Mercedes-AMG has pulled the covers off its new CLA 45 4MATIC+ at the Goodwood Festival of Speed – and speed is one of the big stories behind this small, all-electric saloon that’s been thoroughly reengineered by Mercedes’ sporting AMG division.This is Mercedes-AMG’s new compact electric performance car, and it arrives with the kind of numbers that would have sounded faintly daft not very long ago. There are three electric motors, fully variable four-wheel drive and up to 671bhp. That’s enough for 0-62mph in 2.7 seconds with a one-foot rollout – a drag-racing measurement which means the car starts moving, travels the first foot, then the clock starts. It does a pretty stunning 0-62mph in three seconds using the more familiar measurement from a standing start. Either way, it puts this compact AMG deep into supercar territory.The Mercedes-AMG CLA 45 gets three British-developed electric motors, fully variable four-wheel drive and up to 671bhp (Mercedes-AMG)The new CLA 45 will be available from launch in two body styles: a CLA 45 saloon and a CLA 45 Shooting Brake estate. Both use the same 94kWh battery, the same 800V electrical system and the same three-motor layout, but the slippery saloon just edges ahead when it comes to range. Mercedes claims more than 416 miles for the saloon and more than 398 miles for the Shooting Brake. Charging should be properly quick, too. The CLA 45 can take power at up to 330kW from a fast enough public charger, with a 10 to 80 per cent top-up taking 22 minutes. Mercedes also says a 10-minute stop can add more than 168 miles of range, which is roughly the sort of pause you’d need for a coffee, a snack and a quick chat with fellow fast car fans.The clever bit of the CLA 45 sits underneath. Instead of more familiar electric motors, the CLA 45 uses three axial flux motors: one at the front and two at the rear. They’re much smaller than conventional electric motors, with the front motor just 90mm wide and the rear motors around 80mm each, but Mercedes says they’re able to deliver huge power, again and again, without fading.The new Mercedes-AMG CLA 45's MBUX infotainment system brings together AI from ChatGPT, Microsoft Bing and Google Gemini (Mercedes-AMG)They’re the same type of motors used in the recently-revealed Mercedes-AMG GT, developed by British firm YASA that so impressed the folk at Mercedes-Benz that they bought the company.The rear motors are the main event, with the front motor acting as what Mercedes calls a “booster motor”. It can be disconnected when it isn’t needed to cut mechanical drag and help efficiency, then brought back in almost instantly when the driver asks for more power, or the car needs extra traction. In calmer driving, the CLA 45 can run mainly as rear-wheel-drive, which is exactly the sort of thing AMG fans like to hear.The two rear motors also allow the car to shift power between the back wheels, helping it turn in more sharply and find more grip on the way out of corners. Mercedes calls the system AMG Performance 4MATIC+, but the basic idea is simple enough: it sends power to the wheel or wheels that can use it best, depending on what the driver and the road are doing.AMG has also thrown plenty of chassis tech at the CLA 45. There’s AMG Ride Control sports suspension with steel springs and adaptive damping, with Comfort, Sport and AMGFORCE S+ settings. The car also gets five levels of energy recovery, and Mercedes says most braking can be handled by the electric motors before the conventional brakes step in for emergency stops or hard circuit use.There’s active aero as well, with the saloon getting an active rear spoiler and the Shooting Brake getting an active roof-edge spoiler. Both change position depending on speed and driving mode, either helping the car slip through the air more cleanly or adding stability when things get a bit more serious. The active radiator fins at the front can stay closed almost all the way up to 155mph, helping range and acceleration before opening when the car needs extra cooling.The Mercedes-AMG CLA 45 is strictly a four-seater with two bucket seats in the front and the back (Mercedes-AMG)And because this is AMG, it hasn’t all been left to software and silence. In AMGFORCE S+ mode, the CLA 45 can mimic the sound and feel of an AMG four-cylinder petrol engine. Mercedes says it used an AMG A 45 S as the reference car, recording it with 13 microphones and more than 1,600 measurements before recreating the sound in the electric CLA.The result is more than just a noise through the speakers. The car can also use seat shakers, simulated gear changes and a special driver display with a rev counter and gear indicator. It’s designed to make the electric CLA feel more like an old-school AMG hot saloon than a silent EV, with the paddles behind the steering wheel used for simulated manual shifts rather than actual gears.The Mercedes-AMG CLA 45 gets active rear spoilers on both the saloon and Shooting Brake versions (Mercedes-AMG)There are plenty of drive modes to choose from, too. Comfort is for everyday use, Sport firms things up, ECO cuts back the power and helps maximise range, while Slippery is designed for rain, snow, ice and other low-grip surfaces. The AMG Dynamic Plus package adds a Race mode with launch control and extra track functions, including AMG Track Pace. That system records more than 80 vehicle data points ten times a second, showing lap times, braking points and other track information on the car’s screens and head-up display.The design is fairly serious, as you’d expect. The CLA 45 gets an AMG-specific front grille with vertical struts, wider front wings, a front splitter, bigger side skirts and a rear diffuser-style design. It sits on 19-inch wheels as standard, with 20-inch alloy and forged wheel options available. The saloon has a drag coefficient of 0.23, while the Shooting Brake is 0.26.Inside, the CLA 45 is a strict four-seater, with a centre armrest in the back rather than a middle seat. There are AMG sports seats, an AMG Performance steering wheel and the latest fourth-generation MBUX infotainment system, based on the new Mercedes-Benz Operating System. Mercedes says the system brings together AI from ChatGPT, Microsoft Bing and Google Gemini, while AMG-specific apps show things such as energy flow, motor output, tyre pressures, battery status and driving data.As well as a 390 litres boot in the saloon (450 litres in the Shooting Brake), the CLA 45 gets a 101-litre frunk (Mercedes-AMG)There’s a practical side, too. Boot space is 390 litres in the saloon and 450 litres in the Shooting Brake, and both cars get a 101-litre front boot. Top speed is limited to 155mph, or 168mph with the AMG Dynamic Plus package. Mercedes hasn’t confirmed prices yet, and it hasn’t said exactly when UK cars will arrive. But with 671bhp, a claimed range of more than 400 miles in saloon form, ultra-fast charging and enough electronic theatre to keep AMG fans entertained, the CLA 45 gives a pretty clear idea of where the brand’s electric performance cars are heading next: faster, louder and a lot more dramatic.