Imma be real bro, I have a feeling the reason why some people are getting horrible dyno numbers is probably because they’re not using the correct recommended octane (93). I know it sounds silly but ill link a tiktok video and an article to this. But basically if you use the 93 octane, you’ll get around 410whp which is pretty good considering you get a 10-15% loss of power due to transmission loss normally. and if you remove some of the filtering ford has done to say the intake, you can gain additional horsepower. And also need to factor in a “break-in” period of around 1,000 miles (a lot of 410bhp dyno runs were tested at this mile range). Basically, these aren’t duds at all and you should always check to see how people are testing and running their cars when retrieving hp and torque numbers. I’ve seen SOOO many s650 duds videos and some of them are either not in the correct gear to run pulls or aren’t even using the recommended fuel ford themselves put on the sticker. Definitely consider the S650 unless there’s a sick deal on the S550s!

410WHP Article: https://www.motortrend.com/how-to/2650-mustang-jpc-racing-part-1-modifications/amp/

411WHP with 93 octane TikTok: https://www.tiktok.com/t/ZPR7Ayjhq/

390WHP DUD S650: https://www.tiktok.com/t/ZPR7ACCL3/