How to Reduce Engine Knock
4 min read
To understand how to reduce engine knock, I think it’s useful to first have a good understanding of what engine knock is. Fortunately there is an article that covers engine knock in detail here.
Engine Knock in Short
Spontaneous ignition of the air fuel mixture in the cylinder before the flame front from the spark plug reaches it. The air fuel mixture is ignited due to heat from the compression process and the flame front in other parts of the combustion chamber.
This causes pressures in the cylinder to spike, and at the wrong time which can potentially cause catastrophic engine damage.
Two causes
Fuel air mixture reaching a high enough temp where it can ignite without a flame/spark.
Contaminants in the combustion chamber which are hot enough to ignite the air fuel mixture.
Keys to Reducing Engine Knock
Reduce/eliminate the amount of contaminants in the combustion chamber
Keep the temperature of the air fuel mixture in the combustion chamber lower so it does not reach a high enough temperature to self ignite
Make the air fuel mixture as homogeneous , as mixed, as possible
Contaminants
Where do they come from?
From whatever get’s into the combustion chamber.
Fuel? Possible. If engine is runs very rich we could have carbon build up.
Air? Sure, if the air filter is letting in dust.
Oil? Sure, if engine condition is not what it could be and/or we have excess oil coming from the PCV system.
Reducing all of these will reduce the engine’s tendency to knock which will improve reliability and/or power.
How to reduce these?
Pretty self explanatory.
Run lower AFRs
Use a quality air filter, personally I prefer paper cylinder type of a suitable size.
Use a catch can to trap oil before it’s sucked back into engine. Check gaskets. Rebuild engine if problems mechanical.
Temperature – Environmental Causes
High coolant temps
High intake air temps
High cylinder head temps
Restrictive exhaust
Low octane fuel
Better tune
High combustion chamber temps
Possible solutions
Reduce coolant temps. Add an oil cooler, bigger radiator, lower temp thermostat, add ducting to radiator, more water in coolant mix, better flow through engine bay with undertray, check system operation (air locks, flow and temps)
Reduce intake temps. Shield filter from heat, add ducting to filter, better intercooler, better ducting to intercooler.
Reduce cylinder head temps. See reduce coolant temps. Phenolic intake manifold spacers used to be popular. However insulating the intake manifold from the cylinder head logically will increase the temp of the cylinder head (because the heat can’t be dissipated by the inlet manifold) which could lead to high combustion chamber temps and more knock. I see plus and minuses with these spacers. Personally I would avoid.
Free up exhaust. Check back pressure with sensor. If restriction high check components for operating, design and diameter.
User higher octane fuel. Connected to previous section about reducing fuelling. Higher octane should allow less rich mixtures without detonation
Better tune. Less advance, less boost, more fuel.
Lower combustion chamber temps. Add fuel if AFRs not too bad/excessively smokey. And all of the above.
Reduce Piston Temps. Piston oil squirters are on of the most effective ways of increasing the knock limit of an engine.
Water Injection
Water injection can reduce many of the factors that cause engine knock.
Starting with contaminants.
Water injection will clean the inside of the engine.
Which can allow richer mixtures without the same carbon build up
Port injection definitely preferable but this also means the effect on lowering intake temps is less.
Effectively increases fuel octane, especially if running higher percentages of methanol.
Lower combustion chamber temps. The latent heat of vaporisation sucks massive amounts of heat of the combustion chamber when the water is turned to steam. This vaporisation lowers the chamber temps below what it would normally be, again, increasing the octane of the fuel, reducing the engine’s tendency to knock and allowing ignition timing to be advanced to increase power.
There are many water injection systems available in the aftermarket, however Aquamist, appears to be by far, the most well respected.
Mixing
How to make as much power as possible from a knock limited engine.
(Paraphrasing parts of this podcast)
When we are trying to make as much power as possible but you are knock limited, the priority “completely” changes from getting as much flow as possible to into the chamber, to getting a fantastic air fuel mix as is possible. We want to create as much tumble as possible into the incoming air and we in turn we want the combustion process to be as fast as possible. We want to get to peak cylinder pressure in as little time as possible from the spark plug being fired.
The restriction to airflow is overcome by running loads of boost, that creates a lot of tumble, that creates a uniform air fuel mixture, that burns quickly, which creates higher peak cylinder pressure, over a shorter time as possible.
