Who's for a bit on the side?
By John Dickinson on 22nd Aug 07
IT is not exactly a huge secret at the moment - not since some clown at the factory took a picture of it on his phone and splashed it around - that one of the more adventurous Spanish trials manufacturers is having a play with a side-valve four-stroke motor with an eventual view to production. But here's a tip - don't bang your orders in yet - it won't happen this year!
Now the side-valve - or flat-head - engine hasbeen around since the very first internal combustion engine splutteredinto reluctant life and was actually dominant until around themid-1920s. After that, although it was still widely used as acheap-to-produce bread-and-butter power-plant in all walks of life -automobiles, motor-cycles, industrial use - it was plain to most(Harley Davidson was the exception that proved the rule) that it was adead duck as far as high performance was concerned and those seekingperformance from their engines turned to overhead valve designs.
So, what exactly is a side-valve engine? Check outTecumseh or Briggs and Stratton lawn-mower engines for a start. Theydon't get cheaper or simpler than those...
For the benefit of those who don't know, theside-valve motor has its valves running parallel to the cylinder block,pointing up into the combustion chamber instead of down through thehead. All the workings (camshafts, tappets etc) are situated in thecrankcase so there is nothing at all in the cylinder head, apart fromthe spark-plug (like a two-stroke) - hence the term flathead.
And there's the rub. Because the valves need theirown space in which to operate alongside the cylinder, the spaceavailable for combustion is almost twice as much (if not more) than youactually want. It is impossible to design-in the perfect circularcombustion chamber and damned difficult to get the gases flowingquickly.
There are however several massive designadvantages to the side-valve. Because the head is flat (no valves upthere or camshaft or other gubbins) you get a very low, compact engine,up to six inches less overall height over a conventional overheadcamshaft motor. Also, because what extra weight there is (over atwo-stroke) is low down in the crankcase, the motor is not top-heavy.
The sole problem, as stated, is getting somethinglike an acceptable combustion chamber shape. Side-valve motorcycleengines produced after the mid-1920s were long-stroke, torquey,slogging motors that ran forever due to their low-revving nature, bornout of low compression ratios and relatively poor combustion.Competition engines they most definitely were not.
We explained that Harley Davidson was theexception in dogging away with performance versions of its famousV-twin long after side-valves became unfashionable. Believe it or notbut Harley was still racing at factory level with its KR750 side-valveengine right up into the early 1970s. The special, Dick O'Brien-builtmotors, in the hands of Cal Rayborn, won the famous Daytona 100 miler(road-race) in 1968 and ‘69 at an average speed of 100mph andtopped-out at around 150mph. The 750cc V-twin engine knocked-out around80bhp at 7,500 rpm. Rough maths shows that a 300cc single should haveno trouble putting out 30bhp - more than sufficient for a trialsengine.
To achieve this kind of performance from thevenerable Harley meant some very radical valve-timing with massivevalve overlap - not good for emissions, I'm afraid - and a radicalwedge-shaped combustion chamber gouged and chiselled into the head.
However, 40-years has elapsed since Harley'sside-valve was finally retired but with the massive advances inmaterials and technology in that time who's to say that with somemodern computer-aided design trickery, clever electronic ignition andeven more sophisticated electronic fuel injection and maybe a radicalcylinder head design, the flat-head can't make a comeback?
Given that the design sports some very worthwhilebenefits - low overall height and minimal weight gain being the mostobvious - it almost seems surprising that the major Japanesemanufacturers haven't had at least a look at the side-valve, given whatit could potentially offer in design opportunities.
Then again, maybe they have, and simply not beenable to make it work. No-one outside the factories would ever get tohear about such a project. With their massive research and developmentfacilities they try all sorts of stuff and if it doesn't work, theproject simply disappears.
Wouldn't it be fun though if a tiny Spanishfactory, the equivalent of one man messing around in a shed, turnedengine design on its head (!) and revived the humble flat-head?
If they do, they better have the patents well tied down...
