from the 2.5 L four cylinder engine to a 2.7 L engine having a bore of and stroke of , with a rated power output of (versus for the 1988 2.5 L version) and a significant increase in torque. In addition to the increase in displacement, the new engine featured a siamesed-cylinder block design and a different cylinder head which incorporated larger valves. Callaway 944 In 1983, American tuning company Callaway Cars began offering a turbocharged package for the US-Spec 944 in collaboration with Porsche. The standard 2.5 L inline-four engine was not suitable for forced induction because of the higher compression ratio of 9.5:1 which made the engine prone to failure when subject to forced induction along with the complex Bosch Motronic engine management system. Callaway engineers overcame this problem by increasing the volume of the engine's combustion chambers by milling away metal from both piston heads and chamber walls and by tweaking the Motronic system so it would ensure optimum fuel injection to the turbocharged engine along with installing their own Microfueler unit. This step was highly effective, but required disassembly of the entire engine, leading to the high cost of the package. The resulting engine's compression ratio was of 8.0:1 which was less than the standard engine but ensured linear power delivery. In order to ensure that there were no serious engine breakdowns, Callaway installed an ubiquous internal waste gate recommending the use of 91-octane fuel in order for increased engine reliability. In addition to that, an IHI RHB6 turbocharger was installed on the right hand side of the engine along with a new free flow exhaust system incorporating a larger exhaust pipe for optimum performance. The small turbocharger eliminated turbo-lag thus ensuring linear levels of boost. The turbocharger produced 10 psi of boost, however a boost adjuster knob located on the dashboard was optional. With these modifications, the engine generated a power output of at 6,000 rpm and at 4,000 rpm as opposed of the standard car's at 5,500 rpm. Performance increased over the standard car as well, with a acceleration time of 5.9 seconds and a top speed of . Callaway quoted that the acceleration times would even be lower if the rev limiter was removed. Only 20 cars were produced making it one of the rarest Porsche 944s produced. 944 Turbo (951 LHD/952 RHD) For the 1986 model year, Porsche introduced the 944 Turbo,