Rotary Disc Valve Vespa and Lambretta Conversions for POWER | FEATURE

It’s all about the petals, flower

The reedvalve 2-stroke kit is so pervasive on the scooter scene that anyone would think that it was the most powerful inlet system available. In actual fact, it isn’t. Reedvalve is just easy to incorporate in various positions (crankcase or cylinder mounted) and flows “on demand” rather than at specified timings.

Reedvalve induction is also restrictive. Flexible reed petals must be lifted out of the way, like environmental protesters, before anything flows. Incoming gas must also slalom around the reed cage.

Take one look at the huge un-restricted inlet port that Jesco from SIP Scootershop built into his Vespa sprinter and you’ll see that there is another way. Rotary disc valve induction offers the advantages of a totally unrestricted inlet port together with accurately controlled asymmetric inlet timing.

VIDEO: Jesco demonstrates the internals of his disc-valve conversion

Engine specification:

Casing: Vespa Rally 180
Crankshaft: Mazzuchelli 60mm stroke with a 128mm Samarin conrod.
Clutch: SIP-BFA
Cylinder: Quattrini M232
Ignition: Selettra
Gearbox: Benelli
Exhaust: one off by Truls
Carburettor: VHSF 41,5 Dellorto

Doesn’t a Vespa already have disc valve induction?

From the 1960s onwards, yes, all the 2-stroke ones do. The Vespa system very cleverly uses the edge of the crankshaft web to open and close a small elongated port in the crankcase. It’s brilliant because it requires zero additional moving parts. The down-side of this layout is that the port is far from unobstructed. Even when fully open the port is partially blocked by bits of cutaway crankshaft.

In summary, the Vespa system is great for simplicity but poor for performance, hence why cylinder reedvalve kits usually perform so much better.

It doesn’t have to be that way

Disc valve systems work better with larger diameter discs. The traditional, and easiest, way to do this was to mount a thin disc on the ignition side of the crankshaft and to use that to open and close the inlet port just below one side of the cylinder.

Stealing speed

This layout really came of age with the East German MZ race engineer Walter Kaaden in the late 1950s. Behind the Iron curtain and on a very limited budget, MZ developed small capacity Grand Prix bikes that used modified circular saw blades to open and close the inlet port.

The MZ racers proved to be so simple and effective that Suzuki arranged the defection of Kaaden’s star racer and fellow engineer Ernst Degner in order to steal the secrets of the East German brand. This epic story of cold-war espionage is documented in the book Stealing Speed and more recently by a TV documentary called Missile from the East.

Suffice to say that Suzuki didn’t squander their investment, and the same disc-valve arrangement eventually found its way into the 4-cylinder 500 bike that took Barry Sheene to his Grand Prix championships.

Disc valve induction also powered the Aprilia 250cc bikes at the end of the 2-stroke Grand Prix era. It’s a system that clearly works well.

Suzuki Grand Prix parts were used on the Taffspeed disc-valve motor

Adapting to scooters

Several inventive scooter tuners have dabbled with the disc-valve formula. Steve Moore and Mick Hayman of Team Taffspeed built a disc-valve engine using parts from a Suzuki RG that used to belong to a certain Mr Sheene. At the time Mick worked for Heron Suzuki – the British Suzuki importers and race team – so he had access to both the technology and the parts from old Grand Prix bikes. Ian Frankland of Taffspeed later bought and re-tuned Mick’s disc valve engine.

Another successful application of the same idea came out of rival shop Kegra Racing in Southend, as applied to Graham Best’s Group 6 race Lambretta.

The 1980s Taffspeed disc valve Lambretta, fed by a large Amal carb under the footrest, and cooled by a scoop under the cylinder

Compromises

The main issue with adapting this system to a Vespa or Lambretta is that it takes up the space normally used for the stator, flywheel and fan. As a result the engine must take on another cooling method and be converted to a much smaller type of ignition. In the case of Jesco’s engine, he used a tiny self-generating Selettra ignition and relied simply on passive cooling for short sprint races. The disc-valve Lambrettas previously mentioned were both scoop-cooled.

This type of rotary disc valve arrangement is therefore about as practical for regular street use as a tissue-paper condom.

A belt-driven disc valve sprinter from BSG Corse, manufactured in 2012

Another Way

Of course you don’t necessarily need to mount the disc valve onto the crankshaft and feed the carb in at the side. If you can figure out a method to switch the drive for the disc through 90-degrees then the disc valve can be mounted on the back or front of the engine.

This change of drive angle can be achieved through bevel gears or more simply through a toothed belt that follows an angled path over a set of idler rollers. This is the layout used by BSG (and subsequently BFA) for their disc valve smallframe engines.

Future Shock

Perhaps the most ingenious development is a project by Team Sassin who have adapted the smallframe Vespa engine concept to include a bevel drive under the clutch cover to rotate a valve disc on the rear of the engine. This set-up could leave the flywheel side of the ignition untouched so fan-cooling and conventional ignitions are still a possibility. For more info see Team Sassin on Facebook.

Results

Jesco’s engine is the most powerful modified Piaggio Vespa largeframe casing that he knows of at 56hp on the SIP dyno. People have managed more, but usually with dedicated engine casings.

On the sprint track this output translates to a time of 6.62 seconds for 150 metres and an exit speed of 136 kmh (84.5mph). Bear in mind that a quarter-mile (like we run in the UK) is just over 400 metres; so Jesco is travelling at 85mph even before the half-way mark! He doesn’t know what it would do over a quarter mile but he’s keen to find out…