The AC unit was first shown at the London Motor Show in 1919 and had an aluminium block, a single, chain-driven overhead cam, a crossflow head and triple carbs.
Heady stuff on such a small engine at the time – the chain-driven cam was ridiculed, even.
Sidevalve motors were still common when the engine made its way into the Ace in the early 1950s, and overhead valves were still high-tech.
The AC Ace is a seat-of-your-pants experience
The secret to the longevity of the John Weller-designed ‘light six’, as it was known, was simply having been 30 years ahead of the game.
Immensely long in stroke, at 100mm, with a bore of just 56mm, the first AC ‘sixes’ were limited to 3000rpm.
They powered AC’s pre-war saloons, tourers and coupés, initially with just 40bhp, but by the time the staid 2 Litre saloon arrived in 1947, AC had it up to 76bhp.
Not long after, John Tojeiro was building lightweight sports-racers with Ferrari Barchetta-aping aluminium bodywork, a tubular steel chassis and transverse-leaf independent suspension.
With the 2 Litre looking increasingly crude, AC saw a chance to use Tojeiro’s design and gain a new model with little development cost.
The AC Ace’s alloy head was offered as an option
The Ace hit the 1953 London Motor Show rated at 85bhp, AC by then confident of taking the engine to 4500rpm.
The car’s lightness, at 810kg, was key to its performance, allowing it to top 100mph on a good day.
But it was the fully independent suspension that made it the darling of road testers and racers.
West Sussex AC dealer Ken Rudd took to the track to publicise the new model in this car, the first Ace run by Rudd Racing Teams and the one with which his tuning parts company, Ruddspeed, built its reputation.
It was Rudd who later pioneered the fitting of the Ford Zephyr ‘six’ to the Ace 2.6, hence their ‘Ruddspeed’ nickname.
This was the first AC Ace run by Rudd Racing Teams
Despite its background in motorsport, however, the original Rudd Ace is pretty stock beyond its stripped-out interior, streamlined grille and Perspex wraparound windscreen.
The raw spitting of its exhaust is common to all AC-powered cars, and you can discern, both in the seat of your trousers and in your eardrums, each pulse of the pistons.
The gearbox – another Moss transmission – is a bit easier to use here, but still it requires a slower shift than you’d deploy in a younger classic.
The Ace doesn’t major on straight-line speed or acceleration, but with its incredibly low body, and the way the doors dip lower still by your elbows, it doesn’t feel slow on the road.
‘Our’ AC Ace wears fitting patina
Push on the accelerator and you get a blat of noise and a burst of torque forwards at any revs.
The narrow rubber limits the grip, but it is fun to chuck around.
Although it looks and sounds hardcore, it isn’t tricky to drive, with steering that is light enough – the clutch, too – and it feels perfectly predictable as it corners.
It’s a slightly strange mix, then, the Ace: an old heart in a modern body.
But it’s a beguiling one, maybe even as good to drive as it looks.
The Bristol 405 drophead is softly sprung and not an out-and-out sports car
With at least one of virtually every low-volume ’50s British sports car having been fitted with a Bristol straight-six at some point, you’re spoilt for choice when seeking to sample this engine.
But it is surely best to do so in one of the old aircraft maker’s own products.
The Bristol Aeroplane Company mopped up one of the immediate pre-war period’s best engines – and the rights for the entire BMW 327 model – in a war reparations deal including former BMW importer Frazer Nash.
A glance at the aluminium head’s two rocker covers might identify this adopted Brit as a twin-cam unit, but it is actually cam-in-block, with transverse pushrods acting on the exhaust valves from the intake side.
The Bristol 405’s straight-six needs to be worked hard, but it never feels coarse when pushed
That allows it to squeeze in big valves and hemispherical combustion chambers, and happily necessitated a crossflow design.
It all combined for an engine that breathed very well for its time, while maintaining the simplicity, lightness and compactness of a pushrod design.
It was with the 404/405, however, that Bristol put the last visual vestiges of the BMW 327 behind it.
Twin kidney grilles were replaced by a jet-engine-aping mouth, while the expensive, Italianate Superleggera tubular steel body frame of earlier Bristols was replaced by a steel lower structure, with aluminium panels to the window line and a wooden frame above.
The 404 is a shorter-chassis coupé, the 405 either a saloon or, as here, a four-seater drophead.
Flat seats and refined controls in the Bristol 405
It’s not a small car, the 405, even though all the lightweight aluminium and wood keep the weight at a reasonable 1206kg.
