detonation

[two89w]

my 347 has 600mile on it and was on the dyno and it started to fume out of the rocker covers so they pulled the plugs out and 7 plugs were perfect but number 4 plug was black and had iron filings on it ,so the engine is out and the heads are off.number 4 piston has detonated on the edge of the fly cuts down to the first ring,and all other pistons are ok .i havent seen it yet but i will on monday,can some1 please explain how detonation happens?? and why only on 1 piston??does it have have something to do with the weber set up on that particular cylinder? and why not any other cylinder??

[CobraDan]

Check for a lean fuel mixture. Rich fuel mixtures resist detonation while lean ones do not. Air leaks in vacuum lines, intake manifold gaskets, carburetor gaskets or the induction plumbing downstream of a fuel injection throttle can all admit extra air into the engine and lean out the fuel mixture. Lean mixtures can also be caused by dirty fuel injectors, carburetor jets clogged with fuel deposits or dirt, a restricted fuel filter or a weak fuel pump.
If the fuel mixture becomes too lean, "lean misfire" may occur as the load on the engine increases. This can cause a hesitation, stumble and/or rough idle problem as well.
The air/fuel ratio can also be affected by changes in altitude. As you go up in elevation, the air becomes less dense.
A carburetor that's calibrated for high altitude driving will run too lean if driven at a lower elevation. Altitude changes are generally not a problem with engines that have electronic feedback carburetors or electronic fuel injection because the oxygen and barometric pressure sensors compensate for changes in air density and fuel ratios.

Dan

[Back in Black]

And the piston in question was simply the *weak sister*...

Incorrect timing causes the spark to fire at inopportune times, such as when the piston is still in the upstroke (retarded timing)

This will pound a piston to death.

[CSX 4027]

The cylinder in question could have a bit smaller combustion chamber if they were done by hand as opposed to CNC which could cause that cylinder to have higher compression. Also depending on what stlye of manifold you are using could cause that cylinder to run leaner. The Smokey Yunick book talks about tuning cylinder by cylinder. Get the one that is detonating to a good level and the rest will fall into place. If that requires you to retard timing too far back then something else is wrong.
You have to start somewhere and change one thing at a time.

[HighPlainsDrifter]

You have a weber set-up so all the jets should be the same. However I would look at the fuel lines. Webers draw lots of air for an instant on the intake stroke then sit and do nothing for a long time untill the next intake stroke.That means they need to dump a lot of fuel in in a short time with no help or balance from the other cylinders. Look at the fuel lines and make sure you have 2 separate lines , one to each bank of at least 3/8" inside diameter. Also it would help to have 2 fuel regulators, one for each side. You also need a high volume fuel pump, at least 110 GPH with no more than 2 90 degree elbows in the total fuel system.
Perry.

[Richard Hudgins]

two89w,

there are a couple of things to look at here.

1. As you are using webers, look to the emulsion tubes and air correction and main jets in number four.

2. If one cylinder is detonating and no others I would look to oiling in that cylinder. At 600 miles, the rings are just starting to seat. If you have a bit of oiling (as the black plug indicates, the oil will reduce the octane rating and cause detonation)

3. Timing. This is not your problem. If you were advanced too much, you would see indication of detonation in all cylinders. (ceramic boiling on the plugs etc.) Note: Black in Black. retarded is later in the stroke sequence, advanced is earlier.

4. Fuel flow. Since you have a 347, I will assume that you have 450 hp. At .45 lbs per hour per horsepower, you will need a fuel flow of 450hp X.45 lbs per hour = 202.5 lbs per hour fuel needed. 202.5 lbs divided by 7 lbs per gallon = 28.92 gallons per hour usage. Therefore any normal pump will cover your needs.

If it were me, I would look to oiling as the problem. This is the most common cause of single cylinder detonation.

As an aside, I am quite surprised that the dyno operator did not notice something amiss during the run. When detonation occurs, BMFC goes off the map. Right now. No waiting. Exhaust temp goes goofy. The dyno response goes really woogy as soon as detonation starts. Were they even looking at the instrumentation? I would be really pissed if a dyno guy ran the motor long enough to hurt a piston. For Christ sakes, that is what dyno are for. To find the problems and tune the motor. Not turn it into junk.

Sorry if this seems a bit harsh, but I have stood at the console for many hours and have screwed up enough development motors to know how not to make one trash. Particularly a normal motor such as yours.