[Hib Halverson]

I know this is an old thread, but I'm doing an article for Corvette Magazine about some of the development vehicles which led up to the ZR-1.

The engine in the OP's photos is a 4.3-L V6-90 with splayed-valve heads and a single turbo which was the first engine configuratioan tested for what was known inside GM back in '83 as the "400 horsepower package" an idea which predates the LT5 development by a couple of years.

As Dave McLellan points out in his book, the V6 single-turbo was quickly rejected because: 1) it made not much more power than L98 which was coming for 1985 and 2) the noise and vibration characteristics of a C4 with a 4.3L V6 were awful.

The next step was a V8 twin turbo, of which 14.5* were built. That idea never went to production either because Lloyd Ruess believed it would be rejected as low-tech once it got to market. The technology developed during the factory TT program was given to Reeves Callaway and GM moved on to DOHC heads for the SBV8 and, subsequently, to the LT5.

*I say "14.5" because 14 coupes were built. Then, right at the end of the program, the powertrain from the last coupe was removed and transplanted into a convertible. Of the 14 engines built, 13 were "standard" 440-hp versions, but the last was a 560-hp version with an induction system having larger runners, a bigger plenum and 20-psi boost.

Digging up some of this old stuff is quite educational.