The M12 GTO started life with a Ford-sourced 2595cc Duratec V6 and its associated five-speed manual gearbox. Noble added twin Garret T25 turbochargers to realise 310bhp at 6000rpm from the motor. It also produced 320lb.ft of torque at 3500rpm.
Transversely mid-mounted in the chassis of the M12, the V6 engine and its turbochargers fill almost every inch of space under the clamshell rear bodywork. This has led to some problems with excessive heat, though it tends not to affect the engine itself. More likely to suffer is the air conditioning compressor, which switches off when too hot due to heat soak from the turbos.
Another heat-related problem that afflicted early M12 was simply overheating, but Noble cured this with a shroud to duct air to the radiator rather than having the fan mounted directly on the radiator. Most cars will have had this modification, but check it's been carried out on any potential purchase.
Poor engine idling is likely to be down to fuel starvation, so look at the state of the fuel pipes and budget for replacement if they appear perished. The fuel filter also blocks up over time, so replace this as a matter of course and, for track days, keep the petrol tank topped up above half full.
The 2.5 V6 used a Magnetti Marelli ECU that has proven problematic. Look for cars that have been upgraded with the later MBE ECU, though this does require swapping the complete engine wiring loom to be effective. Careful engine mapping for all M12 variants is vital for smooth running and extracting the best power. Moving the ECU from the engine bay to inside the cockpit helps avoid the perils of excessive heat.
Exhaust manifolds are a weak spot in the engine bay and prone to cracking. Some owners have had the manifolds welded to repair the damage, but this is only a temporary fix and a complete replacement is the only way to solve the problem.
There's no limited slip differential with the five-speed gearbox of the 2.5-litre cars, but it was an option from the factory. Six-speed gearboxes used with the later 3.0-litre engine have an LSD as standard. The five-speed box has a poor shift quality, but this can be improved by adjusting the linkage.
The M12 GTO-3 introduced the 3.0-litre V6, again from the Mondeo, while the more aggressive 3R used a close ratio six-speed 'box and Quaife torque-biasing limited slip diff. The 2968cc V6 produces 352bhp at 6200rpm, 42bhp on the 2.5 it replaced, while torque grew by 30lb.ft to 350lb.ft at 3500rpm. Even with the additional weight of the R, which tipped the scales at 1080kg to the original GTO's 980kg, it offered an increased power-to-weight ratio of 326bhp per tonne.
All of the checks for the 2.5-litre engine apply to the 3.0-litre, though the later engine's ECU is much more reliable. Early 3.0-litre cars could blow the baffles out of the exhaust, but this was sorted on later cars.
Last of the line for the M12 was the M400, which pushed power to 425bhp at 6500rpm with the aid of larger Garret T28 turbochargers, more aggressive camshaft profiles, forged pistons, remapped ECU by Roush Technologies and better engine cooling. Torque also went up to 390lb.ft while overall weight was down to 1060kg, resulting in 0-62mph in 3.5 seconds. The M400's rev limiter was also raised from the 6700rpm of the 3R to 7200rpm.
Noble M12 Buying Guide Index: