11 things you need to know about Bentley's 6.75-litre V8

By topgear, 29 June 2020

Today the last ever Bentley Mulsanne rolls down the production line. Within it lies the very last six-and-three-quarter V8. And, somewhat oddly, it’s the engine rather than the car that we’ll mourn most.

The Mulsanne is ten years old. It replaced the Arnage, which came after the Turbo R, which dovetailed with the first Mulsanne, which supplanted the T-Series cars, which superseded the S-Series.

That’s a lot of history – over sixty years of grand Bentley sedans in fact, and the same V8 engine has thrummed, roared, rumbled and sung under the bonnet of each and every one of them.

Here are the 11 things you need to know about the death of the engine… and the car that now dies with it.

STORY Ollie Marriage

It now develops over 200 per cent more power

The L-Series engine (technically L410, after the 4.1-inch bore size) first appeared in the 1959 S2, and back then it wasn’t a six-and-three-quarter at all, but a six-and-one-quarter.

The capacity of experimental versions went as high as seven-and-a-half litres, before settling at six-and-three-quarters in 1971 (the stroke was lengthened from 3.6 to 3.9 inches).

It’s been developed of course, but when Bentley says it’s the same engine, the aluminium block still has the same bore measurements and cylinder spacing, and the heads retain two valves per cylinder driven by pushrods on a single camshaft. 

Originally the L410 developed around 180hp. In its latest guise, under the broader but shorter bonnet of the Mulsanne, it develops 530hp and a massive 1050Nm. Almost three times the power, but here’s the stat I really love. So much cleaner is the engine now (emissions are down 99 per cent), that the new engine could idle on the tailpipe emissions of the old one.

(Click HERE to read about the final Bentley Mulsanne, the '6.75 Edition by Mulliner')

It replaced a straight six

Here’s one being assembled sometime in the Sixties. You can see the pushrod-actuated valves and the beefy springs that supported them. The engine had a tough brief when it was first developed.

It was replacing a straight six that dated back to 1922 (two engines spanning almost a century of car development), and the new V8 was tasked with boosting power and torque by 50 per cent, matching it for noise, smoothness and reliability and being no heavier or larger – it had to fit in the same space without styling or structural alterations.

It managed all of that (casting the block from aluminium than iron certainly helped, as it was 13+kg lighter), and I think we can confidently say that it exceeded the final part of the brief, which was that “bearing surfaces should be large enough to allow a 15-20 per cent increase in power and torque over the lifetime of the engine”. Nailed it.

No other car produces more torque this low down

There’s much to be said for engines that don’t rev. Of course we all love revs in sportscars, but frantic, fizzing engines just aren’t right for luxury cars. In those you want effortless torque as low as possible – and none delivers more of it, or lower down than the six-and-three-quarter.

In its latest, mightiest iteration, the twin turbo V8 delivers a metric 1100Nm at 1,750rpm, although everything else about this car is imperial. It’s why it’s definitely a six-and-three-quarter engine, not a 6.75…

And yes, the engine really does only rev to 4,500rpm (plus the needles sweep smoothly south). Why do you need more, though? The Mulsanne is long geared, but has so much shove from 1,750 that you rarely find yourself going beyond 3,000rpm. These days almost every diesel revs higher. Which brings me on to another point.

Now, I might have this wrong, because this goes back over 15 years, but I’m sure I remember Bentley’s then engineering director, Ulrich Eichorn, telling me that the 6.75 was so strong that they tested it without a spark plug, whacked up the compression and ran it on diesel – and the block took it.

It was an early adopter of turbocharging

In 1982, Bentley turbocharged the six-and-three-quarter. It was the first ‘blower’ Bentley since the Birkin era and pushed up power and torque by 50 per cent. Bentley didn’t talk about such uncouth specifics as power and torque back then, instead suggesting it developed something in the order of 300hp and 610Nm – enough to yield 0-95km/h in 7secs. 

They had looked at fitting a Garrett turbo per bank, but there wasn’t enough room in the engine bay. The original Mulsanne Turbo carried no suspension or bodywork changes, just a small badge and a bigger price tag. But it was an instant success, and it’s no stretch to say this is the car that saved the company.

