McLaren 765LT produces far more power than claimed

One of McLaren’s last exclusively combustion-powered models, the McLaren 765LT boasts an impressive calling card: a 4.0-litre twin-turbo V8 - already making us miss an era that is rapidly drawing to a close - officially rated at 765 hp and 800 Nm.

Although those figures are certainly substantial, they almost seem modest when the British supercar’s proven performance is taken into account…

There is only one way to find out whether it has hidden horsepower: put the 765LT on a rolling road. That is exactly what the DragTimes YouTube channel and Hennessey Performance chose to do.

The moment of truth

Rolling-road results can always attract some scepticism, as the equipment may not be correctly calibrated. In this case, however, two separate tests were carried out on different dynamometers using two different 765LT models, giving the results greater credibility.

The DragTimes YouTube channel completed three runs. The first two were conducted in fifth gear, with the opening attempt recording 776 hp at the wheels and 808 Nm of torque.

On the second run, wheel horsepower rose to 780 hp while torque remained at 808 Nm. Finally, a third attempt in sixth gear produced 768 hp, with torque climbing slightly further to 822 Nm.

Hennessey Performance, meanwhile, carried out its run in fifth gear and measured 791 hp at the wheels - again, a figure considerably higher than the official claim.

There is an important caveat, though: neither result was obtained with the McLaren 765LT running on “normal” petrol. In both cases, the British supercar was fuelled with racing petrol, meaning higher-octane fuel, which clearly affected the measurements.

So, what does this mean for the McLaren 765LT?

At this point, you may be thinking: “well, with some fancy petrol even my car makes more power”. That is not always the case, and we recall this article, which may help clear up a few doubts. To settle the matter, Hennessey Performance also put the 765LT on a rolling road using “normal” petrol - in other words, the fuel recommended for this model, equivalent to our 98 RON petrol and 93 in the US.

And what was the result on regular fuel? The McLaren 765LT recorded 758 hp at the wheels, suggesting that it most likely produces more than the claimed 765 hp at the crankshaft.

Why? The explanation is simple: engine output measured at the crankshaft is always higher than the figure measured at the wheels because drivetrain losses are unavoidable. Between the crankshaft and the wheels, power must travel through the gearbox, driveshaft, differential and more. Some force is always lost along the way.

With a conventional automatic transmission, power losses across the drivetrain are estimated at 25%. However, the 765LT uses a modern dual-clutch automatic gearbox and a rear-mid-engine layout, removing the need for a long driveshaft. Taking into account other cars with a similar architecture that it has tested, DragTimes estimates a loss of only 13%.

Based on that calculation, if this is the proportion of power being lost while using normal petrol, the 765LT’s twin-turbo V8 should be producing around 857 hp - 90 hp more than the official figure! With higher-octane racing petrol, that output should lie between 866 hp and 890 hp. Impressive!

Comparison with the 720S

Another detail that stands out after this test is that, based on the figures achieved, the power gap between the McLaren 765LT and the 720S is far greater than officially stated.

Consider this: on a previous occasion, the same YouTube channel put the 720S on a rolling road and measured 669 hp and 734 Nm at the wheels. Doing the maths, that indicates the difference in power between the two models should be around 100 hp, rather than the official 45 hp.

That may help explain just how much quicker the McLaren 765LT can be than the already ballistic 720S, as this Hennessey Performance drag race demonstrates: