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Re: [T3] Engine noise

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 

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
Madison, WI 53711-3054

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