The phrase "monstrous engine" refers to a very big and powerful machine or device. It implies that the engine is so huge and strong that it can accomplish extraordinary tasks. Full definition
A lot of people wish for F1 to return to the noisy, monstrous engines of the past but Todt said that'll never happen. (wtf1.com)
What Shelby has managed to do with the Dodge Charger may not be so overwhelming as his Cobra - Mustang achievements, but that was another day, when all performance needs were answered by monstrous engines. (articles.orlandosentinel.com)
For the ultimate trackday monster you will need a suitably monstrous engine, either by tuning the existing engine or by transplanting in something considerably bigger. (evo.co.uk)