Detonation, Pre-ignition, and the Danger of a Hot Spark Plug in an Ethanol Fuel Racing Application

By: Eric Niefert, Owner Horsepower Innovations LLC

I think there is a lot of confusion on the difference between Detonation (knock) and pre-ignition, especially when talking about spark plug heat range. Knowing and understanding these two abnormal combustion events helps one understand how to be a better tuner and a more competitive racer. It may also save your engine one day.

Just remember that detonation (also referred to as knock) usually happens before pre-ignition. An engine knocking is sort of a warning sign that pre-ignition is just around the corner. Knock tells you that you are having abnormal combustion events taking place and if you let it continue, you may get pre-ignition. With carbon rich fuels (i.e. gasoline) detonation can be seen on the plug's porcelain insulator as black specs like pepper. More severe cases show shiny aluminum specs on the porcelain or rounded edges of the electrode. That being said, ethanol is not a carbon rich fuel and it is very knock resistant so seldom do you see evidence of detonation on a spark plug in an ethanol burning engine.

This is how detonation happens. It takes place when there are two simultaneous flame fronts in the combustion chamber that collide. One flame front is from the ignition spark and one is from a hot spot somewhere else in the chamber (like an overheated exhaust valve or sharp edge due to lean mixture, overheated coolant, or too low octane fuel). Since both events happen at approximately the time of the ignition spark you get a "ping" caused by the pressure spike that happens when the two flame fronts collide. These pressure spikes are harmful to the engine over time, but are not usually an "instantly fatal" event.

On engine tear down detonation can be seen as a rough surface on top of a previously smooth piston or cylinder head quench pad. It also can be observed on bearings. The tip-off is that the bearing shells fall right out of the saddle when a bearing cap is removed. Fretting of the surface under the main bearing caps and loose cap fitment is another sign of more significant detonation. If you see this when freshening your race engine, NOW is the time to get the problem solved because if knock goes on it usually progresses to Pre-ignition.

Pre-ignition is when a hot spot (over-heated exhaust valve, carbon deposits, glowing spark plug) lights off the charge during the compression stroke long before the spark plug fires. Because the flame is burning way too soon and the piston is still moving upward pressure and temperature in the combustion chamber immediately and radically increase. Temperature will rise to the point where it exceeds the melting point of aluminum and piston damage results, sometimes in as little as one or two cycles. If you are lucky the plug tip burns off first which kills the cylinder before the piston is damaged.

The extreme heat of pre-ignition can also expand the top compression ring until the ring gap closes. When this happens the ring ends butt together, but the ring continues to grow and must go somewhere. A broken ring land is the result. Quick engine destruction happens in a split second from here.

While detonation is not as common in an Ethanol powered race engine, they are not immune. They are definitely not immune to pre-ignition either! Since there is less evidence of detonation in an Ethanol engine, there is not a large window of "knock warning" that is easily heard (rarely is it ever heard with any fuel on a loud race car!) or seen on a spark plug like with gasoline. Ethanol is a great fuel but you cannot expect to see detonation on a plug tip or porcelain to warn you before pre-ignition occurs.

What you are doing when you follow the "hot plug, rich mix" strategy is using a hot heat range plug (like an NGK -5) to help overcome the difficulty of lighting a very rich ethanol charge. It is debatable as to whether there is more power to be gained when operating in that very rich range -- I believe NO, but that is another subject for another day. However, it is not really debatable that a very hot spark plug heat range does in fact make an engine more prone to pre-ignition. It most certainly does!

Here is why very hot spark plugs are especially dangerous in an Ethanol fueled race engine. Using a very hot spark plug will NOT cause detonation. You heard me right -- the hot plug will not cause knock. If the plug tip glows it does not cause a secondary flame front because it originates at the plug, IT CAUSES PRE-IGNITION! When a glowing plug tip kicks off the mix in the chamber before the spark happens no flame fronts are colliding, but the combustion event is starting too early. The lack of detonation is irrelevant because we now have a much more severe form of abnormal combustion, pre-ignition.

Even if you can keep the hot heat-range plug cool enough to stave off pre-ignition with excess fuel, the first time the engine goes lean due to a missed tune, leaner fuel blend, plugged fuel filter, failing fuel pump or regulator, chunk of fuel cell foam in a needle and seat, (I've seen them all!) you are now operating with a very hot plug and insufficient fuel to cool it. Here comes pre-ignition without any knock warning at all. You are literally playing with fire.

An alternative strategy to the hot plug technique (if you decide you must run in that tune area) is to close the plug gap a little, use a more powerful ignition system, and use a cooler (safer) heat range spark plug. I use an MSD digital 7 box with an HVC coil and have been able to light off just about any ethanol tune imaginable with an NGK -7 or -8 plug gapped at .028". There have been thousands of tunes tested on my engines successfully with this ignition system and a moderate heat range plug! If a high energy box and coil doesn't do it for you (extreme boost , big nitrous, or very high compression), you may have to go to a magneto as often used on methanol race engines.

To be clear, I would suggest that anyone that is having trouble lighting off their mix (you're having misfire problems) and a hot heat range plug helps, I would look at a more powerful ignition system and stick with a cooler plug. Your pistons and head gaskets will thank you!