Renault Sport CLIO 2 RS MINICUP - Ouverture du moteur F4R730 après la casse, AÏE ! :-(

Video part 1:    • Renault Sport CLIO 2  RS MINICUP, la fin d...   Possible causes? Piston 4 melted and broke (hot spot or connecting rod hitting it?). Furthermore, spectators told me afterward in the parking lot that flames came out of the exhaust on the pit straight just before the breakdown, and on my track bike, these "backfires" are not normal. This fact (which I had forgotten before writing this message :p) is important because it's a consequence that could contribute to and explain some of the causes listed below! So, when we look for possible causes, we find these: Bad injectors: Replaced with new ones of the same model as the previous ones from the reputable Magneti Marelli on April 17, 2021. It's been a while, and since I race almost every month, if the problem came from them, I think I would have experienced this breakdown sooner. So I don't think that's the cause. Incorrect injection quantity: Following a dyno test that revealed a mixture that was too lean, especially at low RPMs, I installed a 3.5 bar fuel pressure regulator on October 5, 2021, instead of the stock 3 bar one. This corrected the lean mixture somewhat. It's been a while since I've been driving it every month, so I don't think it's the cause of the engine failure. Especially since the flames coming out of the exhaust, seen by some spectators on the pit straight just before the engine failure, are usually due to the opposite: a mixture that's too rich. BUT if the mixture remained too lean, or even worsened for various reasons, it could cause a piston hole, as happened to engine #4. So it's POSSIBLE in the end, even if it's not the primary cause in my opinion. Incorrect injection timing: POSSIBLE, but why and how can I diagnose it now? Insufficient compression: I check cylinder compression every 6 months at most. The last one was on November 23, 2023. The compression in cylinder 4 was, as always, excellent, especially considering its age and mileage (24 years of hard work and soon 178,000 km) at 13.5 bar. So I don't think that's the cause. Pre-ignition delay: POSSIBLE, I'd even put it first, partly because of the flames coming out of the exhaust (abnormal on my track car) that spectators told me they saw on the pit straight, just before the engine failure. But again, why did it happen all of a sudden, and how can I diagnose it now? Vibrations in the fuel injection lines: POSSIBLE, although I don't believe it without being able to prove it. Ultimately, it's probably due to successive over-revving over the last 10 years on the track, which gradually caused two valves in cylinder 4 (one intake and one exhaust) to twist, culminating in the exhaust valve head breaking on that fateful day in February 2024. This caused a hot spot in the combustion chamber, melting and burning a hole in the head of piston 4. :-(