackly? If it is, then the sparkplug is good and hot. If its too hot, it’ll detonate. If its too cold, it’ll foul out. Hence the balancing act begins.
So in short, here’s what to look for: A spark plug that is too hot will show excessive electrode erosion and “blueing”. This excessive tip temperature heat will burn the fuel in two stages. First, the excessively hot tip will light or smolder the incoming mixture (match in the forest), then the ignition throws the Molatov Cocktail. You end up with an erratic burn and a loss of mph in lower performance engines (typically engines that operate in the .45-.50 BSFC range), and detonation and piston holes in higher performance engines (typically engines that operate in the .37-.45 BSFC range). You can look for this condition by the air fuel line at the base of the porcelain. If the lines not sharp, the cylinder isn’t either.
If the plug is too cold, the engine will be lazy and blubbery and you’ll be cleaning out the plugs as you travel down the track.
Listen to the engine, IMEP numbers, A/F ratios and E.G.T.’s are only references to get you base-lined and can only be used as a rule AFTER the engine is tuned for best performance
So use the human factor to your best ability. We’re a pretty versatile machine, you and I. Sure the numbers are great but sight, sound and smell isn’t overrated.
Thanks for the business, its been a great year at Pro-Systems and thanks for reading. Hope this helps out.