Twin test: Lotus Emira i4 vs Mercedes-AMG A45 S

Engine transplants (much like heart, liver or kidney swaps, I presume with a healthy lack of medical expertise) are a tricky business. 

There are many great engines. There are some sublime chassis. But not all of them are necessarily happy playmates. We all adore the Lexus LFA’s wailing V10, don't we? But I don’t want one disturbing the peace in a Rolls-Royce Phantom. A Range Rover feels born for a diesel V8. I don’t think they’d sell many if a shock Volkswagen joint-venture plopped in the thrummy little tyke from under the bonnet of an Up GTI.

So what happens when you take literally the maddest road-legal four-cylinder engine in the known universe, and plumb it into the back of a British sports car? That’s what we’re here to figure out with the new Lotus Emira i4, powered by AMG. And just to spice up the first impression, and make life a bit trickier for the Brit, we’ve brought along the donor car. The Mercedes-AMG A45 S. The Saturn V rocket of hot hatchbacks. 

What with all the hoo-hah over AMG deciding this engine was fit to replace the V8 in the C63, it’s become a bit of a villain of late. And that’s a pity, because it’s not the engine’s fault where its masters decide to send it to work. And this powerplant, unsexily codenamed ‘M139’ is a masterpiece of improbable ‘ve vil make it vork’ German bloody-mindedness. 

The turbocharger’s turbine spins on roller bearings to reduce friction and quicken-up spool time. The cylinders are coated in the same hyper-slippery linings as a Mercedes F1 car’s V6. The whole engine weighs just 160kg ready to go, yet in the A45 S it generates 415bhp. That’s over 200bhp per litre. Over 100 horsepower per cylinder. And yet humming away in eighth gear, ensconced in a leather-and-metal-lined A-Class cabin, you could almost fool yourself into thinking it’s a normal engine. 

Or you could select Race mode and it’ll pull your head off and eat it. Boy has this engine got a temper. It’s buzzy and revvy and doesn’t deliver maximum urgency until it’s going berserk beyond 5,000rpm. It’s got a Japanese element of tuner fury to it. It’s also flawed: there’s enough lag to get caught out by a turbodiesel van if you’re sleeping, and the noise sounds overly augmented. But it’s hugely exciting. Experience this and the engine in a Golf R or Audi S3 is about as invigorating as indoor exam revision on a balmy summer afternoon. 

So, big pay rise ahoy for whoever at Lotus cleverly negotiated the use of AMG’s furious four-pot? Hmm. Temper your frothing excitement for just a moment, because here come The Caveats. 

Firstly, this engine develops 415bhp in the A45 S. In the plain ol’ A45 ‘non-S’ that isn’t sold in the UK, it’s down at 382bhp. That’s strictly the engine that Lotus has employed. And because it required a new intake and exhaust to breathe through now it lives in the middle of a car, it’s been detuned (or retuned, if you prefer) to make ‘just’ 360bhp. 

That’s still a lot of poke from an engine capacity smaller than the bottle of milk in my fridge. But I could understand if you were left underwhelmed, knowing just how much more this motor’s got to give.

Then again, with more torque on tap than the V6 and a faster-shifting gearbox as standard, the Emira i4 almost embarrasses its supercharged six-cylinder stablemate by getting from 0-62mph in 4.4sec – just a tenth slower than the 3.5-litre version.

Both cars will do over 170mph and in the time it takes an Emira V6 driver to heel-and-toe a downchange with the awkward pedals, an Emira i4 driver will have summoned the kickdown and been hurled forward on a wave of boost. 

So don’t worry: it’s fast enough. And my hunch is that Lotus will leave the range like this for a year or two, then start turning up the wick on the i4, deleting the V6 in the process. 

If and when that happens, hopefully it’ll recapture some of the lost fury of the A45 S. The Emira’s application of the same oily bits is more sanitised, less banzai. But what you lose on sheer lunatic factor you get back in nuance: the turbo is much more of a personality in the Emira, wheezing and whooping and countering and hollering. It whooshes menacingly as you trundle about town and the wastegate sighs as you back off are brilliant. Part old-school Noble, part early Audi RS car. 

Pity about the engine bay though, right? Lotus says the rubber bath mat it has used to hide the Mercedes bits acts as a cooling feature. I reckon it acts as a 'hide all the Mercedes badges' feature.

So which would be best to live with? Well the Emira uses less fuel: we found it averaged around 32mpg, with the A45 always 1-2mpg worse off. At a 70mph motorway schlep the Lotus was doing almost 50mpg. Impressive. 

This Emira had the Sports chassis set-up, with pointier front geometry and stickier tyres. It was hungry for cambers and quite restless on back roads. Meanwhile we agreed Mercedes appears to have softened off the A45 just a smidge. It must have, because the white car was a couple of thousand miles old and the trim hasn’t all started rattling yet, like most A-Classes. So the A45, despite its 3am McDonalds drive-thru image, is the more easy-going car.

And overall – in a result that shocked me, I must say – it’s the more entertaining drive. Where the Lotus has grip and composure but lacks steering feedback and delicacy, the A45 gives you more options. It feels more up on its toes in the corners, and with lower limits on the exit, you can exploit the chassis a bit harder. The gearbox is much crisper on home turf in the Merc, and you can’t get away from the fact the engine is outrageous. A swivel-eyed, bark-at-the-moon madman for the ages. 

You also can’t escape the fact the A45 S – though hugely pricey for an A-Class at over £63,000 – is almost £20,000 less expensive than an Emira i4 First Edition. Which is all you’re able to buy for the next 12 months. Cheaper trim levels come later. 

The Lotus will turn more heads. People will wave it out of side turnings more often. Fewer oxygen-thieves in tastelessly modified VW Group tat will try to engage you in an inner-city race to the death. It’s an extremely desirable, seemingly well-finished product.

But sat here writing this a few days after they both went away, I can’t quite justify the Emira’s price, or get away from the sense the best of that car is yet to come, whereas with the A45, it’s already here. 

Photography: Johnny Fleetwood