2021 McLaren 720S GT3X Unleashed With 740 Horsepower From Hand-Built Engine 15 Mar 2021, 16:35 UTC · by Under GT3 regulations, the race-ready 720S isn’t on par with the road-going 720S in terms of horsepower. What the British automaker calls GT3X is a track-only machine that doesn’t follow the rulebook to a T, which is why the twin-turbo engine develops 740 horsepower in push-to-pass mode. 12 photos Factory test driver Rob Bell mentions “an extra 200 horsepower” over the GT3, which is obviously a lot for a car that isn’t hindered by the weight of airbags, ESP, and other nannies. Every force-fed engine is assembled by hand, and the biggest differences over the 720S in GT3 specification come courtesy of blueprinted cylinder heads, upgraded cooling, bigger turbos, and manifolds.