The point of the water-cooled GT300 Rotax motor – Eden explained – was to build a Lambretta engine that would hold 80mph for hour after hour. Even in my short stint on the motorway it was clear after a couple of junctions that Gran Turismo had hit their target. What’s more, they’ve done it with an engine that feels civilised, despite its obvious potency.
This isn’t my first go on Eden’s GT300. In early 2020 I managed to grab his GP for a day as part of a feature for my (long-awaited) Complete Spanner’s Lambretta Kit Book. You should expect that book early this summer by the way…
The GT300 is based on one cylinder from a Rotax 580 twin. At the time the engine was wearing Gran Turismo’s first development exhaust for the motor. It was built by Charlie Edmonds of Performance Tuning and looked visually similar to his design for the Super Monza. In that configuration Eden’s scooter almost caught me out. As I left his house in first gear it hit 5,500rpm and the front wheel suddenly went skywards. Beyond the sudden kick in the pants was a broad spread of power that made the GT300 great fun; even with a 4-speed gearbox.
Do you want to pull out to overtake a lorry climbing a hill and accelerate up to 80mph in one gear? Not a problem.
What was a problem, it turned out, was exhaust supply. As a one-man-band with lots of tuning work to do, Charlie Edmonds is not set up for large production runs of exhausts. This forced Richard Taylor of Gran Turismo to look at other more affordable options.
Next stop for a production GT300 pipe was Lancashire-based exhaust specialists PM Tuning. The resulting development resembles their Fat Mamba pipe but differs in many details. It obviously needed a special 4-stud flange to fit the Rotax cylinder, but there’s also a special tapered downpipe made from pressed steel halves. Even the traditional PM Tuning muffler has been modified. What is more, the muffler now has a strap and dedicated support bracket (hallelujah!) though it is only supported by two chaincase studs (boo!).
While Eden was waiting for the PM exhaust, he also took the opportunity to fit an AF Rayspeed Clo5e 5-speed box and a massive 22-tooth front sprocket giving 3.82 : 1 final drive ratio. That translates to 80mph at a smidge over 6,000rpm in 5th gear.
What does it ride like?
The first thing I noticed about Eden’s engine was that it wasn’t hard to kick over. It’s hard to know whether the ease of starting is intrinsic to the GT300 or simply due to the top end being worn due to miles of testing*; including a trip to Euro-Lambretta Poland. It’s already done more hard riding than Katie Price.
The second observation is how good it sounds with the PM exhaust fitted. Together with the open-mouthed carb breathing through a hole in the kickstart-side panel, the sound is a traditional 2-stroke symphony of induction and exhaust. It’s great without being intrusively loud.
*it turns out that Eden’s engine was well and truly worn out (it was a used top end when first fitted) so the PM Tuning exhaust graph is actually taken from a customer’s engine.
On the road the acceleration was swift and effortless, without the sudden kick in the pants that came with the Performance Tuning exhaust. You can feel that the PM exhaust sacrifices some peak power and torque at higher revs in favour of a beefier low-range. In my opinion, it’s a safer and more practical ride but at the cost of some excitement.
What more than makes up for the loss of power is the addition of an extra gear. There’s no waiting around for the engine to get to the power because there’s always a cog to suit. It pulls cleanly in top gear from very low revs but if you want to accelerate more swiftly, or hit a hill, you can always drop a gear and the GT300 lifts up its petticoat to reveal a clean pair of trainers.
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On the motorway it pulls 80mph in 5th gear with ease, both into and out of the wind. You don’t really want to ‘cruise’ any faster than that on a Lambretta without some form of windscreen because it’s actually quite tiring to hold on with that much wind-blast on your torso.
With a steady temperature on the toolbox-mounted Aprilia RS125 gauge, and a relatively low level of vibration, the scooter felt like it would comfortably hold this pace until it ran out of fuel.
The only thing that was clearly struggling to cope was the BGM 6-plate clutch; running a mixed combination of medium and strong clutch springs. For a motor as torquey as this, a full set of strong springs would have been better. By that stage the lever-action gets stiff; making either the easy-pull CasaCover side casing a good idea, or Chiselspeed’s rack-and-pinion conversion for standard side covers.
