A lot depends on how the engine went "pop".
In motor-trade law "the condition of the motor vehicle is the owner's responsibility at all times, unless separate warranty is entered into".
It's all down to "duty of care", whether with or without negligence.
"With" would have been a situation such as Jim mentioned.
"Without" would be if there was an underlying problem with the engine that the tuner wasn't aware of.
The arguments in court would be based around whether the tuner "being an expert or specialist in his field, SHOULD have known or ascertained"

Hope this helps