Dodging Detonation - Part Two

This article was first published in 2004.

In Part One of this two-part series we had a look at the background of detonation – when you might experience it, how to detect it, what type of detonation to look out for and the causes of intolerable detonation (detonation that is severe enough to lead to engine damage).

In this article we’ll offer you the solutions to detonation problems.

No/Low Cost Detonation Solutions

There are several no/low cost ways to solve an engine’s detonation problems.

The easiest way to avoid detonation is to use the highest octane fuel available in your area. Sure, high octane fuel might cost a few cents more per litre than the low-grade stuff but the long-term saving is potentially thousands of dollars. Using premium unleaded may also release slightly more power from your car’s engine.

If your car’s engine continues to detonate on the highest grade fuel that’s available, it may be worth adding octane booster to each full tank. However, at a cost of more than AUD$20 per bottle, there are more cost-effective solutions.

Spark plug choice can play an important part in avoiding detonation. If an engine is detonating when using the standard heat-range spark plugs, you might want to try a set of plugs 1 or 2 ranges colder. Colder spark plugs have a shorter centre electrode, which serves to more effectively transfer heat away from the plug. Note that spark plug brand can also make a difference to detonation. The only downside of colder plugs is potential spark-plug fouling.

A relatively easy and cheap mechanical modification that helps avoid detonation is the fitment of a cold air intake. Many modern vehicles draw hot induction air from inside the engine bay. The likelihood of detonation is reduced by relocating the air intake pick-up to an area isolated from engine bay heat. Inside the wheel arch is a popular choice.

Air-fuel ratio and ignition timing are also critical factors in avoiding detonation.

Air-fuel ratios can be cheaply adjusted in any EFI car that uses a voltage type airflow meter or MAP sensor load input. The voltage output of an airflow meter or MAP sensor can be altered using the technique discussed in a previous article (which we’ll come to). By increasing the output of the load sensor, you will achieve a richer air-fuel ratio. Ignition timing may also be slightly retarded. Note, however, light-load closed-loop running will not be affected with just these modifications.

Another way to increase fuelling is to feed the ECU a false coolant temperature reading. Use an adjustable potentiometer on the coolant temperature circuit to fool the ECU into thinking the coolant is colder than it really is. This will almost exclusively richen the air-fuel ratio. Note, however, this enrichment might occur only at relatively low revs and/or load.

And what about the all-important ignition timing?

A zero cost – but crude - way to prevent detonation on some cars is to simply retard the distributor or crank angle sensor slightly. This has the effect of retarding ignition timing a few degrees across the board. The downside is reduced engine performance at all revs.

Another way to retard ignition timing is to alter the output of the engine’s intake air temperature sensor using a potentiometer. By fooling the ECU into thinking intake air temperature is hotter than it really is, it will give retarded ignition timing.

Note that the engine management tweaks mentioned above are described at DIY Modification of Car Electronic Systems - Part 2

Sophisticated Detonation Solutions

The actions described above are perfectly adequate approaches to eliminating detonation. However, if none of these work – or you simply don’t want to go down these tracks – you can turn to these higher cost alternatives...

For rpm and load specific changes to air-fuel ratio and ignition timing you’ll need an aftermarket interceptor, an ECU remap or you can start from scratch with an off-the-shelf programmable ECU. The costs involved may run to thousands of dollars but, on the upside, you’ll have the tuning flexibility to make the most of the vehicle’s existing mods. These approaches also allow the engine to be retuned to suit later set-ups.

Turbo and supercharged cars – in particular those that are modified – rely heavily on an intercooler to eliminate detonation. If money isn’t a problem, you can purchase a large-capacity aftermarket air-to-air intercooler from anywhere from AUD$400 to AUD$2000. These gigantic intercoolers offer a huge thermal mass, large frontal cooling area and sophisticated internal finning to reduce charge-temperatures to near ambient. These are an all-out solution to charge-air cooling.

Note, however, the heat exchange performance of your vehicle’s intercooler can be improved in a couple of ways - there’s often no need to buy an aftermarket replacement.

The most popular intercooler enhancement is the fitment of a water spray. A water spray removes heat from the charge-air through the evaporation of water on the core. An intercooler water spray performs best when there’s always cooling airflow through the core (eg induced by a fan) and when a water atomising nozzle is employed. See Intelligent Intercooler Water Spray - Part 1 for details.

If detonation problems are evident only in traffic conditions, your solution might be the fitment of an electric fan to draw cooling air through the intercooler core when the vehicle is stationary. This will remove the intake air temperature spikes that occur in heavy traffic. See Fan-Forcing Your Intercooler, Part 1.

An interesting approach that can reduce the likelihood of detonation on some engines is the use of intake manifold insulators. Manifold insulators are intended for use with alloy intake systems, which are prone to heat-soak as the result of under-bonnet heat. These spacers – usually made from phenolic material – are sandwiched between the manifold and engine and serve to reduce the amount of heat conducted to the alloy manifold. This can reduce intake air temperatures by several degrees Celsius.

See Cool Stuff - Manifold Insulators for details.

Another approach to intake system heat insulation is to use a ceramic coating on relevant parts of the engine. Ceramic coatings can also be used on piston crowns. Of course, this necessitates an engine tear-down to access the parts.

Water injection is arguably the most effective solution for detonation.

Water injection eliminates detonation in a number of ways – it substantially cools the induction air and, when the water turns to steam upon reaching the combustion chamber, it acts as an anti-detonant and cleans the inside of the engine. This cleaning action helps eliminate detonation-inducing combustion ‘hot spots’.

Note that water injection systems sometimes include a mix of methanol or methylated sprits. See Water Injection for more information on water injection.

So there you have it - with so many ways to tackle detonation problems, there’s really no excuse for being its next victim. Play it safe. Play it smart.

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