Do split tip sparkplugs really work?

When it comes to performance and race engines, the short answer is... no.

Electricity follows the path of least resistance. It has nothing to do with a split tip or a fancy U groove! The spark plug does one thing only. It acts as the grounding source for the ignition's electricity and that's it, period, end of story. The only difference with a spark plug is that there is a gap in the current path which makes the spark. The size of the spark is dictated by the voltage and amperage output of your ignition system (your module, coil, etc), and the output of your coil is dictated by the saturation (dwell) time of your ignition system. You'll hear claims like, "it made more horsepower" or "I got better gas mileage." Well of course they did! Their engine needed a tune-up or they wouldn't have been changing the spark plugs in the first place! ANY engine will get better gas mileage and make more horsepower after a tune-up! I mean, that's pretty much the entire reason why you give your engine a tune-up is because it isn't running very well and isn't getting very good gas mileage, right, so of course after installing a new set of plugs you see better performance... it'll happen with ANY new plugs. 

One really bad thing split tipped spark plugs can do is promote pre-ignition and/or detonation, ESPECIALLY in a high performance engine. How? Try this; take a paper clip and heat the end up with a typical Bic lighter. It gets red hot real fast and glows cherry red... just like a glow plug! The last thing you need (or want) in any kind of performance engine is a glow plug because the quickest way to destroy a performance engine is from detonation, and that's exactly what you'll get if you have anything inside the cylinders acting like a glow plug. 

Now take something a little thicker, like a piece of welding rod and try to heat it up with the same Bic lighter. Oh yeah, it gets hot, but not hot enough to glow cherry red in 2 seconds and become a glow plug. It's simple; it takes more heat to heat-up larger / thicker objects than it does smaller / thinner objects. What do you think those tiny little thin pieces of split tip ground strap are doing inside your engine? Glowing, because the mass of the ground strap has been cut by 50% when they split it! Now add more compression and burn more fuel which causes more heat, like what you get in any typical performance or race engine, and those thin split ends act like dog gone glow plugs causing pre-ignition which leads to detonation, which in turn leads to... ka-boom! In pretty much that order.

Now, there ARE things you can do to help the burn in the cylinder (which actually will increase power), such as indexing the spark plugs. It's easier to show you how to do this than to tell you, but here's a quick overview so you'll at least get the basic idea.

Indexing a spark plug puts the open end of the gap towards the intake valve, or towards the center of the cylinder. This way when it sparks, the ground strap isn't in the way, blocking the flame path into the cylinder. It exposes the open end of the gap towards the fresh air / fuel mixture coming through the intake valve and into the cylinder, which helps light the cylinder faster, better and more evenly. A better burn = more power. There's a little more to it, and like I said, it is easier to show you than it is to write it out.

You can see in the picture what a typical good index looks like and how the open-end of the gap is exposed towards where all of the fuel and air is at to promote a better flame travel. You can enlarge the picture by clicking on it if you want to see it better.

To index your plugs, you start by marking the outside of the spark plug with a Sharpie pen in line with the back side of the ground strap. When you screw the plug into the head, try to get the mark to point away from where the intake valve would be. This puts the back side of that ground strap pointing at the exhaust valve or at the back side of the combustion chamber, which in turn points the open-end of the gap more towards the intake valve and into the open area of the cylinder for a clean, unobstructed flame path. They actually make special crush sleeve "washers" that go onto the spark plugs so you can snug the plug up, yet continue to turn it to "index" the plug to where you want it. You can order them at any good performance parts supplier. They're just called "Spark Plug Indexing Kits".

Obviously heat range has a little to do with how a plug fires too, but that subject is always a mystery to many people. Many guy's run way too cold of a plug in their performance engines because "their buddy" told them to. I always get a kick out of that. If I had a nickel for every time I heard someone say "my buddy told me to do that" for something that totally didn't work, I'd be a millionaire! Anyway, cold plugs don't retain enough cylinder heat to burn-off deposits and they foul quickly, thus making your engine run like crap. A hot plug runs cleaner because nothing can stick to it. When you clean greasy dishes in your sink, you use hot water to cut the grease, right? Well, spark plugs aren't much different. Now, the wives tale is that a hot plug will make your engine run hot, and that is about the most ass-backwards thing as you can get. Hold a new spark plug in your hand. Is it hot? NO! So how can something that isn't hot, and that doesn't create heat make your engine run hot? It can't. The cylinder temperature inside your engine heats the plug up, not the other way around. All a hot or cold plug means is how hot the core temperature (or running temperature of THE PLUG) will be. It has NOTHING to do with engine temperature, period! If you run too hot of a plug, it will allow it to retain too much heat and it will begin to melt off the electrode, thus causing the engine to misfire. The trick is, you want the hottest plug you can possibly run WITHOUT melting off the electrodes.

Now, a cool trick for nitrous and blower guy's is when you run a plug that is on the hot side, you can actually use the plug as a "safety fuse". In other words; if your plugs run just under the melting point, then if you have the misfortune of leaning the engine out, (which is THE worst thing you can do to a nitrous or supercharged engine), it will increase the plugs core heat and melt off the electrode, thus causing that cylinder to not fire anymore. No more fire = no heat. No heat means no lean-out melt down. Obviously it takes time and testing to figure out exactly what to run, but these tips give you some good ideas and starting points anyway.

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