How much is enough?
For the vast majority of riders on 10-inch tyres, this power level – around 32hp – is certainly enough. Once you get beyond these sorts of figures, peak power becomes less important than the way it’s delivered.
With a tyre contact patch about the size of a thumbnail and a massive rear weight bias it doesn’t pay to have sudden steps in a scooter’s power delivery. In the dry that means unintended wheelies and in the wet it means uninvited wheelspin.
With the PM Tuning exhaust, the GT300 feels like a good compromise. It is undemanding to ride, almost like a 4-stroke. Fast but smooth: capable of lolloping along in low revs at high speeds. The big advantage of this exhaust’s boost in power at 5,000rpm is when it comes to riding with slower scooters or around town. It takes that in its stride.
The problem with the GT300 is not one of the engine itself, but of human nature. Once you’ve got used to this, then you’ll want a bit more; particularly if you have a friend with an even faster scooter. After the dyno test Eden was already talking about getting the grinder out to add some more exhaust timing to the otherwise standard Rotax cylinder.
So, in answer to the question, for some people, enough is never enough…
The graph above shows a GT300 with the PM Tuning exhaust (blue) vs the Performance Tuning version (red) for this engine. The dotted curves are torque, measured against the right hand scale.
This graph shows Eden’s own GT300 and PM Tuning exhaust combination in a graph of power and torque vs speed. This is a ‘through the gears’ run with deliberately slow and careful gearchanges. Each of the five ‘lumps’ represents one gear of the AF Clo5e 5-speed gearbox. There’s never time to transfer full power to the dyno in 1st gear, hence the low reading in that gear.
The GT300 is based around a specially-modified set of GT engine casings which house a dedicated 60mm-wide GT crank with a stroke of 64mm. The cylinder for this one is a non-powervalve Rotax/Bombardier from a Seadoo twin-cylinder watercraft with a 76mm piston giving 290.3cc on the standard bore size. There are also later versions of this cylinder which feature a pneumatically-controlled Guillotine power-valve.
The wider crankshaft used for the GT300 forces some compromises in the transmission because the front sprocket and chain are positioned outwards by 5mm. This means that a special rear sprocket and chain tensioner are needed in order to make everything line up again.
The special sprocket used still follows the Lambretta plate layout, so the engine will accept any Lambretta-based clutch solution; such as the BGM Superstrong fitted to the development motor.
The reed inlet is fed directly into the crankcase between the crank-webs using a slightly oversized 4-petal reed block similar to that used on the GT kits.
The flywheel side of the casing is unique, with a special mag housing to both accommodate the larger crankshaft and external-pick-up Aprilia ignition. This mag housing needs no fan, so it is finished with an anodised billet aluminium flywheel cover.
The Aprilia ignition features a pre-set advance-retard map and produces 180 Watts of electrical output; which has so far proved ample for permanently running an electric water-pump.
The theoretical advantage of water-cooled engines like this is an improvement in thermal stability, reducing the risk of heat-seizures while also allowing closer piston to bore clearances. This means the engine can feel smoother.
To know whether water-cooling is worthwhile you simply have to look at all the cars and motorcycles produced today. Liquid is very obviously a superior cooling system to air but only when it works 100% reliably. In production engines this is almost always with a mechanical water-pump driven by the engine.
On the GT300 the water-pump is electric, powered by a battery which is charged by a highly reliable Aprilia RS125 ignition system. All the components are good quality and have proved themselves in the 6,000 miles that Eden has so far done in development.
Equally, with vibration that is comparable to a normal Lambretta, you are only one broken wire or failed battery away from a non-functioning cooling system. As such, with any water-cooled motor running an electrically-pumped system, you’ll need to keep an eye on the temperature gauge while riding.
The other compromise forced by water-cooling is finding somewhere for a radiator on a Lambretta that doesn’t completely spoil the lines of the machine.
Eden’s initial rig had the radiator mounted on the outside of the legshields but the production solution has been to mount the radiator in a fabricated aluminium belly pan under the floor of the scooter. It is the same solution eventually chosen for the water-cooled Scomadi models too. This folded belly pan is aesthetically less disturbing and provides a mount for the water-pump. From an aerodynamic perspective the belly pan does cause some additional drag at high speeds but it also improves the stability of the scooter at speed. Everything is a compromise.
