The original Bizzarrini-designed 3.5 liter V12 was essentially a race motor, potentially developing 400 hp at 11,000rpm.[6] In order to fit his grand touring car with a smoother, more pleasant, longer-lasting engine, that was "good for 40,000 hard miles between services"[4] Ferruccio had Dallara and Wallace detune a version of this prototype "GTV" motor for street use.[3] This included:[4]
replacing the elaborate and costly racing type dry sump oiling system with a conventional wet sump oil pan
reducing the compression from 11.0:1 or above to 9.4:1
cutting back on the exotic materials specified for the crankshaft and other components to reduce cost
relocating the distributors to more accessible positions on the fronts of the exhaust camshafts
adoption of a single, very tall Lamborghini-made oil filter
replacing the expensive 36 mm vertical racing Webers with conventional sidedraft 40 DCOE Webers
softening the cam profiles
This first "detuned" L350 engine was tested on October 3, 1963.[7] The result—later fit in the 350 GT—was a very capable 270 bhp power plant[4] that could reach 254 kilometres per hour (158 mph) in top form.[8]
While this 350 GT design work continued, the prototype was rushed to completion for the upcoming October 26 press meeting[5] and the subsequent inauguration of the Turin Auto Show on the 30th.[3] The Lamborghini 350GTV was shown at the Turin Auto Show with the original Bizzarrini "racing" V-12 engine—with its downdraft webers, rear distributors, etc.—displayed alongside as it was not "adapted to the chassis."[5] The car was a "non-runner"[4] with the suspension arms simply tack-welded in place[5] and the engine not installed.[4] Lukewarm reaction to the car[4] caused Ferruccio to postpone plans for immediate production[4] and move on to the new 350 GT design.
In March 1964, only 5 months after the debut of the GTV in Turin, the "redesigned GTV"—now called the 350 GT—was debuted at the Geneva Auto Show.[3] It was greeted with sufficient enthusiasm[4] that Ferruccio decided to proceed with production at Sant'Agata in May 1964.[3]