Millbrook’s Hill Route is essentially a private B-road, full rollercoaster in spec - and closed just for you. You might argue that makes any car fun, but at this year’s SMMT Test Day, where I had my pick of piston-powered and electric performance cars, two stood out: the Abarth 600e and the Polestar 3 Performance.

Healthy power outputs and impressive straight-line pace aren’t anything new for performance EVs, but what happened mid-bend set these two apart. In the Abarth, the steering wheel tugged as it tightened my line; in the Polestar, torque shuffled side to side, making scything through S-bends both swift and smooth. Despite their contrasting formats - front-wheel drive hot hatch versus all-wheel drive SUV - both share a critical piece of hardware: a mechanical limited-slip differential.

Almost all cars have differentials - even EVs - but not all diffs are created equal. A regular (open) differential allows the driven wheels on a given axle to turn at different speeds. Torque distributes itself naturally, but it’s not without drawbacks. The limited-slip differential restricts the independent movement of each driveshaft to a degree, sending torque to the wheel that can best use it.

This not only increases agility, but maintains forward thrust rather than wheelspinning the excess away. It also subtly - or not so subtly - affects the car's line by shifting it toward the tyre with the most traction. Especially in the hot hatch market, the great and the good have been dynamically separated by the inclusion of an LSD.

In recent years, ESP systems have evolved to mimic the effects of an LSD, removing the need for a mechanical diff, saving manufacturers money and reducing drivetrain losses. While many brands have moved toward brake-based torque vectoring to achieve a similar result, this reduces torque to one wheel rather than transferring it. The end result may look similar, but a dialled-in driver can tell the difference straight away. Recently, manufacturers like Audi have been introducing mechanical torque-splitting tech into the S3 and RS3, transforming their dynamic reputation in the process.

EVs have largely gone without LSDs, as adding one to the drivetrain can reduce efficiency due to increased transmission losses and a drop in range. And for EVs, efficiency is everything. But does that matter in a performance car? Sensations are more important than sensibilities in this segment, and improved handling at the cost of a few WLTP miles is a worthy trade-off. Nobody buys a hot hatch to hypermile.

Driver engagement has become important enough that old tech is returning to performance cars – even as the piston-powered era nears its end. In-wheel motors are coming, multi-speed motors are already here, and so are LSD-equipped performance EVs. One of which is the Abarth 600e.

The Scorpionissima may blur the line between compact crossover and hot hatch, sharing a silhouette and chassis with the Fiat 600, but look closer and the lowered suspension, bespoke bumpers and sills, enlarged brakes, 20” wheels, and Michelin Pilot Sport EV tyres stand out. It retains a single-motor, front-wheel-drive setup with 254lb-ft (345Nm) of torque, and a choice of 235bhp or 276bhp.

What it’s got that none of its rivals currently offer is a Torsen mechanical limited-slip differential - and the result is profound. The chassis setup is ripe for tackling B-roads yet compliant enough not to fuse my vertebrae over a shallow rut. The same couldn’t be said for the 595 that came before it.

The steering wheel feels thinner than a USB-C cable, and is more triangle than circle, but it’s wrapped in grippy Alcantara, much like the fabulous two-piece seats. There’s only a small dead zone during initial turn-in, but beyond that it weights up nicely and doesn’t require mid-corner adjustment if you’re balancing the throttle. At seven tenths, the Abarth corners with confidence and enthusiasm.

Stand on the throttle however, and there’s no sense of washing out, tyres losing grip, or the power being cut by traction control. The car tightens its line and accelerates as quickly as if it were a straight section of road.

EVs are known for their low centre of gravity, courtesy of floor-mounted batteries, but many require encouragement to shift their mass when cornering. The Abarth can be thrown into corners and keeps its composure where rivals would understeer. It drives like a proper hot hatch - fuel type, ride height, and 1,640kg kerb weight be damned.

The LSD reacts quicker and more decisively than that of the Mk8 Fiesta ST, the enduring benchmark of the hot hatch segment. It may not match the outright power of the new electric hyper-hatches, but the fun doesn’t stop when the road gets twisty - thanks to the LSD.

