Q-40 was developed as a replacement for 15 size Quarter Midget, which at the time had become a regional event. Formula-1 was in decline also, at the time, and Q-40 replaced it too. So it wasn't a rules change, but rather a new event replacing one that was dying. F-1 was very loud, and some people thought if mufflers were required, it might save the event, but others argued that tuned pipes would just add another level of difficulty. Q-40 engine rules were based on AMA Quickie 500, Event 428: side exhaust engines with mufflers (tuned mufflers!) on 15% nitro. It didn't take long for modelers, and engine manufacturers to learn that the cleaner Q-40 air-frames could benefit from a slightly different port timing, so we ended up with two different engines for event's 422 (Q-40),and 428 (Q-500). Without looking at your engine, I couldn't say for sure, but it's either a F-1 engine, or an alcohol FAI engine, and I'm about 90% convinced it's a Formula-1 engine. Greg