It has the upright driving position and stance of a Bentley or an Alvis, rather than an Aston or Jag – so is a stately tourer rather than a roadster.
In that tradition, performance is adequate.
The engine’s strong torque allows it to pull out of junctions eagerly, and it gets up to a 50mph cruise happily.
It’s only after that point the motor feels somewhat overwhelmed by the size of the car.
Classic Bristol lines and small fins adorn the 405 drophead
Nonetheless, the character of this gem of an engine shines through, much as it might be masked by the weighty body.
All of the units here are smooth, of course – it comes with the straight-six territory – but the Bristol really is serene.
You’ll be revving the ‘six’ hard trying to make rapid progress, but it is difficult to tell you’re doing so, such is the absence of vibration.
Indeed, the main obstacle to your speed with the Bristol is its soft suspension set-up and high centre of gravity: it rolls a fair amount in the corners and, with little bolstering on the plush seats, you’re left clinging on to the wheel.
This isn’t that sort of car, clearly. The effort has gone into refinement instead, and every control, be it the steering, pedals or gearshift, is perfectly placed and easy to use.
The rorty AC Ace and refined Bristol 405 represent diverging applications of the straight-six motor
Never flashy, the Bristol nonetheless feels a high-quality product in everything it does.
There’s no better word for it than ‘pleasant’, complete with the connotations of mid-century Home Counties respectability that the label brings.
Although its British credentials are dubious, the BMW/Bristol unit fits the profile of a classic British straight-six well.
It’s easy to see why so many, beyond Bristol’s Filton factory, were so keen to put the engine in their own cars.
Thanks to: SLJ Hackett; AC Heritage
Austin-Healey 3000 vs Triumph TR5
The Austin-Healey 3000 (left) and Triumph TR5’s straight-sixes were shared with saloon cars
One very important reason why so many British sports cars were powered by straight-six engines is, of course, that such powerplants were readily available.
Cheaper mass-market sports cars couldn’t afford to have an engine design of their own, and had to take their running gear from high-volume vehicles elsewhere in the range.
Few British manufacturers went beyond six cylinders – especially those trying to push significant volumes, such as BMC.
The Austin-Healey collaboration began with the four-cylinder 100 of 1952 – Austin’s only six-cylinder motor at the time was the D-series, originally a truck engine, and it was deemed far too large.
The Austin-Healey 3000 offers lots of grip and a playful chassis
The arrival of the C-series, however, provided an appropriately sized six-pot that BMC insisted the Austin-Healey should use, even if, at first, the six-cylinder Healey was little faster than the old ‘four’.
From the Nuffield half of the BMC merger, the engine’s development started around the time of the two rivals’ 1953 marriage.
On paper, at least, it looked to be a bit of a backward step.
Unlike Austin, Nuffield already had a ‘six’ of roughly the right size, but the single-overhead-cam Morris/Wolseley 2.2 unit had a habit of burning valves, and that wasn’t in line with BMC’s 1950s brand positioning as a maker of slightly conservative but reliable cars.
The Austin-Healey 3000 switched back from triple to twin SU carburettors in 1962
Although unrelated to the Austin-developed A- and B-series, the new unit followed them in rough outline with all-iron construction, a long-stroke layout and a pushrod head. BMC wasn’t taking any risks with this engine.
From the 100/6 of 1956 on, then, ‘Morris-Healey’ would have been more appropriate, especially after production moved to MG’s Abingdon works in 1957.
Somehow it doesn’t have quite the same ring, but perhaps that’s just unfamiliarity.
Girling front discs and a boost of capacity in ’59 created the 3000, which, while consistently updated, finally settled the model’s name until its discontinuation in ’67.
The Austin-Healey 3000’s handsome cabin
This car is a ‘BJ7’, which is Austin-Healey speak for a 130bhp twin-carburettor model – the first to gain winding windows and with a much easier to use convertible hood.
As it grew older, the Healey increasingly shifted from a bare-knuckle hardcore roadster to a more civilised sports tourer, with the final BJ8 even featuring a wood-veneered dashboard.
So eager and strong from low revs is the Healey that it takes a delicate foot on the clutch to avoid flinging every loose bit of asphalt under the car’s rear tyres as you set off out of a junction.
The 3000’s other inputs are more forgiving, however: the chromed gearlever clicks satisfyingly between ratios, while the steering is direct and feelsome – if heavy at parking speeds.