Similar technology appears higher up the food chain. The Polestar 3 employs a more advanced differential on its rear motor: a twin-clutch, electronically controlled unit that adjusts torque vectoring during both acceleration and braking. It promises enhanced agility and greater cornering stability under load. But does it deliver in a 4.9-metre, 2.5-tonne family SUV?

It does, just not in the same way as the 600e. The Polestar 3 Performance provides 510bhp and 671lb-ft (910Nm) from twin electric motors, and the resultant acceleration means corners arrive quickly. Braking and turning into a downhill hairpin, the car is stable, with the diff subtly tightening the arc during energy recovery.

The same is true through a double S-bend, but here the sensation of torque shifting side to side is more pronounced. The front axle holds true to my commands as the pace increases, with no sense of washing out or excessive body roll.

Where the Abarth demands to be driven like a hot hatch, the Polestar prefers considered, consistent inputs to match its more relaxed nature; it flows along undulating roads at an impressive rate. Grip comes courtesy of bespoke Pirelli P Zero tyres, and its stable cornering stems from the firmest steering and dual-chamber air suspension settings I selected before departure.

Plenty of performance EVs pair strong grip with heavy steering, but few combine that with the ability to confidently gain and shed speed through tight corners while feeling as if the car is rotating around the driver. The BMW iX and Mercedes-Benz EQE SUV don’t handle like this, nor do they encourage the same exploration of limits in such an approachable way. The Polestar may not change direction as quickly as the four-wheel steer-equipped AMG GLC 63 S E hybrid, but its natural, fluid motion gives it an edge.

Traction control doesn’t interfere when the ‘Performance’ badge is put to the test. It allows the car to press on without losing composure. It’s not just satisfying to move something this big quickly – it’s rewarding to ask more of it and get an enthusiastic response.

The Polestar 3 Performance is the best-handling car in its class, and certainly my personal favourite. Likewise, the Abarth 600e is the RUSH benchmark for compact electric hot hatches and crossovers we've tested. While EVs are often criticised for feeling one-dimensional and lacking the mechanical feedback of great piston-powered models, something seems to have been unlocked with the return of LSDs.

There’s still some work to do though. Neither the Abarth nor the Polestar features regen paddles. The 600e uses a button in the centre console, while the 3 buries its regen settings in the infotainment. While presets allow for consistency, steering wheel-mounted control would add an extra layer of engagement.

Would the Virtual Gearshift Technology from Hyundai and Kia’s performance EV range improve engagement further? Perhaps, but equally the relative simplicity of the set-ups across both cars - merely selecting the drive mode and regen level - made it easy to dial into the chassis and powertrain personalities, without getting distracted by a simulated gearbox.

I digress. Performance car stats - power, torque, acceleration, range, charging times - don’t lie. But they don’t tell the whole story, either. Feel and feedback can be engineered in, but what can’t be measured is the feelgood factor. The mid-corner adjustability of both cars feels real and unfiltered. It hasn’t been predetermined by a software engineer who, despite their best intentions, isn’t sitting next to you, experiencing the same road, and sharing your desire to press on.

Both Abarth and Polestar have prioritised feel over force. None of us wanted more power – just more cars like these. Performance EVs aren’t new, but LSD-equipped drivetrains are becoming more common, following in the footsteps of the Audi RS e-tron GT and Porsche Taycan GTS.

Multi-motor setups are also on the way. The Alpine A390 uses a tri-motor layout with individual motors for each rear wheel, while BMW’s VDX prototype has one per corner. These systems promise faster response times and reduced mechanical drag compared to traditional LSDs.

The mechanical setup is always the most important thing in any performance car, and while some effects can be replicated and rendered by electronics, there’s no substitute for mechanical torque vectoring. It's the oldest cliché in the book, but it's like comparing a digital wristwatch to a timeless chronograph. Intricate mechanics and a sense of craftsmanship will always conquer lines of code, however accurate.

The sensation of the Abarth’s motor grabbing at the steering wheel, and the precise vectoring of the Polestar’s torque whether using or harvesting electricity add agility, fluidity, and that immeasurable feelgood factor. In the case of performance EVs, the limited slip diff makes more than a difference; it makes them transcend the binary parameters of the coding, and begin to come alive.