Chamber Chat - Blackbourn 395

By: Rob Blackbourn

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'A thing of beauty is a joy forever’ or it bloody well should be, according to Rob


Rob Blackbourn's Chamber Chat

Surviving your mid-teenage years can involve grabbing on to a few ‘certainties’ as props while you try to make sense of the confusing world around you (and of your confusing self). For me it wasn’t memorising test-cricket scores, or train spotting, or mastering the sitar – I chose a heavy diet of engine design theory.

While many of the early ‘certainties’ from these youthful obsessions go by the board as you finally stumble your way into adult-life’s realities, some stick with you against the contrary evidence.

I managed to let go of stuff like the supremacy of Stromberg 97 carbies for all applications and the universal efficacy of the Scintilla Vertex magneto. A trickier one to jettison was the notion that if God designed the perfect engine, it would definitely have hemispherical combustion chambers.

To make things easier I’ll call the blessed set-up ‘HCC’ from this point – ‘Hemi’ is a little confusing, being actually a registered Mopar brand.

While HCC engines have been around as long as cars, Oz petrol-heads started to take notice when Jaguar’s twin-cam XK engine burst on to the scene powering the glamorous XK120, then when the humble Peugeot 203, with a push-rod HCC motor, won the Redex Trial.

No question, then – HCC was the holy grail design. How could it not be with its cross-flow set-up and the space it provided for bigger valves?

Those of us who were besotted with old Henry’s flathead V8 knew that no amount spent on Offy or Edelbrock heads could match the performance of an Ardun OHV converted side-whacker. The amazing American engineer Zora Arkus-Duntov (yes, the ‘Father of the Corvette’) had produced these gorgeous HCC heads from 1947.

Okay, Ardun kits were too thin on the ground here for the ordinary bloke to afford them – but he still recognised that they represented the ultimate.

Then another amazing engineer, our own Phil Irving (of Vincent motorcycle-fame and later Repco Brabham V8-fame), produced an HCC head for ‘Grey’ Holden motors – the Repco Highpower head. A Repco-equipped black FE Holden, an ex-taxi raced by Bob Holden, got our attention when it recorded a top speed of 123mph.

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Then there were Chrysler’s Hemis. If we hadn’t noticed them before, we certainly did when ‘Big Daddy’, Florida’s Don Garlits, topped 200mph in 1964 in his blown-Hemi powered ‘Swamp Rat’ rail.

Credit where it’s due, though, to the alternatives to the HCC. Side-valve motors could be made to do good work, especially with pioneering Brit engineer Harry Ricardo’s ‘turbulent chamber’ design – Chrysler’s flathead sixes and Harley-Davidson’s flathead V-twins used Ricardo’s expertise successfully. And ‘bathtub’ and ‘wedge’ chambers have their own worthy place in the scheme of things.

The one that I just can’t salute, though, is the widely used ‘Heron’ chamber – the ‘Claytons’ chamber. With a dead flat Heron head you need a bowl in the top of the piston to let the fuel-air charge do its energy-release thing.

Then there’s the complicating reality of the need in modern engines to achieve high efficiency across the rev-range, not just the excellent breathing an HCC offers at high revs. And fitting four valves in a hemispherical space is just too tricky. And you need generous ‘squish’ areas and ‘swirl’ effects. The list goes on…

So sadly the HCC just doesn’t cut today’s more complex mustard as well as a pentroof chamber. But some of us still worship at its altar...

Finally, imagine my distress with what unfolded at Jaguar in 1971. The factory that had been a champion for the prince of combustion chambers in its XK-series engines, introduced an exciting new 5.3-litre V12 for the E-type – exciting until they revealed that it featured cheap ‘n’ cheerful Heron combustion chambers.

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