Detonation: A violent explosion; also called combustion knock. This usually occurs near the end of the combustion process when highly compressed, high-temperature end gases spontaneously ignite, radically increasing the cylinder pressure. This pressure spike moves at the speed of sound in the combustion chamber, and the pressure can cause damage to pistons, cylinder walls, and the head gasket.
Pre-ignition: The onset of combustion before the spark plug fires. This is generally caused by some type of glowing ignition source such as a hot exhaust valve, too-hot spark plug, or carbon residue. Pre-ignition is especially damaging to engine components like pistons and head gaskets, since excessive cylinder pressures can occur even before the piston reaches top dead center (TDC).
These are the classic definitions of detonation and pre-ignition. Perhaps a more fun definition of detonation would be to imagine the piston screaming up to TDC while you whack that piston as hard as you can with a 10-pound sledgehammer. The clang that you would hear is the same noise that occurs when your engine goes into detonation. Even if detonation doesn’t break any parts, as soon as an engine experiences detonation, the power drops way off. A secondary definition of detonation is uncontrolled combustion, which means that power suffers.
If you get the idea that detonation and pre-ignition are bad, that’s good. Of all the things that can kill an engine, detonation should be right at the top of the hot rodder’s Public Enemy Number One list. The quickest and easiest way to cure detonation is to use a high-quality, higher-octane gasoline. But this isn’t the only solution. Since we’re all in search of more horsepower, especially if we don’t have to pay more to get it, we thought we’d take a look at some solutions to all the rattling and banging going on that’s killing engines and power. We have some elegant solutions that may allow you to make 400 hp with 87-octane gas instead of that more expensive 92-octane stuff. The key to this whole story is the sidebar “Tips to Reduce Detonation” .
We’ve broken these tips down into some examples that are cheap and easy, and others that require more effort and money. We’ll take these ideas and explain why each one could offer some real opportunities to use less expensive, lower-octane fuel for your street runner.
Perhaps the easiest and least- expensive way to reduce an engine’s sensitivity to detonation, which would allow you to run 87- or 89-octane fuel, is to cool the engine-inlet air. Not only is cooler air more dense, which makes more power, but cooler air is also less prone to detonate. The classic performance rule-of-thumb is that for every 10 degrees you reduce the inlet air temperature, the engine makes 1 percent more power. This is why drag racers use ice to cool the intake manifold and why all those cold-air inlet systems work on late-model cars. Forcing your engine to breathe hot underhood air will also make it more prone to detonate, so seal that cowl-induction hood to your carb.
If you are running a stock or mild dual-plane intake manifold, blocking off that heat-crossover passage that ducts hot exhaust gas underneath the carburetor will also reduce inlet air temperature. Of course, this will increase the warm-up time when the engine’s cold, so this may not be a good idea for a daily driven street car in the wintertime.
Ignition timing is another cheap and easy area to work on. If your engine detonates at low engine speeds at part throttle, consider retarding the initial timing by 2 or 3 degrees and then adding that amount back into the total by increasing the mechanical-advance curve. For example, let’s say you have 18 degrees initial timing with a total of 36 degrees and your engine rattles a little at part throttle, especially right off idle. You could cut the initial back to 15 degrees and add 3 degrees to the mechanical advance. The total remains at 36, but now the engine doesn’t death rattle every time you let the clutch out.
Also experiment with the vacuum-advance timing. Crane, Mr. Gasket, and others sell adjustable vacuum-advance cans that allow you to customize both the start point and the rate at which it comes in. Let’s say you want to experiment with 87-octane instead of 92. This will probably require a slower rate of advance, but the total at cruise could remain the same. That’s something that would require some tuning—but very little money.
Camshaft timing also plays a huge role in dynamic cylinder pressure, especially with street-driven performance engines. As you increase intake duration, this means the intake valve now closes later than it does with a shorter-duration cam. This later-closing intake valve bleeds some cylinder pressure back into the intake manifold at lower engine speeds. The longer the duration of the cam, the later the intake closes. This reduces cylinder pressure at lower engine speeds, which reduces the tendency for the engine to detonate.
Late closing of the intake can also be accomplished by retarding the camshaft’s installed point. For example, many small-block Chevy cams are installed with the intake centerline at 106 degrees after top dead center (ATDC). This tends to close the intake valve sooner, which improves low-speed torque by increasing cylinder pressure at low speeds. But if the engine rattles at low speeds, retarding the closing point of the intake valve can by 3 or 4 degrees (from 106 to 110 degrees ATDC) softens the engine’s need for higher-octane fuel.
Obviously, this is a little more difficult to do than playing with ignition timing but may pay off by allowing you to run a lower-octane fuel. If you do retard the cam, it’s important to go back and perhaps add a degree or two of initial ignition timing.
You can also experiment with camshaft overlap. Unfortunately, this requires a new camshaft. Tightening the lobe separation angle, from 114 degrees to 110 degrees, for example, increases the amount of overlap since the exhaust valve closes slightly later and the intake valve opens a little sooner. This tends to bleed off cylinder pressure at lower engine speeds, which could be beneficial since this is a little like built-in exhaust gas recirculation (EGR) in the intake manifold.
There are several other ideas that you can try to reduce your engine’s sensitivity to detonation and allow it to live on lower-octane fuel. Any kind of oil contamination in the combustion chamber is bad news. Oil is a great breeding ground for creating detonation. The best way to avoid this is to ensure your combustion space enjoys the benefits of tight valve-to-guide clearances and good leak-free valve guide seals. Of course, you want to seal up that intake so it doesn’t suck oil into the cylinders, and your short-block should be in good shape.
We’ve just skipped over the surface of this rather complex subject matter. In fact, we decided to not even get into fuels and how they do what they do. If there’s enough interest in this subject, we can dive into that, at a later date. As you can see, there’s plenty to cover.
