The red log blows.
Presto logs (originally Pres-to-Logs) were an artificial fuel for wood-burning stoves developed in the 1930's as a means of recycling the sawdust from sawmills. They were made of clean, dry sawdust, wood shavings and green waste that was formed into logs by machines under great pressure without any binders or glues. Other brands of artificial logs used paraffin or natural binders.
Doc Brown developed his own presto logs sometime in 1885 from pressed wood and anthracite (a hard and dense variety of coal), treated with chemicals only known to himself. He color coded the logs green, yellow and red in increasing order of the amount of heat produced. He would use them in his blacksmithing furnace so that he wouldn't have to continually feed wood to the fire.
These logs were used to power the steam locomotive that pushed the DeLorean time machine up to 88 mph. Each colored log would react differently, raising the boiler pressure and getting the fire hotter, thus making the train run faster than it ever could. With each colored log, the train would emit steam of that color.
The boiler temperature readout gage.
The green log pushed the train up to 35 mph and around 1250 degrees. The yellow log increased the speed to 40 mph and all the way to 70 mph, as well as increasing the boiler pressure to 1500+ degrees. When the red log ignited, the boiler hit 2000 degrees and exploded. The speed increased to 80 mph and the car reached 88 mph before the end of the track, entering temporal displacement. This left the locomotive in a wreck of a condition before its final destruction in Shonash Ravine.