... detonation is actually when the mixture in the combustion chamber explodes violently instead of burning smoothly. It is often caused by conditions being too hot in there, and lean running is a contributory factor. High boost pressures and ignition being too far advanced can also contribute. Another consequence of high boost is that you are getting more air into the cylinders and therefore also need more fuel. If your fuel system cannot deliver this much extra fuel, then you are running lean, which again can lead to detonation.

You don't say which car you have, but the C900s are known to have only enough fuelling capacity for standard running. 9000s have plenty. I don't know much about later Saabs, but I believe they also have decent fuelling.

If you're lucky, you can hear detonation at low revs as a "pinging" or rattling sound. It sounds like someone banging the tops of your pistons with a hammer, and has a similar effect on the pistons. If you're unlucky, it only happens at high RPM and you won't hear it. This is the most destructive type of detonation and is unfortunately when it is more likely to occur.

The APC system is supposed to detect detonation and do something about it. On older APC systems, this dials back the boost to base boost (the lowest it can go) and I think it may also try to dump in more fuel to cool the combustion process, but this only works if the fuel system has enough capacity - if the detonation is because the system is running short of fuel, then this can't do anything. On later cars with Direct Ignition, the ignition will be retarded as well.

This is one good reason not to bypass the knock (detonation) protection provided by the APC system.

posted by 62.253....

• Stock pistons or Je Pistons more....., Dylan, Thu, 1 Aug 2002 11:31:04
  • Re: Stock pistons or Je Pistons more....., cutic, Thu, 1 Aug 2002 20:04:32
  • Sounds like detonation, BillJ, Thu, 1 Aug 2002 12:25:02
    • What is detonation? running too lean?, Dylan, Thu, 1 Aug 2002 14:07:26
      • Running lean is one possible cause, but..., BillJ, Thu, 1 Aug 2002 14:44:31 <-- Viewing This Message
        • Actually I thought....., Nuts, Thu, 1 Aug 2002 19:36:02
          • That is the usual cause..., BillJ, Thu, 1 Aug 2002 19:57:37
        • What my engine set up is....., Dylan, Thu, 1 Aug 2002 17:33:32
          • Ohh and I use 93 octane highest in the US. nm, Dylan, Thu, 1 Aug 2002 23:38:29
          • A really good thing to do would be ..., BillJ, Thu, 1 Aug 2002 19:15:50
        • Re: Running lean is one possible cause, but..., Thorir Jonsson, Thu, 1 Aug 2002 15:49:22
  • if you can afford them, nickR, Thu, 1 Aug 2002 12:13:34

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