With open tubes the engine gets damaged from cold inrush of air following wide open throttle then shutdown of engine. Cold inrushing air can warp valves. With mufflers, the inrushing air is heated sufficiently to prevent any chance of damage. With cherry bombs there is little chance of a problem if exhaust piping reaches the rear axle. Chances increase as the plumbing gets shorter and shorter. I would start to worry if the total plumbing length is less than 36".
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