Detonation is a great thing if you’re taking in a fireworks show or maybe watching MacGyver.
Inside your engine? Not so much.
In fact, it’s probably best if you avoid detonation at all costs where your engine is concerned. Detonation occurs when excessive heat and pressure in the combustion chamber causes the air/fuel mixture to ignite on its own. Instead of a typical single flame kernel within the chamber, this creates multiple flames which collide with explosive force. This causes a sharp, sudden rise in cylinder pressure which subjects engine internals—pistons, rings, bearings, gaskets, etc.—to severe overload and creates a pinging or knocking sound. Worst case scenario: you’re looking at costly, if not catastrophic, engine damage.
#1. Up Your Octane
The higher the octane number, the better the fuel’s ability to resist detonation.
Most engines are just fine on standard 87 octane; however, engines with high compression (9.0:1 and over) or forced induction (blowers or turbos) may require 89 or higher octane. Also, applications where the engine sees increased load or stress, such as towing or heavy hauling, may require additional octane levels. Basically, anything that causes higher combustion temperature and pressure or causes the engine to run hotter than normal can lead to detonation.
It may be time to up the octane.
#2. Keep Compression Reasonable
A static compression of 9.0:1 is typically the recommended limit for naturally aspirated street engines (although engines with knock sensors may be able to handle higher compression). For forced induction, a static ratio of 8.0:1 or less may be required depending on the amount of boost. A compression ratio over 10.5:1 can create detonation even with 93 premium gasoline.
The trick is to keep the compression ratio within a reasonable range for pump gas unless your engine is being built to operate on racing fuel. To accomplish this, you may need to use lower-compression pistons, opt for cylinder heads with larger combustion chambers, or try using a copper head gasket shim with the stock gasket to reduce compression. Also, if you’ve bored the engine cylinders or milled the cylinder heads, this increases compression and you may need to make accommodations.
#3. Check Your Timing
Over-advanced ignition timing can cause cylinder pressures to rise too rapidly and eventually lead to detonation. Reset your timing to stock specifications. If that doesn’t work, retard the timing a couple of degrees or try re-calibrating the distributor advance curve to keep detonation under control.
#4. Manage Your Boost
Controlling the amount of boost in a forced induction engine is critical.
Too much boost can lead to detonation, so you’ll need to either A) scale back boost, or B) outfit your engine to handle more boost. For example, in a turbocharged application, you’ll need to make sure your wastegate is operating properly to bleed off excess boost pressure. Leaks in vacuum connections, a defective intake manifold pressure sensor, or poorly performing wastegate solenoid control can cause the turbo to deliver too much boost. These things should be remedied. And you may also want to add a better performing intercooler while you’re at it.
#5. Monitor the Mixture
Lean air/fuel mixtures are prone to detonation.
Check your air/fuel mixture and adjust accordingly. A lean condition could be a symptom of a larger problem such as air leaks in vacuum lines or poor-performing gaskets. It could also be caused by dirty fuel injectors, clogged carb jets, or a restricted fuel filter. If your engine is experiencing hesitation or rough idle, you may be dealing with a lean fuel condition and will want to make appropriate adjustments or fixes before detonation occurs.
#6. Blow out the Carbon
Carbon deposits are a common cause of detonation in high-mileage engines.
Essentially, carbon deposits can accumulate in the combustion chamber and on top of pistons until the overall compression of the engine is altered. In addition, the deposits can create an insulating effect that slows heat transfer from the combustion chamber to the cylinder head. If deposits build up enough (and compression goes up enough), detonation can occur.
Like the lean fuel ratio above, carbon deposits could be a sign of another problem: worn valve guides, cylinder wear, broken piston rings, or infrequently changed oil. Check for the root cause of the deposits, fix any issues, and then remove the deposits with a chemical cleaner or via wire brush or scraper (requires removal of heads).
#7. Examine Your Knock Sensor
Many late model engines have a knock sensor which may become defective.
The knock sensor responds to vibrations within a certain frequency range. When the frequencies, which are typically produced by detonation, are detected, the knock sensor tells the vehicle’s computer to momentarily retard the ignition until detonation stops. If defective, this sensor would become ineffective.
If your vehicle’s “check engine” light is on, you may have a bad knock sensor (among other things). You could check the onboard computer system by reading the engine’s trouble code with the right tools. Or you can test the knock sensor by tapping a wrench on the manifold near the sensor and watching for a timing change. If the timing doesn’t retard, the sensor may be bad. You will need to find the proper diagnostic chart in a service manual for your vehicle to determine the cause.
#8. Read Your Spark Plugs
Be sure to read our earlier post on how to read spark plugs.
You can tell a lot about your engine’s performance by reading your plugs. For example, if your spark plugs appear yellowish, blistered, or broken, they may be too hot for the application. Try spark plugs with a colder heat range to avoid potential detonation. See our post on spark plug heat range for more tips.
#9. Consider Your Cooling System
If your engine is overheating, it is more likely to suffer spark knock. That’s why you should make sure your cooling system is in good order. Check your coolant level and fill if necessary. Make sure your fan is properly sized for the occasion. And look out for a bad water pump, missing fan shroud, too hot a thermostat, slipping fan clutch—basically anything that can prevent your cooling system from operating efficiently.