what is the biggest engine that will fit into a 1991 Honda Civic DX?

Whatever you can fit under the hood- think outside the box- new engine, tranny, drive train, suspension, body mods, etc. Might as well go all the way- I would cram a Chevy 454 bad boy in there- who wants to come in 2nd?

Dusty, My grandma used to say if you dress a monkey in silk it is still a monkey! But If your twin brother is Richie Rich go ahead and drop a HEMI, 454, or M3 engine then after redesigning new motor mounts, cooling, relocating battery on a 23 year old car likely with about 200K on the clock you'll have to reinforce its tired suspension, braking and exhaust system because none of them originally designed for 117 horses will be able to handle the new engines' cooling, acceleration, torque nor braking power and after all that dough you'll still have a 23year old HONDA. The smart thing would be to sell it and get yourself a Mustang, Challenger or M3 and save a ton of money!