Project Aprilia SR50: 100 mpg Madness Part 1, Basic Bolt on Stuff

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The lean burn condition helps give the SR50 exceptional fuel economy, our bike returning an average of 115 mpg. Emissions are really low, this is one two stroke with absolutely no smoke and no smell. The orbital engine uses ECU controlled electronically metered oil injection and burns about the same amount of oil as a 4 stroke. Considering that it displaces only 50cc, the engine has a lot of power in stock form. Even though this scooter is very high tech, it is brimming with performance potential and is a very high quality and solidly made scooter. The Aprilia SR50 is considered to be the Ferrari or Ducati of small displacement scooters.

The travel limiting tabs on the restrictor ring held to the variator by the three philips screws must be bent upward to allow the backing plate to raise all the way, allowing the CVT to get into its full high gear position.  A washer on the shaft on the other side of the variator that spaces it away from the face plate must also be removed to let the variator and face plate come closer together so the drive belt can ride higher in the high gear position.  This lets your bike go about 5 mph faster!

Our Aprilia featured good hydraulic disc brakes on both wheels and decent real working hydraulically damped suspension stock with quality solid construction everywhere. The SR50 has near motorcycle handling and braking. Your typical Taiwanese or Chinese scooter isn’t even close in quality to the Aprilia. In imported form the SR50 is restricted to around 35 mph by the computer software, this is so it can be registered as a moped in certain states and can be ridden by 16 year olds in Europe. So for the first stage of modification, we took Project SR50 to our local Aprilia dealer to derestrict it. This consists of the reflashing the ECU to get rid of the speed limiter and retuning the CVT transmission by bending some back plate limiter stops in the variator, and removing a washer that keeps the belt on the CVT from moving all the way to the higher ratio position on the pulley.

Remove the restrictor ring and the backplate lifts off to reveal the rollers.  The stock rollers weigh about 11 grams and we replaced them with 6.8 gram rollers.  As the variator spins, the rollers roll up the ramps in the variator, pushing the back plate up.  This moves the variator closer to the face plate and forces the drive belt higher on the face plate increasing the gear ratio.  Lighter rollers delay upshifting by not rolling up the ramps until higher rpm is reached.  This keeps the CVT in a lower gear ratio longer, improving acceleration.  Changing roller weights is one of the easiest ways to tune a scooter transmission.

Derestricted our SR50 was tons better. It had better acceleration and driveability. The top speed was now easily around 47 mph and it could occasionally hit 50 mph under ideal conditions. Of course we were never ones to leave stuff alone so we set out to improve things. In our search for answers we found www.apriliaforum.com. This forum is sponsored by AF1 racing, a hardcore performance oriented Aprilia dealer. This forum has a very active SR50 area full of very knowledgeable people. Our research lead us to our first steps in modding Project SR50.  The general consensus among the forum members is that the Leo Vince ZX pipe is the best pipe for a stock or near stock engine.

We bought a Leo Vince ZX expansion chamber from AF1.  AF1 has dyno tested the ZX pipe and found it to produce an honest one hp gain over stock.  This is nearly a 20% difference, a huge gain for just an exhaust. The Leo Vince pipe comes with restrictors to make it legal for 16 year olds to ride in Europe with the pipe as a fashion statement and these must be removed for the pipe to actually work as intended. This involves grinding some spot welds and a little clean up work with a die grinder. AF1 sells the pipe in derestricted form saving us some work.

The Leo Vince expansion chamber is a lot lighter than the stock exhaust and is tuned for more top end power.  It is beautifully made and has a cool carbon kevlar silencer.