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On 9 Jun 2005 at 21:39, Daniel Baum wrote: > It's a kind of rustling or rattling noise that starts when going up a steep > hill at almost-full throttle in third (i.e. top) gear. If I lift off the > throttle slightly it will stop. Also, downshifting will stop it altogether. > Once it has started doing it, it will carry on doing it whenever I have use > slightly higher than usual throttle - even on much less steep hills - until > I stop the car. Again, downshifting will stop it. The engine doesn't seem to > be running too hot. Detonation, pre-ignition, and knock are all different degrees of the same thing, and this may be what you are hearing. It's a bad thing and can damage the internal parts of the engine. The reason that it's bad is that the detonation is the result of an exploding mixture which produces very high and very sudden peak combustion chamber pressures. The shock of this amounts to hammer blows against the piston face and can damage pistons, and rod and main bearings. Of course there are different degrees of detonation, and yours are probably mild, but you shouldn't take a chance with it. My ME professor friend once described it as the "sound of a handfull of ball bearings being slowly dropped into a metal wastebasket." My '68 did it, but I didn't know what it was at the time. When I took that engine apart to rebuild it, I didn't find any real damage that I could say was caused by this. Detonation is the spontaneous ignition (like a Diesel) of the mixture that occurs before the spark. It should not be confused with the other kind of preignition that is caused by a piece of hot something glowing in the combustion champer. Detonation is caused by the heating that occurs in the mixture just by virtue of compressing it. Detonation is generally a wide open throttle, low rpm phenomena. At high rpm there's not enough time for it to occur much ahead of the spark, and at low throttle not enough air gets into the chamber for sufficient compression to occur. There are 3 solutions: 1) Don't do those things which cause it: WOT at low rpm. 2) Use higher octane gas or add an octane booster. This is the exact reason that higher octane fuel is made. 3) Rebuild the engine and reduce the compression ratio. I'd try # 2 first, just as a check that this is really the problem. -- Jim Adney jadney@vwtype3.org Madison, WI 53711-3054 USA ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ List info at http://www.vwtype3.org/list | mailto:gregm@vwtype3.org ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~