Yamaha have issued an official explanation for the problem they suffered at Mugello, which saw Valentino Rossi's engine blow up during the race, and Jorge Lorenzo's engine blow up during warm up on Sunday morning.
The cause given is exactly in line with the reasoning in MotoMatters.com's Mugello Sunday post-race round up: the engine overrevving as the rear wheel lifted at the end of the Mugello straight. At that point in the track, with the bike hitting 350 km/h and nearing peak speed at top gear and at full throttle, when the rear wheel lifts over the crest at the end of the straight, the engine spins up too quickly for the rev limiter to catch.
Yamaha MotoGP project leader Kouji Tsuya acknowledged they had been caught out by the new unified software package: they had used similar settings to last year, but the rev limiter with the 2016 electronics had not reacted as quickly as Yamaha's proprietary software was capable of in previous years. That caused engine damage at the end of the straight, eventually causing the engine to fail completely with piston and valve damage.
Because of the nature of the damage, and the difficulty in locating the precise nature of the problem, Yamaha were unable to put in a fix for both factory Yamahas for the race at Mugello after Jorge Lorenzo's bike had blown an engine on Sunday morning. The engine which Rossi blew up was his third engine, a relatively fresh engine, as was Lorenzo's engine that blew. Lorenzo eventually raced his second engine, which had been in action since Qatar, and had 25 practice sessions and 2 races on it.
Yamaha's statement says that they have identified the problem and have a fix in place from now on. In part, the risk is lower, as there are no other tracks coming up with such unique conditions as Mugello. But it is entirely possible that part of the fix is lowering the maximum revs on the engine a fraction (perhaps just 50 or 100 revs) to avoid a similar problem. That should not have a material effect on the performance of the bike.
Below is Yamaha's press release:
OFFICIAL STATEMENT FROM YAMAHA MOTOR CO. LTD.‘S 2016 YZR-M1 PROJECT LEADER, KOUJI TSUYA, FOLLOWING MUGELLO TECHNICAL INCIDENT
Barcelona (Spain), 2nd June 2016
2016 YAMAHA YZR-M1 PROJECT LEADER, YAMAHA MOTOR CO., LTD
After the technical problems in Mugello the two defective engines were returned to YMC for investigation. Following our detailed investigation of the engines, telemetry data and related systems we found the cause of the failures. The failures were caused by an electronic issue related to the rev limiter which ultimately resulted in valve and piston damage. The cause for both Jorge‘s and Valentino‘s engine failures was the same. To be clear, there was neither an engine component nor a structural failure, it was purely an electronic control issue.
Valentino‘s failure was caused by an accidental over rev in acceleration that occurred jumping over a crest with full throttle at the end of the straight. This failure was not in any way related to the mistake made by Valentino at the San Donato corner on the lap before.
There were no special mapping settings used for Mugello; we used the same precise mapping as always. We have now withdrawn both engines from the allocation for the season.
We have a strong history of engine reliability and this fact does not change after this incident; the engines had no problems, but we were not aware of the different behaviour of the standard ECU software, that made the rev limiter work in a different way compared to last year. We set the rev limiter using last year‘s data in exactly the same way as we did last year, but we could not be aware that the software worked in a different way.
Valentino‘s engine was the freshest of the three that were sealed so far from his allocated engines, therefore after Jorge‘s Warm Up engine failure there was no reason to consider replacing it. Furthermore we couldn‘t find out the electronic issue in Jorge‘s engine in such a short timeframe.
Mugello is one of the most critical circuits because of jumping over a crest with full throttle on the straight, and engine RPM becoming higher. We have learnt from this incident and already modified the rev limiter setting, so it will be OK in Catalunya. As a precautionary measure, the other engines used by both Valentino and Jorge in Mugello will be used only for practice sessions, until their life cycles are completed.
We understand an electronic issue caused the failures in Mugello, therefore we do not need to intervene with our remaining engines that are not yet sealed. We still have enough engines for the remainder of the season. We have already devised a countermeasure, so we are confident that the failures in Mugello will not recur.