The rarest 20th Anniversary Turbo Trans Ams are the hardtops and convertibles. Only 40 hardtops were made. Only three convertible TTA's were built by ASC, one of which was bought by the president of PAS, with the other two falling into private hands. Only two are generally known to exist, however, as the ASC order log shows records for only two TTA convertibles. The five test TTA's, because they were for pre-production use, were pulled off of the assembly line without regard to color, and therefore were not necessarily white. At least two of these test cars were sold to private individuals, so there are at least two extremely rare, non-white, factory TTA's in existence; in fact, one is known to be red. As for the rest of the Firebird line, GM made a new dual catalytic converter exhaust system (RPO code N10) available, freeing up 13% more power from some LB9- and L98-equipped Formulas and Trans Ams; so equipped, LB9 engine output was increased to 225 hp while the L98 increased to 235 hp. The N10 option remained available throughout the balance of the Third Generation production run, however the L98 powerplant was only available with an automatic transmission, whereas the LB9/N10 combination could only be coupled to a 5-speed manual (RPO code MM5) and a limited-slip differential (RPO code G80) using a 3.45 performance axle ratio (RPO code GM3). According to a March 1990 (Vol. 35, No. 9) "Car and Driver" article, when the latter set of options were combined into the relatively unassuming (and 300-pounds-lighter) Formula body, which shared the same WS6 suspension with the top-end Trans Am GTA, it created a "sleeper" Firebird that could out-perform the heavier Trans Am GTA (even when equipped with the L98 engine) – at a roughly 30% lower sticker price. "Car and Driver" also reported that (aside from the TTA, offered only in 1989) these "sleeper" Formulas were the fastest Third-Generation Firebirds – capable of accelerating from 0–60 mph in under 7 seconds and boasting a top speed of more than (quite impressive for the day). Although not an exhaustive comparison, these claims would seem to be supported by a simple examination of the weight-to-power ratios of both models: a 225 hp Formula at a curb weight of versus a 235 hp Trans Am GTA at a curb weight of ; the weight-to-power ratio for the Formula is 14.67 lb/hp compared to 15.32 lb/hp for the Trans Am GTA.