What was missing from a conventional water-cooling perspective on Eden’s scooter, was any sort of fan to keep the engine cool while the radiator is not experiencing air-flow; such as sitting in traffic. Eden found on the trip to Poland that the engine temperature would rise near to maximum in traffic but never boiled-over.
For the system to be totally practical in every condition, I’d prefer to have a fan to draw air through the radiator when the scooter isn’t moving.
VIDEO | Road and dyno...
Gran Turismo’s Richard Taylor has done his best to keep this conversion economical by permitting the use of used but high-quality components. The Rotax cylinders are plentiful in used condition. Aprilia RS125s abused by teenagers lie forlorn in breakers yards waiting to donate their water cooling and electrical organs to a good cause.
In the end there are so many other specific parts required for the conversion that savings on the cylinder and ignition are soon swallowed up elsewhere.
The estimated price of all the parts to build a complete engine based on a non-powervalve Rotax 580 cylinder is around £6,500, not including labour.
This price includes:
- Special rear sprocket
- Reconditioned Rotax top end
- Aluminium cylinder head and temperature sensor
- Custom wiring loom
- Special fabricated mounts for the electrical components
- Dedicated PM Tuning exhaust system
To fit this conversion you may also need the following items:
- Special pressed aluminium belly pan and LF Brothers electric pump (P.O.A)
- A selection of water hoses and connectors (approx. £100)
- Specific GT300 fuel tank – offered direct by Oiltek (P.O.A)
- Donor Aprilia RS125 ignition, clocks and cooling components
- Ignition switch
If you want a Lambretta engine that works both as a motorway mile-munching rally machine and a relatively civilised round town ride then the GT300 fits the bill, but there are provisos.
There are clear advantages to some of the component choices in this engine. The cast iron replaceable cylinder liner and off the shelf piston mean that the kit potentially has more oversizes than Eamonn Holmes’ wardrobe. Similarly, the Aprilia RS125 ignition is cheap to replace and second-hand is still probably more reliable than half of the new Lambretta ignitions on sale today.
That said; the GT300 recipe also introduces new compromises such as the need for a unique rear sprocket every time you wear one out or want to change sizes. The off-the-shelf clutch solution used on the test scooter was clearly at its limits. Richard Taylor has drawn up an 8-plate clutch conversion that might well go into production, but even with proprietary plates it still won’t be cheap.
My main reservation is that this is not an engine or scooter build for the ambitious bodger. For water-cooling to be any more reliable than simpler air-cooled alternatives then your electrical and plumbing work needs to be pristine. Greater complexity actually provides more opportunity for things to go wrong so ten-thumbed technophobes are better off sticking with the hairdryer principle.
Coincidentally, the air-cooled GT conversion using a Simonini microlight cylinder (like the KillerCase and KR Automation engines) has been running and on the dyno at PM Tuning made peak power not far shy of the GT300 Rotax. That’s the next engine on my test-ride list…
SPEC AS TESTED
- Casing: Sand cast GT Intercontinental machined for Rotax cylinder and wide crankshaft. Crankcase reed induction using oversized version of the shallow GT reedblock. Special CNC mag housing and aluminium ignition cover
- Crankshaft: 64mm stroke GT ‘Monster’ crank with oversized 60mm-wide crankwebs and flywheel taper to suit Aprilia RS125 flywheel. YZ250 125mm rod
- Cylinder: From a Rotax 580 twin. Aluminium water-cooled with a reboreable cast iron liner
- Head: Aluminium GT water-cooled head with centre plug
- Ignition: Used – from Aprilia RS125
- Clutch: BGM Superstrong with special GT offset 47T crownwheel
- Gearbox: AF Rayspeed Clo5e with optional taller 34T 5th gear
- Radiator: Aprilia RS125 mounted in specially-constructed GT aluminium belly pan
- Electrics: Full RS125 ignition and gauges connected with custom-made GT wiring loom mounted on fabricated steel brackets. Electric water pump
- Carb: Dellorto VHSB 34
- Exhaust: PM Tuning for GT300
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