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How timing affects engine temp: First, fuel doesn't 'explode' in the cylinder, it burns. There is an actual 'flame front' that progresses across the combustion chamber from the spark plug after the plug fires. If the 'fire' is started at the proper time, the burn is completed by the time the exhaust valve opens and just hot exhaust gasses flow out through the exhaust port and manifold. But if the fire is started late due to retarded timing, the fire is still burning when the exhaust valve opens and now there is a live flame blowing out past the valve, adding a lot of excess het to the valve, port, and exhaust manifold. The extra heat in the valve and port is transferred to the cooling system, possibly overtaxing it. The excess heat in the exhaust manifold turns it red and warps it.