Hi, recently I was told about a newly built 1275 that was about to be dynoed and the tuner noted it was misfiring. Inspection noted a brand new distributor had been fitted , at around 12 degrees base timing, but total timing at 4000rpm was over 50 degrees. An inspection then noted it most likely had a melted piston.
This would have been caused by whats really known as abnormal combustion., where the spark or heat of combustion is trying to force the piston down well before its got to the top, and as it is still going up, the gas that is in there is being compressed much more than normal, and also gets super hot. Voila , blown head gasket , melted piston, broken rings, seized piston, softening of alloy head..... many of these disasters can occur.
Often people think its pinking, where you may hear a tinny rattle under light load caused by too much ignition timing. That can be a cause.
The cause is usually multiple things
Too high a compression ratio for the fuel octane rating available, or the one chosen because it was cheaper.
Lean or excessively rich fuel mixtures
A poor cooling system or running excessively hot
Often the major part of of the problem is ignition timing, not the base or idle amount, but the grand mechanical total , often checked at 3500rpm, but some distributors still advance to 8000rpm.
Generally speaking an A series (mini) engine likes a max between 28 - 32 degrees . Less is more efficient, more is not generally desirable.
So that total of 50 meant 18 degrees of crankshaft rotation earlier before top dead centre the plug fired. If it got the mixture burning then that gas got super hot and softened the alloy to the point it failed. It may also have had other factors.
The point of this conversation is when tuning the car, check the grand total, thats when the damage is done, not at idle, or light load.
And I can assure you there are distributors available that have too much total, and need to be modified
Also you may be able to check your dissy by removing the rotor button, fitting a 360 degree marked wheel and levering cam forward to measure its angular movement.
Cheers Lindsay Siebler
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