[bobcat]

If you read back through some of my posts ive explained what knock is and what causes it.

also heres some basic background on how you car detects knock.

On the t5 engine fitted to the 850's etc, you have 2 knock sensors fitted on the front of the block. they are nothing more than a microphone tuned to the resonance frequency of you engine. when the fuel burns you shouldnt get any noise, however when it explodes you get knock. this is like hitting the engine with a spanner, it will vibrate at its resonance frequency like a tuning fork. the amplitude of this is proportional to the amount of knock.

also knock isnt just knock, there is varying degrees of knock. imagine drawing a line on the floor by pouring out some fuel etc, imagine setting fire to it and watching the flame work its way alone (this is a normal burn), however image as it gets to the end the rest of the fuel explodes - this is knock. so depending on how much of the fuel explodes will affect the knock readings.

so as you can see from that when applied to an engine, and engine\ecu can see knock starting to happen, say for example as boost pressure is increasing.

knock normally dosent just happen suddendly and blow the engine up!!!

Originally Posted by ttacornwall

Hi Conehead,

My T5 is M reg so my handbook may be slightly outdated as to the information/advice contained within. Although I did say "For the B5234T5 Volvo recommends 98 RON lead-free petrol for the best performance, but will run on any fuel with an octane rating of 91-98." So going with Volvo's information the 850 T5 can run on 91 RON, which is something I would not do, however there are certain countries that have poor quality petrol so Volvo and other car manufacturers I pressume have to a cater for those countries and detune the engine for a world wide market.

As Volvo states that 91 RON fuel can be used, why do owners suffer from detination or knocking and worst case senario bent con rods? I think Volvo would be comprimising themselves re litigation if the vehicle could not be driven spirited using a lower RON fuel after stating in their handbook you the T5 can run on such a lower rated fuel.

So another question arises, what causes the knock? A fault or failure in the sensors, ECU, fueling, timing not able to change to appropriate conditions or is it the fuel?

This is one of those subjects where the more you think you are learning something you suddenly realise more questions are being raised than answered!