The Austin-Healey 3000’s sleek details
The 3000 is very low in its ride height and will corner with real grip, although it never lets you forget that this is a small car bolted to a big engine: push and it’ll understeer on the way into a bend, then waggle its rear on the way out.
Like its pre-war ancestors, this is a car that requires technique – a trail-brake here, a bit of steering on the throttle there – in order to get a fluent drive out of it.
With 90% of Austin-Healeys making their way across the Atlantic, it was Federal safety and emissions laws that killed off the car, not any lack of demand.
Despite the model’s age, 1966 had been one of its best years in terms of sales.
The Austin-Healey’s charms, like its engine, were a bit old-school even at the time, but that didn’t prevent it from garnering a consistent following and success.
The Triumph TR5 was short-lived but popular
With no carburettors to be seen in the engine bay of the Triumph TR5, you could be tempted to think that this is where the six-cylinder British sports car went high-tech.
The TR5 was the first British production car with fuel injection, and Triumph used a Lucas II mechanical set-up to get 150bhp from its new 2.5-litre straight-six – although it wasn’t all-new, and it stayed true to the traditions created by its compatriots.
When looking to improve upon the TR4’s 104bhp ‘four’, Triumph reached for the ‘six’ of the range-topping 2000 saloon.
That engine was itself based on the Standard ‘SC’ four-cylinder, first launched with 803cc in the 1953 Standard Eight – so to say it had been stretched would be an understatement.
Now 2498cc, this forced another undersquare British straight-six, even though the old tax horsepower system was long gone.
The Triumph TR5’s fuel-injection issues are largely resolved today
Triumph boss Harry Webster was still a great believer in the importance of the flexibility of a long-stroke engine, however, so he didn’t mind the compromise.
The addition of fuel injection was a happy accident: Triumph first looked at the system as a potential solution to the increasingly harsh emissions legislation coming in the USA, its largest market.
After further development, however, it worked out the other way around.
US-bound cars kept twin Zenith-Strombergs, which couldn’t be beaten for cleanliness after careful tuning to 105bhp, while the rest of the world gained the injected set-up, which was providing a much healthier output.
Despite the poor breathing of the Triumph’s reverse-flow cylinder head, the accurate metering of fuel allowed a wilder cam that would have been otherwise too rough on a carburetted motor.
The Triumph TR5’s contrasting trim and glossy veneer
For all that, however, this still feels like a car with a lumpy cam as the Triumph bursts into life and settles to a rambunctious idle.
Many owners love this characterful rumble, of course, and it certainly marks out the TR5 in this group.
Full of torque and pushing one of the lightest cars here, it helps the Triumph to feel particularly quick, with each prod of the throttle eliciting a rorty growl from the engine and a slight squat on to the rear axle.
It’s a nimble-feeling car, too. The steering is quicker and more precise than that of any other rival here: the wheel’s small diameter enhances a sense of agility, and the brakes are particularly strong.
The Austin-Healey 3000 (left) and Triumph TR5 are charming sports cars
That said, there are some traditional Triumph separate-chassis squeaks and rattles as the firm suspension rides over cracks and bumps.
It’s this aspect that earns the car its ‘hairy-chested’ reputation when compared with more refined choices such as the MGC.
It’s an odd mix of modernity and tradition, then, the TR5.
It makes an event out of every journey, with its vociferous engine, bucking suspension and swift steering, whether the driver is looking for one or not.
Thanks to: Sarah Cox; Paul West; Rawles Motorsport; The Triumph Sports Six Club
British straight-six showdown: the verdict
The AC Ace’s six-cylinder engine can be traced back to 1919
What makes a great engine? If it’s technological progress then the AC’s alloy block, crossflow head and overhead cam back in 1919, allowing it to retain post-war relevance, gets the nod.
But it was expensive. Only 220 Aces had this unit, with far more ending up with Bristol ‘sixes’ – or Ford V8s in the Cobra – which were hardly accessible cars, either.
If instead your measure is quantity, you have to turn to the gruff, simple charms of the Austin-Healey and Triumph.
Both these sturdy straight-sixes emphasise the configuration’s inherent durability, as they remixed saloon-car mechanicals into accessible sporting experiences.
The archetypal British sports car, then. But are they the sweetest-driving or best-sounding engines here today? Not quite.
The Triumph TR5 is an archetypal British sports car
When it comes to the appeal of the motor itself, the Bristol ‘six’ is the one that leaves a lasting impact as the real peach.
Even over-encumbered in the 405, it is sonorous, so very smooth and admirably punchy for its size.
