Project Aprilia SR50: 100 MPG Madness Part 2, Engine Internals

Project Aprilia SR50: 100 MPG Madness Part 2, Engine Internals

By Mike Kojima

In our last edition of Project SR50, we had reached the limits of simple bolt on modifications which left us with a nicely running Scooter, capable of just over 50 mph with acceleration good enough to keep up with traffic and 115 mpg. With more time, we are really taking a liking to the refined Aprilia SR50, which rides, handles and brakes better than any scooter in this displacement category. We also like its sporty looks. The SR50 is the king of urban commuter scooters and we are going to take it to the next level.

See Part 1 Here!

The Malossi big bore cylinder has a 47mm bore as opposed to the stock 40mm.  The water jackets are also much bigger as are the ports.

Our next goal was to make a big difference in our scooters ability, we opted to install a Malossi big bore kit. If you don’t know, Malossi is like the HKS of the scooter world. They make very high quality racing components with exceptional engineering. The quality and engineering of most Malossi components is done to near OEM levels. Malossi cylinder kits can be purchased from High-Gain Tuning in Colorado.

The difference in water jacketing, bore and port size is very apparent when you compare the stock Left and Malossi Right cylinders.  You can see the hole to feed the boost ports on the Malossi cylinder.

The Malossi big bore piston is on the left is obviously a lot bigger than the stocker on the right.

The Malossi piston has a big window to feed an additional boost port to improve cylinder scavenging.  The lustrous sheen is from the friction reducing, wear increasing, WPC process we used on the piston, rings, pin and cylinder.

The complete Malossi kit includes a cylinder, cylinder head, piston, pin, rings and a tuned ECU. The Malossi piston is 47mm in diameter up from the stock 40mm increasing displacement from 49cc to 68cc. The Malossi kit also bumps the compression ratio up from the stock 11.5:1 to 12.6:1. The Malossi cylinder is iron like the stock cylinder but has significantly larger cooling jackets and much larger ports.

The Malossi cylinder head has much more generous water jacketing around the combustion chamber.  The smaller combustion volume bumps the compression ratio to 12.6:1 up from the stock 11.5:1.

We sent our cylinder to Aprilia guru and AF1’s head tuner Micah Shoemaker for some mild porting. As the high tech direct injection stratified charge Ditech Orbital engine’s full workings is not entirely understood by us, Micah recommended some very mild porting. He deburred, blue printed and squared up the ports only removing a minimal amount of material. Never the less, as is the case with small two strokes, this clean up amounted to about a 15% increase in port window size and around a 10% increase in port timing. Without direct control of the engine mapping and no experience with Orbital tuning, this is as far as we dared to go in our first round of testing. In Micah’s experience, additional porting did not produce measurable results.

AF1's porting consists of mostly a cleanup, matching all of the transfer port and boost ports height, chamfering with minor straightening of the windows.  Even thous this is seemingly mild, it increases the port window area by about 15% and the port timing by 10% on this small engine.  The lustrous look is WPC treatment.

The exhaust port is also subjected to the same chamfering, straightening and deburring.