Gran Turismo Lambretta Rotax GT300 prototype | FEATURE
The last few years in the Lambretta tuning world have changed everything. New ideas, new technologies and new capacities. BSG broke the 300 barrier for a small-batch single cylinder Lambretta engine made from scratch.
Now Richard Taylor of Gran Turismo is initiating testing of his 290cc GT300 engine which potentially can be expanded to a whopping 380cc.
What is it?
You’ve read about the GT engine casings on SLUK before. Along with the CasaCase they are highly-adaptable engine casings that can be used as a direct replacement for a normal Lambretta engine, but they can also take anything up to 72mm stroke cranks and a massive variety of potential cylinder conversions.
The GT300 is Richard’s solution to producing a reliable big-bore engine for a Lambretta as affordably as possible. Not so much about the biggest numbers but about good value for money.
What is the cylinder?
The barrel fitted to the running prototype engine is from a Bombardier Sea-Doo 580cc or 587cc watercraft, but similar barrels were used by Austrian 2-stroke specialists Rotax for many different applications.
Richard chose this cylinder as a way of making a relatively affordable big-bore water-cooled Lambretta engine because the barrels – which were made in the mid-90s – are easy and cheap to find on eBay second-hand.
Other advantages include:
- Removable, replaceable and reboreable cast iron liners
- Relatively small gasket area easily accommodated in GT engine casing
- Crankcase reed inlet easily accommodated in GT engine casing
- Simple, single-exhaust port layout with no exhaust bridge
Beyond this there are even bigger Rotax cylinders that are theoretically capable of being shoehorned in, like that of the 787cc Sea-Doo twin. One of these barrels could make a 380cc Lambretta single, but it would require a different angle on the crankcase mouth to angle the barrel down enough to clear the frame (as was done by engineer Chris Sadd on the DACTek Lambretta engines).
The advantage of the 787 barrel is a built-in guillotine-type power-valve, offering the potential of far more low-rpm grunt.
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What’s the target?
Richard’s aim with this motor is a not unreasonable 35hp output, but easy to ride and reliable for long distances.
You can make similar outputs with tuned air-cooled Lambrettas and the right exhaust, but air-cooled motors simply do not have the same thermal stability or safety margin.
The advantage of using a production cylinder with a cast iron liner is that not only are the barrels cheap to buy, but they are also easy to fix and rebore when they go wrong or wear out.
These cylinders were not designed with ultimate power in mind, but for bullet-proof reliability in arduous conditions like at sea, in the snow or even in the air. In all those conditions, reliability and simplicity go hand in hand.
Where are the compromises?
In theory a water-cooled engine is far preferable for motorway use than air-cooled, but the main compromise is not so much the cost but the complexity of a water-cooled conversion to an air-cooled scooter.
In addition to having to find somewhere to put the radiators, plumbing and 12-volt water pump, there’s also the need for a dedicated wiring loom and an ignition system to reliably power it. The prototype has one of Richard’s Aprilia RS125 ignition conversions fitted, which offers plenty of electrical power output.
The main compromise on this test scooter is the exhaust – a reworked JL3 with an extended downpipe. This was chosen more to have an easy test pipe that fits a Lambretta than one that suits that cylinder configuration. Not only is it a limiting factor on performance but it also sits very close to the ground. At the moment Richard is considering various options to develop a more suitable exhaust system.
What is the cost?
How long is a piece of string? The objective is clearly to produce this engine cost effectively, at least relative to exotica like the BSG305 and the DACtek, but at the moment Richard is too early in the development phase to be able to offer anything concrete in terms of dates or costs.
What does it ride like?
Now we come to the exciting part. It’s difficult not to be impressed with how much Richard has achieved, largely on his own, in the design and production of so many parts for Lambretta.
As such, it’s something of an honour to be the first person to give it a short blast up the road. It would only ever be a short test given that the radiator was tucked behind full legshields, but it would at least give me a taste.
The engine takes a good kick to get going. The Aprilia flywheel is relatively low inertia so you need to be determined about who is the boss; you or the kickstart pedal. If Richard succeeds in making a 370cc version then I think some form of decompressor may be required in order to prevent broken kickstart shafts and lightweight GT370 owners being found in trees.
To be frank, this was a very short test on a residential road, so there was no chance to really open the 290 up. In the current configuration the engine felt strongest from 6,000-7,000rpm but probably still shy of Richard’s target of around 35hp. I see no reason why this engine can’t reach that target given the right components, but there’s still a ton of work to do in ignition timing, jetting and mostly in exhaust design.
VIDEO | Richard Taylor explains about the GT300
Does the world need a GT300?
Power only makes people greedy for more power, so naturally there’s an obsession with having the fastest Lambretta in the land. If that’s what you want then the GT300 will never be the answer. For ultimate performance you’d be better off fitting a much more modern motorcycle cylinder to a set of GT engine casings.
Meanwhile, in the real world, 35hp is probably a sensible figure that can actually be used in anger on a scooter, but how that power is delivered is vitally important when 10-inch tyres are involved.
Where the GT300 potentially scores will be in having a powerful engine for a Lambretta with a cylinder design that is rugged and simple enough for life in the fast lane. Ultimately though, the reliability of one of these engines now rests not with the proven cylinder but on ancillaries like the water pump, ignition, crankshaft and exhaust design.
If Richard wants to supply complete GT300 engines rather than a kit of parts then he’s still at the bottom of a very steep hill. But if any one person in the scooter scene can see this project to the end largely unassisted, then Richard is the man…
If you would like to know more about GT product or to buy them, then you can find all the parts via Eden’s Lambretta Images Archive Gran Turismo page. Alternatively they can be purchased from Disco Dez Scooters.