How much difference is a 454 and 502. My boat is a twin 502. One engine went missing from a shop. Havent found another 502 yet but a friend of mine has an extra fuel injected 454 he said I could have. I'm wondering how the boat would react with two different size engines just to get it back in the water or should I hold out for a 502

You will notice the difference. I went from 454 to brand new 502s picked up 7mph. Was not cheap but I didn’t have a choice.

I have twin ~550HP 454's. One of the engines the block cracked during the middle of the summer. I had a spare Merc 420HP (454) laying around so I threw that in to finish out the summer. I believe I lost 4mph and 600 RPM, but the engines were still within 150 RPM at WOT. During cruise speed the throttles were way different. Not really a big deal to keep me boating the rest of the summer.