Achieve red line by 1/2 throttle for manuals, and higher revs than usual for autos!

How To Reduce Engine Knocking

Photo: An example of the Chevrolet Captiva

Case study: Chevrolet Captiva 2.4

Having installed the Surbo, the owner of the above car immediately noticed that the knocking noise during acceleration was gone. It is possible that for more power, car manufacturers are increasing engine compression to the limit. Vehicles have also become heavier due to added equipment and the load causes the engine to work harder and so the chance of knocking goes up.

A Nissan Latio 1.5 CVT turned up with an extra hot engine. The owner confirmed that there were knocking noises when he accelerated. After the Surbo was installed, he noted that the knocking noises were gone. The accelerator felt lighter, and he was actually pressing less so the tendency for over-fueling, and therefore knocking, had decreased. Through a corner, he said that the power did not slump as before (reason being that the Surbo charges air when the accelerator is held still). The gearbox would also find a lower ratio more readily when he accelerated while cruising.

Analysis

A petrol engine knocks when compression is so high that the air fuel mixture ignites by itself before the spark occurs. The result is a flame front coming down at the piston that is still on its way up, and this causes noise and power loss, and even damage to the cylinders. When is knocking (otherwise known as pinking or detonation) likely to occur, and what can you do?

1. When too low an octane is used. Normally, petrol with octane number (RON) that is too low can't be used in high compression engines, because the petrol under pressure can ignite itself prematurely. However, if you use a Surbo, you can use petrol with a lower octane than recommended by the car manufacturer, because the accelerator is often pressed less, resulting in less fuel injected, so the chance of knocking is reduced.

2. When some fuel additives are used. These may make the fuel too easy to burn and so knocking occurs, audibly as a "clack clack" sound. If you use a Surbo, this may be less likely, because the accelerator will not have to be pressed as much for the same acceleration. If the purpose of using the additive is to save fuel, then it may not be required as the Surbo saves at least 10% fuel.

3. When too high a gear is used and the engine cannot turn fast enough, and the accelerator depressed too much in an effort to move the vehicle. As a result, the fuel that is injected is excessive and can cause knocking. For example, in second gear at a low rpm, it is often difficult or even impossible to shift down to first gear, acceleration is poor and overfueling and knocking can occur easily. If a Surbo is used, the air intake pressure is raised, and low rpm torque significantly increased, so the tendency for knocking is reduced. For automatic tranmissions, in such instances the gearbox is more able to kick down to first gear with slightly added pressure on the accelerator, which sends more of the Surbo's pressured air into the engine, allowing for higher rpm required for a lower gear.

4. Whenever the accelerator is pressed too much. With a Surbo, the driver may never have to floor the accelerator, because the Surbo sends the engine to its rpm limit with just 1/2 accelerator pressure. The engine with Surbo does not have to work as hard as before, as it is aided by the air pressure from the Surbo. The increased torque means that the accelerator does not have to be pressed as much as before, so the fuel injected and heat created in the engine is less, and the likelihood of engine knocking is reduced.

Compression graph of a conventional petrol engine versus one with Surbo

This graph shows how the difference between a conventional engine and one with Surbo. At idle, due to a fully-closed throttle, the Surbo, which is throttle back pressure activated, is already pre-compressing air into the engine. As the vehicle moves, the Surbo can be further activated at 1/2 throttle for more air compression, so that the rpm limit is reached by 1/2 throttle. At this instance, the reading on a vacuum/boost meter attached to the manifold shows that the absolute air pressure is far lower than at full throttle. As the fuel pressure is only half at 1/2 throttle, and in the presence of a lower absolute air pressure, the likelihood of a knock is greatly reduced.

Compression graph of a conventional petrol engine versus one with Surbo