 Complete the following phrase:  BMW pulled out of Formula One and the engines were badged Megatron through a deal with Arrows major sponsor USFG, but the British team had their best seasons yet, finishing seventh in 1987 and fifth in the final year for turbocharged engines thanks to frequent points finishes by drivers Eddie Cheever and Derek Warwick. While 1987 and 1988 were Arrows best years in F1, they were also the cause of frustration for the team and its drivers Warwick and Cheever. At the start of 1987 the sports ruling body FIA mandated that all turbo powered cars were to use a pop-off valve in order to restrict turbo boost. This was done not only to slow the cars down for safety reasons, but it was an effort to curb the rapidly rising costs of Formula One. The problem for Arrows was that the valve would regularly cut in lower than the set limit 4.0 bar in 1987, 2.5 bar in 1988. This meant that the Megatron engines were not producing their full power. It took the teams engine designer Heini Mader until just before the 