Firebird Convertibles prior to 1991.
It is known that Pontiac did not officially offer a convertible from 1970 thru 1990 model years. Starting in 1987 The Chevrolet Camaro was offered as a Convertible, but the Pontiac firebird was not. There are probably many speculations to why or why not they did not offer a convertible the real reason is probably lost to history, and the truth would probably never be told.
It is my speculation that Pontiac wanted a Convertible, but were unable to. Some of the other speculations I have heard is the ASC conversion process was not up to snuff to the quality that Pontiac wanted. Another speculation was that Pontiac did not see a market for the cars. I do not believe that either of these scenarios are true.
The following may be quite lengthy and some of it may not appear to be relevant to the convertible but if you read all of it the reasons for my conclusions will become clear.
When the LB9 (305 TPI) engine was introduced in 1985 the new 1986 Emissions laws had not yet taken effect, the 1985 LB9 with the Automatic was probably one of the hottest new engines to be introduced since the early 70's. The benefits were better power (estimates suggest 20%) over the same engine with a 4bbl carb. Also better economy was a huge plus. In 1986 the L69 engine was dropped from both Pontiac and Chevrolet on the F-car, although it appears that the engine was still offered on the G-body, the reason was due to a vapor lock problem with the engine, why the problem did not exist on the G-body is unclear. At any rate the engine was dropped, 26 L69 engines were sold in Trans Am's, most of which were sent to Canada for the Players challenge series.
Although the LB9 was more efficient the emissions were not quite up to snuff for the 1986 model year. The engine was detuned drastically, for emissions and fuel economy reasons. The Cam was detuned, the ECM was upgraded from an 8 bit to 16 bit computer which allowed for more computations per second and better efficiency.
In the 70's the Environmental Protection agency (EPA) adopted a means of regulating the fuel efficiency of the vehicles produced for sale in the US. Every car was to be tested and the economy of the car is listed on the Window sticker indicating the city and Highway mileage of that particular vehicle. Although cars are rated by model and engine, the EPA used a Corporate Average Fuel Economy (CAFE). To learn more about CAFE go to the Epa's web site www.epa.gov, this is the link to the Fuel economy page: http://www.epa.gov/fueleconomy/420f04053.htm.
In 1987 the LB9 was for the first time available with a M5 Borg & Warner Transmission. Also the L98 (350) engine was introduced with only the Automatic transmission. When research began on the 1987 GTA one of the concerns was that the economy of the car may not be up to snuff. Many options which were available on the Firebird & Formula were NA on the Trans Am or GTA when equipped with a 350. For example, Both the subwoofer control and the rear shade were NA when a GTA had the 350 engine. Pontiac was trying to raise the fuel efficiency of the cars to an acceptable level so that the government would not slap the car with a "Gas Guzzler tax". It should also be noted that from 1987 & 1988 the Firebird was NA with t-top and a 350, although there are several cars which have the combination the vast majority are either C&C conversion or they happened to get out the door when they were not intended to. The Camaro on the other hand apparently had more T-top 350 cars in those year.
In 1989 T-tops with a 350 were very available, so available that there were more 350 engines produced that year than the 305 LB9. So much so that more than twice the number of 350 engines were made to the LB9, the only other year the 350 would exceed LB9 production was 1988 when the figures are about identical.
Since Pontiac was already concerned with the efficiency of the Trans Am and its weight, the same car in convertible form would have been impossible since convertibles are not as aerodynamic and they weigh more. I would imagine there would have been many confused customers if Pontiac offered a Base firebird in convertible form and not a Trans Am. Ultimately Pontiac decided not to directly offer a convertible firebird until 1991 when the development of the LB9 engine had matured, and the F-car was made slightly lighter and more aero friendly with new fascias and ground effects. Also some of the up-scale trim pieces which were commonly found on the GTA were gone like the Steering wheel controls, the Formed Door panels etc.
By 1986 Chevrolet & Pontiac for sure would have taken notice that there were several companies which were making convertibles from their T-top Camaro's and Firebirds. To name a few, Matrix3, Hi-Line Customs and Autoform had made around 2000 Convertibles total. Chevorlet & Pontiac definately wanted that business as the conversions were around $4000. The most striking of those three was the Autoform Convertible which was a 2 seat Firebird or Camaro. American Specialty Cars (ASC) (originally called American Sunroof Company) made a quality convertible in 1986, they used stamped metal for both the tonneau cover and the trunk lid. When the top was up or down the car was sexy, clean and affordable. ASC did conversions for dealerships for around $4000 a car.
As a note, Not only did ASC make the 87-92 F-body convertibles for GM, ASC also did the 1998-2002 WS6 conversions on the Trans Am & Formula, the Chevrolet SSR was almost completely made for GM by ASC, and many other projects.
ASC became the choice of GM to make the F-body convertibles in 1987 although only Chevrolet got an official convertible the Firebird was similar enough to the Camaro it was not a stretch to make those as well for dealerships. Since firebird convertible production was low enough, ASC would not have had to provide fuel efficiency ratings for the Firebird convertibles. IIRC 500 is the minimum before you are forced to submit Fuel efficiency ratings to the EPA.
As an interesting side note, according to some early owners when you purchased a Firebird convertible from 1987 - 1990, any work done to the Top, whether it was a recall, or broken piece work would have been preformed by the Pontiac dealership. All service was preformed on the cars as if they were officially licensed by Pontiac. Parts were available through the dealerships, the Panels, clips and all other pieces installed on the cars have a GM part number.
To the best of my knowledge ASC made the following Firebird convertibles:
1987 - 173
1988 - 104
1989 - see below
38 - LB8 Firebird
58 - LO3 Firebird or Formula
16 - LB9 Formula - 16
42 - L98 Formula - 42
4 - LO3 Trans Am - 4
10 - LB9 Trans Am or GTA - 10
139 - L98 Tran Sm or GTA
2 - LC0 Turbo Trans Am (TTA)
15 - Unknown
1990 - Unknown
Although the Convertible firebird was not an Official Product of GM, and if
your considering purchasing one and are worried about this fact keep the
following in mind.
1) the ASC Firebird convertibles from 1986 - 1990 are basically the same as a Camaro Convertible for those years, parts are plentiful.
2) In the late 60's a dealerships like Yenko & Berger made specialty cars for sale, these cars were not authorized by GM either but they have a good resale today. Just because it was not Official does it make it worthless, there was definitely enough of a production to make them valuable. Also consider that on many occasions I have seen auctions side by side where the 1989 GTA 350 convertible outsold the "Offical" 1991 or 1992 Trans Am convertible with similar features and condion & miles.
3) GM did not make a single Production convertible for the F-body line from 1987 thru 1992, all cars were conversions by ASC whether they had a convertible VIN or not. The ONLY difference between an Official Convertible and a non Official convertible in those years is a single number in the VIN, structurally they are identical.
In short, I believe the reason why Pontiac did not offer a convertible until 1991 was Fuel efficiency, there is no other logical reason. In the same vein the reason why NO PRODUCTION Camaro or Firebird Convertible was available with a 350 had nothing to do with the torque of the engine, rather it had to do with the efficiency of the car as it would not have met the EPA standards.