It’s not exactly responsive

Remember what I said about revs? You don’t need ‘em. Same goes for response. You just need to plan better. The six and three-quarter has a huge amount of inertia in it – you’re overcoming not just turbo lag, but piston weight, flywheel effect, crankshaft – so building up a head of steam takes a second or two. But then it does the full runaway locomotive, and it’s chuffing fast.

Speed builds quickly, and when you lift off, doesn’t diminish at all – the Mulsanne has unstoppable rolling momentum. So you find yourself feeding the engine gobs of torque via the long travel throttle, there’s a distant churn of noise, the nose picks up and after a short while you lift off, the noise dies away and you sweep serenely on.

It’s not the smoothest or most refined these days (a Merc V12 is superior in both regards), but a Bentley shouldn’t be silent, it should have charisma. The engine doesn’t have a particularly memorable noise signature, there’s no quaking V8 roar, instead it has presence. You’re always aware of a distant rumble and hum, giving it more in common with a cross-channel ferry than you might expect.

What’s wrong with body roll anyway?

The engine defines the car. So it rolls like a nautical vessel as well. There’s nothing wrong with that. Roll lets you know a car is working. Sure, there’s something semi-delinquent about going fast in a Mulsanne, even one with a Speed suffix, and initially it feels quite ponderous.

Throttle and brake pedals have to be pushed a long way, the steering (with an unfashionable 3.5 turns between locks) needs a good long sweep. It comes across as lazy and reluctant. 

So I went and drove it for 160km, and I completely changed my mind about it. It’s not lazy, it’s just deliberate. It gives you time to correct your mistakes, and convinces you not to rush about, but instead to follow the car’s lead. Not slow down, but realise that there’s a certain pace at which the Mulsanne is perfect, a certain way it likes to be driven.

You slow down earlier, calmly select the correct gear (you rarely need to call on one lower than fourth) and then gently introduce some power back in before you arrive at the apex. The car adopts its chosen roll angle, settles and then surges onwards. It’s far more composed than you expect.

It generates enough heat to warm a stately home

Lift the bonnet after any drive (and you will because you’ll want to admire the work done by – in this case – Ian Allcock), and you will inadvertently take a step back as you realise that what you’ve just opened appears to be a giant oven door. It gets hot under there, even after short, modest trots out and even though the engine only pulls 1,550rpm at 112km/h in top. 

Much heat is the result of much fuel being burned. Yes, you should get over 640km per tank, but that tank is almost 100 litres. At a stately cruise you’ll get 11.8-12.3l/100km, which is good, seeing as that soars to over 20l/100km if you get carried away.

Nothing else looks or feels like this

Except maybe an aircraft – there are a lot of dials, buttons and switches about the place. You sit high, at crossover altitude, and survey the outside over that proud bonnet. Yes, you look like it’s the late Eighties and you’re chauffeuring Tiny Rowland around, but you won’t care a jot because it feels good.

It feels nothing like a Volkswagen

The Mulsanne doesn’t pre-date VW’s ownership of Bentley. In fact, the Crewe marque had been under German ownership for 12 years when the Mulsanne appeared, but for some reason this proud machine, with its signature motor, seems to have been immune to the effects of Wolfsburg. While the Conti GT, Bentayga and Flying Spur all have matched platforms, engines and components with other cars across the group, the Mulsanne stands alone. 

That’s not to say the Mulsanne is a bespoke one-off – it’s not, but the shared componentry is better hidden and disguised here. Often, it must be said, by being quirkily flawed. I searched high and low for a recognisable VW part. This was literally all I could find – the handle for the compartment under the boot carpet.

It has a drawer for your phone

Maybe not so much quirkily flawed as idiosyncratic, but here’s an example of what sets the Mulsanne apart. It has a USB slot. Just the one, and it’s tucked away in a drawer. Which means that’s where your phone has to go, because if you shut the drawer with the lead poking out, the drawer gets stuck. Phone too big to fit? Most of them are – deal with it.

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