But it’s very much of its time, doomed to be overtaken by engines capable of much higher rev limits. It is an ultimate evolution but not a revolutionary one, like some of the others here today.
The Tadek Marek straight-six of the Aston is also a brilliant unit and the most modern on the spec sheet.
All the cars in which it found itself, but especially the DB5, are beautiful and compelling.
The engine is flexible, powerful and as aurally satisfying as you could want.
Like the AC, this was an expensive engine in an extravagant car built in small quantities.
Jaguar’s XK engine, as found in the XK150, is significant and accessible
Which is what is most impressive about the Jaguar XK: it does all of those things but was, and is, far more accessible.
It was cutting-edge in 1948, yet the XK quickly found its way into models built by the thousands, including not only sports cars but also saloons such as the Mk1, Mk2 and XJ – and even the odd armoured vehicle.
It’s a brilliant engine to use, too, fantastic in its booming exhaust note yet quite adaptable and happy to either slog along in a high gear or rev out to its redline.
Greatness is subjective, and some will say vulgarities such as economics and quantities shouldn’t come into it.
I have to admit that, were cost no concern, there would be a DB5 in my garage, and the Bristol ‘six’ lingers as a jewel of an engine.
However, and with an eye on highfalutin ideals of historical impact and legacy, as well as how it drives today, it’s hard to argue against the Jaguar XK as the greatest of the British straight-sixes.
Factfiles
Jaguar XK150 3.8 DHC
- Sold/number built 1959-’61/586
- Engine iron-block, alloy-head, dohc 3442cc straight-six, twin SU carbs
- Max power 220bhp @ 5500rpm
- Max torque 240lb ft @ 3000rpm
- Transmission four-speed manual with overdrive, RWD
- Weight 3139lb (1424kg)
- Mpg 19
- 0-60mph 7.6 secs
- Top speed 125mph
- Price new £1942 (1960)
- Price now £60-100,000*
Aston Martin DB5 Convertible
- Sold/number built 1963-’65/123
- Engine all-alloy, dohc 3995cc straight-six, triple SU carburettors
- Max power 282bhp @ 5500rpm
- Max torque 280lb ft @ 4500rpm
- Transmission five-speed manual, RWD
- Weight 3284lb (1490kg)
- Mpg 17
- 0-60mph 8.1 secs
- Top speed 142mph
- Price new £4726 (1965)
- Price now £650,000-2m*
AC Ace
- Sold/number built 1953-’63/220
- Engine alloy-block, iron-head, sohc 1991cc straight-six, triple SU carburettors
- Max power 85bhp @ 4500rpm
- Max torque 105lb ft @ 2750rpm
- Transmission four-speed manual, RWD
- Weight 1785lb (810kg)
- Mpg 25
- 0-60mph 11 secs
- Top speed 100mph
- Price new £1439 (1954)
- Price now £100-250,000*
Bristol 405 DHC
- Sold/number built 1954-’56/43
- Engine iron-block, alloy-head, ohv 1971cc straight-six, triple Solex carburettors
- Max power 105bhp @ 5000rpm
- Max torque 123lb ft @ 3650rpm
- Transmission four-speed manual with overdrive, RWD
- Weight 2660lb (1206kg)
- Mpg 24
- 0-60mph 13 secs
- Top speed 110mph
- Price new £3188 (1955)
- Price now £150-200,000*
Austin-Healey 3000
- Sold/number built 1962-’63/6113 (BJ7)
- Engine all-iron, ohv 2912cc straight-six, twin SU carburettors
- Max power 130bhp @ 4750rpm
- Max torque 167lb ft @ 2700rpm
- Transmission four-speed manual with optional overdrive, RWD
- Weight 2465lb (1118kg)
- Mpg 19
- 0-60mph 11 secs
- Top speed 117mph
- Price new £1190 (1962)
- Price now £25-60,000*
Triumph TR5
- Sold/number built 1967-’68/2947
- Engine all-iron, ohv 2498cc straight-six, Lucas II mechanical fuel injection
- Max power 150bhp @ 5500rpm
- Max torque 164lb ft @ 3500pm
- Transmission four-speed manual with overdrive, RWD
- Weight 2271lb (1030kg)
- Mpg 25
- 0-60mph 8.8 secs
- Top speed 120mph
- Price new £985 (1967)
- Price now £35-60,000*
*Prices correct at date of original publication
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Charlie Calderwood
Charlie Calderwood is Classic & Sports Car’s Features Editor
