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A Treatise About Engine Knock

A Testing Report by Roger Gerl (Team3S Admin - Switzerland)


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NOTE: Knocking is the self detonation of the unburned fuel in the combustion chamber toward the end of combustion. The flame burns around ten times faster than normal, causing huge pressures. Possible cures for knocking are: cooler intake air, increased intake humidity, reduce load, increase RPM, enrich or lean engine from lambda stoichiometric, improve spark, increase octane level of fuel, reduce total ignition advance, etc...


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I ran my car up to 1.26bars where I got fuel cut. After adding the AFC the fuel cut was cured by leaning it out around 5600. The measures on the dyno showed that we were still on the rich side, but we also found out that the timing got heavily retarded. We then took the old "garden hose knock detector" and went down with the boost until knock disappeared. Unfortunately, boost was then only 1.05bars :( and power went down from 465hp to 397hp (DIN)

The hesitation can be caused by the ECU retarding the timing as it sees knock. The more or longer you're running in this situation, the more the ECU thinks that there's something wrong and finally jumps into the emergency program. This then only lets you drive around 2600 and doesn't allow you for more for about 30 seconds or more (can't remember). The knock is mostly caused by a lean situation in a combustion chamber. Unfortunately an EGT meter cannot 100% see this if not on the dyno holding the car at 5500 under full load. Only then we were able to see short lean spikes that caused knock and retarded the timing (see my homepage). Well, the lean situation can be caused by different things :

a) less fuel (add more fuel)
b) badly gapped plugs (regap the plugs)
c) backpressure (the biggest problem)
d) too much heat in the chamber (a better intercooler)
e) badly burnt gases (clogged up valves that do not close good)

Unfortunately, a lot of people think that we can just add fuel, fuel, and more fuel and the problem will be cured. This is wrong ! Of course the symptom goes away as the additional fuel will cool down the combustion chamber and therefore help to avoid knock. The drawback of this is that we then run way too rich as well as we'll lose some horses due to this. Sure we are able to increase boost then to regain the ponies but we have still the original problem. We only cured the sympton.

On our cars the exhaust manifold, the turbos and the pre-cats are the main restrictions. The biggest problem is that the turbine cannot get rid off the boost and heat that builds before the turbo. Therefore the gases cannot go out of the combustion chamber and will be burst again in the next cycle ... causing a lean situation ! As then the pressure in the manifold is reduced because one of three didn't work well, it could be that in the next cycle the problem is gone. Experts call this "high speed knock". Of course it'll come back pretty soon and therefore causes your hesitation. Therefore a bigger turbo does not only mean a bigger intake  but a bigger turbine and housing as well as this should help to get rid off the additional pressure and heat. Also, a well-designed exhaust manifold as well as removed pre-cats would free up the flow. Only then adding a bigger DP and adding a free flow cat together with a good cat-back will do the full job for us. The turbos are sitting on the floor while I expect the manifold next week. Also I know that David Buschur just developed replacement pipes for the pre-cats (I'm in contact with him) and he's testing them out. I'll probably go with them to have the less possible restriction to lower the backpressure as well as to lower the heat in the exhaust manifold.

I'm currently working on my diagnostic tool and it will have a knock monitor as this is only reading a voltage coming to the ECU (like others, O2, idle speed, etc.). I currently don't know what level is knock but I'll have it ready when I'm (Ahem, my car) is again on the dyno the next time :)

I know I had some hesitation with hard acceleration a while back and cleared it up with a simple fuel additive. Increasing the octane always helps until it's used up but unfortunately, there's no octane booster available here :(

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