x Abu Dhabi, UAESaturday 22 July 2017

In the eccentric world of supercars, is there one right engine location?

David Booth looks at the science behind modern supercar design as he pits the forward-engine Mercedes SLS AMG against Audi's mid-engine R8 GT.

Despite the Audi having a 5.2L V10 engine and the Mercedes having a larger capacity 6.2L V8, there is but two tenths of a second between the cars when accelerating from zero to 100kph.
Despite the Audi having a 5.2L V10 engine and the Mercedes having a larger capacity 6.2L V8, there is but two tenths of a second between the cars when accelerating from zero to 100kph.

If you're reading this article for anything other than a pretty picture of two exotic cars, then you probably know that these cars share a few basic common attributes. Both boast about the same horsepower - the Audi R8 GT's newly tweaked 5.2L V10 pumps out 560 ponies; the SLS's big 6.2L V6, 563. They accelerate at roughly the same pace - the R8 GT takes just 3.6 seconds to scamper to 100kph from rest; the Merc an eye-blink more at 3.8. And give or take a Fiat Punto or two, they cost about the same, at - or slightly north of - Dh740,000.

But the similarities end there. Indeed, unlike budget subcompact runabouts where cost-cutting rigorously reinforces homogeneity, supercar engineering is the wild, wild west of automotive design and it would be difficult to find two more different solutions to the same problem - going fast and looking good doing it.

The Mercedes-Benz, for instance, is the classically constructed gran turismo - its engine is (nominally) in front, the drive wheels are in the rear and somewhere in between are the transmission and driveshaft. The Audi, meanwhile, is mid-engined; its entire powertrain crowded behind the rear of the cabin in one great lump. The R8 throws in all-wheel-drive just to make matters a little more confusing.

Armchair enthusiasts hold that the difference between the handling of these two designs is a result of weight distribution; the front-engined SLS having slightly more weight (47 per cent) over the front wheels than the R8 (which has a 43/57 per cent front-to-rear weight distribution). But the plot is far more complex and, while how much weight is on the wheels is of significance, it doesn't totally explain why most modern supercars have gone to the mid-engined format. More important to the driving dynamics is something called "polar moments of inertia".

Imagine you have two dumbbells; both weigh 10kg, each with 5kg weights at their end. But one places those weights a convenient hand-width apart while the second has them separated by, say, 90cm. Both will require the same amount of energy to raise above your head but, if you were to try to rotate them, their behaviours suddenly become vastly different. Grab the shorter dumbbell in the middle and it'll twist in your hand with little effort. Grab the longer barbell in the middle and any rotation will take far greater effort. The farther away from the point of rotation, the greater is the effect of all that weight. That's polar moment of inertia.

The same applies to cars. If you locate much of the car's actual weight as close as possible to the centre (the engineers call this mass centralisation), theoretically, the car should be far more responsive to steering inputs, more easily controlled through corners and easier to toss from one corner to the next.

At least that's the theory. Like all things theoretical, perfection must give way to practical, everyday engineering, so just as Audi's engine is not situated perfectly in the middle of the R8, nor is the SLS's V8 truly in the front. That great mass of R8 V10 is actually a little more rearward than perfectly centralised and Mercedes has taken great pains to shove its 6.2L V8 as far rearward as the firewall and passengers' feet would tolerate. The question then becomes: which of these imperfect designs - front mid-engineish or mid-with-a-tinge-to-the-rear - works best.

If you were judging from racetrack performance alone you'd have to give Audi's engineers their due. And even though the R8, even in GT form, isn't as stiffly suspended as single-focus supercars such as the Ferrari 458 and McLaren's new MP4-12C, it's quite a racetrack tool. Of course, the grip is incredible (the tyres are sticky Pirelli P Zeros, 235/35ZR19 in front and monstrous 305/30ZR19s in the rear); the R8's all-wheel-drive system is tailored towards sporty driving (85 per cent of the V10's torque is distributed to the rear); and the roll well mitigated. But the real joy of the R8 is that it feels like the entire car is rotating about a point directly under your seat. Rather than steering a 4,435mm-long car, the sensation is that you are controlling something not much larger than the seat you're driving in.

The SLS, meanwhile, is a little more unwieldy. Its grip is just as prodigious (it too wears mucho rubber; 265/35ZR19 in front and 295/30ZR20 in the rear) and the big 390mm front discs almost as powerful as the R8 carbon-ceramic items, but the front-engined Merc never feels quite as responsive as the R8. Some of that is because a) it's a Mercedes and, therefore, b) it's more of a GT than true supercar so its relatively (only by comparison) soft suspension allows more roll. But despite lighter steering, the SLS always reminds you that there's a substantial mass to change direction. It'll fly around a racetrack, certainly, but like the barbell, it takes more work.

"So that was easy", I hear you say, "we should all just buy the R8 and ignore the Mercedes."

Not quite so fast. Besides the obvious impediment of forking out more than 700 large for any car, the R8, at least in this guise, has a few faults of its own. Actually, it has one fault. But it is glaring and so any recommendation for its purchase has to come with the caveat that you can deal with its often cranky automated manual transmission.

Unlike the rest of Audi's semi-automatic transmissions, the R8's R-Tronic uses a single automated clutch. Simply put, in automatic mode, the shifts take too long, slowing forward momentum long enough to be a real impediment to comfort. Use the paddle-shifters and things happen more quickly, but then shifting becomes fairly abrupt.

It works well enough at the racetrack, where comfort is not a primary concern, but at least one of our road testers found it problematic enough on the street to eliminate the R8 GT from his list of must-haves. And while other versions of the R8 have a six-speed manual option, the GT is only available with the R-Tronic. However, salvation is pending as Audi is working on installing a dual-clutch unit.

The SLS has its own transmission issues, but prospective owners will be happy to find that they only show up at the racetrack. When the pedal is to the metal in the AMG-tuned seven-speed, it can be slow to shift. Particularly annoying is its hesitation to downshift as it often screws up corner entry. Of course, it's only an issue on the racetrack - otherwise, the SLS's SpeedShift transmission is all sweetness and light. As is the rest of its comportment.

In the end, unless you're planning to spend all your time racing, the difference in actual speed and handling between these two cars is inconsequential. If you're actually shopping in this snack bracket, the decision will be more qualitative than quantitative. How they feel is far more important than how fast they go.

So here's the easy answer for which one is better for you.

The Mercedes has all the traditional attributes of a classic, big-boned GT, albeit with sophistication and an elegance that rivets the attention of passers-by.

The R8, on the other hand, is the quintessential mid-engined sprinter. Flawed in its powertrain perhaps, but otherwise athletic in the extreme. And equally alluring.