By "pinging" do you mean timing of spark too advanced relative to valve opening at end of compression stroke... or what?
My thoughts:
You may have to retard the spark a degree or two at 3k rpm... since spec's are designated for high octane gasoline --- before the lead free stuff came along. OR check valve tappet clearances first before you change timing. If tappet clearance is insufficient (less than spec clearance) than valve's open sooner than they should... equivalent to spark being too advanced relative to valve opening. OR, valve seating's not right on newly rebuilt head (assuming you're talking Long Block motor rebuild). Also, the spec 30 degr BTDC @ 3k rpm applies
without vacuum control.
I get pinging under heavy loads or heavy acceleration on my Chevy's 327 when I advance the spark too much... which I do more often than not since I'm always trying to maximize power and acceleration to hairy edge. The vacuum advances (hence the term "vacuum advance" as opposed to "mechanical advance") the rotation of the points, so if you're setting the timing (strobing it) at 3k rpm with vacuum applied then it's more advanced than it should be. The same 30 degr BTDC applies at 4500 rpm without vacuum as well... in other words, there should be no change in timing from 3k to 4.5k rpm. If there is then you're advancing the distributor points (from top of distributor cam lobe) or you have a wobble in the distributor cam shaft --- meaning the shaft's bushing is warn so that it's location under higher torque is different than at lower torques.. speeds. I can't recall whether the vacuum is adequet to advance anything at 3k or not, but it should be. The spec's (BBB ==> Big Blue Book ==> MB Shop Service Manual) insist in
BOLD TYPE that the vacuum control must be in-0p during strobe setting of timing.
I assume you've set the point gap to spec's (0.3 - 0.4 mm ==> 11.8 - 15.7 mils or 12 - 15 mils in round figures... I always shoot for 14 - 15 mils since the gap will decrease with use and the preverbial '****' that builds up on the points eventually, and that therefore decreases the dwell angle... which also advances the ignition) before you started any timing measures, right?
FWIW -- the rest of this explains the pinging sound source... read only to get a general handle on why this occurs. Also you can google "engine pinging" or other words to that effect. Wikipedia has a reasonable starting explanation.
http://en.wikipedia.org/wiki/Engine_knocking After ignition of spark it takes a finite amount of time for the fuel/air mixture of compressed gases to "burn" (explode ==> combust)... creating the expansion of the gases during the reaction (oxidation) that provides the power to push the piston. The peak of the explosive power occurs roughly mid-way in the duration of combustion. Since the peak power occurs during the peak part of the combustion phase, the maximum expansion occurs well after ignition (spark).
Because the piston operates on a sinusoidal displacement curve, it's maximum leverage point occurs half-way down the power stroke, so the maximum advantage occurs from approx. 1/3'rd down the power stroke (60 degr ATDC) to 2/3's down the power stroke (120 degr ATDC). It is over this approx. range that the combustion is designed to be maximized ... each engine design has it's unique optimal position in terms of peak explosive power to maximum mechanical advantage position of the piston during the power stroke.
Since the combustion takes time, ignition has to occur well enough in advance of this maximum leverage condition so that the maximum expansion occurs during the approx. max. mechanical leverage of the power stroke (which is AFTER Compression TDC).
When ignition occurs too soon, here's what goes wrong...
The maximum expansion (explosive power) occurs before the piston is at it's designed maximum mechanical leverage point... so that most of the power is pushing on a piston that has relatively much larger mechanical resistance to movement.... restricting the advance of the explosion progress and wave-front. This creates a pocket or pockets of unburned fuel somewhere in the combustion chamber that ignite explosively creating a sudden shock to the piston, valves, rod and the sound you hear as a "pinging". I'm not sure though if the ping is the sound of the shock (explosion) itself or the effect of that shock on the piston, valves, and rod that create vibration in one / more of them that results in the audible sound. I'd guess it's the latter since the ping sound is very high frequency sound... unlike what you hear during pre-detonation (when hot carbon on the valves or some other portion of the combustion chamber ignites unburned gasses in a random time pattern causing the sounds you hear during pre-detonation.
Look at it slightly differently. If the piston were at TDC when max expansion occurred, the maximum energy is fully resisted by the fact that the piston is pushing straight down at that point directly on the crankshaft with zero mechanical leverage (crank lobe is directly beneith and in-line with the piston rod... i.e. totally wasted energy.... likely to warp/bend the rod or blow a hole in the top of the piston.... that energy has to go somewhere no matter what. It's that fully expanded gas that has no-where to go that creates a shock to the piston/rod & valves... hence the ping.
So, to avoid this problem of course, the explosion / expansion has to start early enough before TDC so that the maximum expansion ocurrs AFTER TDC ... as I said above something around 50-60 degrees ATDC... but don't take these angles for gospel.... tweeking the maximum amount of net energy out of the explosion is an art form (highly specialized engineering) for any given specific engine / piston / valve port & size design.
When the engine's under load, the resistance to mechanical leverage of the piston/rod on the crank is increased independant of rpm. ... so the maximum explosive power (expansion of gasses) occurs at the same time in the stroke. If the explosion too advanced, but still is o.k. (no pinging sounds) with no load, then when under load at the same rpm it's too advanced for the mechanical advantage required... and it pings when the explosion has no where to go (expansion is being resisted too much by the piston's relation to the crankshaft lobe/rod alignment)... creating the shock... hence pinging sound. The reason dropping to a lower gear reduces he pinging is because the resistance on the crank/rod/piston from the wheels thru the tranny is reduced.... reducing the resistance toward the 'no load' condition.
Longtooth
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