Project Aprilia SR50

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!

Malossi big bore kit cylinder for aprilia SR50
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.

Malossi vs stock Aprilia SR50 cylinders
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.


Malossi big bore piston for aprilia SR50
The Malossi big bore piston is on the left is obviously a lot bigger than the stocker on the right.
Aprilia SR50 Malossi big bore piston
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.

Malossi big bore cylinder head for aprilia SR50
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.

Micah Shoemaker porterd Aprilia SR50 big bore cylinder

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 Micah Shoemaker porting of Aprilia SR50 big bore cylinder
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.
AF1 Micah Shoemaker porting of Aprilia SR50 big bore cylinder
The exhaust port is also subjected to the same chamfering, straightening and deburring.


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Tuesday, December 29, 2009 2:49 AM
This is such a cool project. I think bike modification is a radical project in our current society (obviously US society). It really is unheard of, while 'mor powa' is the craze around my parts. Everyone is all about the 1/4 mile time, what a drag.....pun intended.
Wednesday, December 30, 2009 11:17 AM
With all the respect, while to many this type of tuning is "unheard of".....Malossi, Polini, Proma, Gianelli have been in the business for God knows how long.
I used to mess around with my moped and scooter in my early teens
For those of you that are interested, look up Piaggio Ciao and Piaggio Si, the most tuned moped of all time. Sort of the European version of the US trend Civic EF/EG/EK...with the B-Series swap....great stuff for sure, but nothing new....
Again, no offenses.
Mike Kojima
Mike Kojimalink
Thursday, December 31, 2009 7:02 AM
Scooter tuning is old hat in most of the world, we are trying to introduce it to the USA. Its pretty fun and practical as well.
Friday, January 01, 2010 8:34 PM
I guess...it's always fun to learn something new. I eventually grew out of it, and moved into cars....but I can see where you come from.
On a side note: the old school Piaggio Vespa had always been a great platform for tuning as well.
Sunday, January 10, 2010 1:24 PM
not to mention how economical it is to tune, when was the last time you saw a non-knock-off exhaust for around $100?
Sunday, January 10, 2010 9:15 PM
thanks for the avatar tip!
Mike Kojima
Mike Kojimalink
Monday, January 11, 2010 4:51 AM
What kind of bike is that? Its not a Monkey but looks sorta like a modded one.
Monday, January 11, 2010 1:13 PM
1977 GS 550 E, Bought it last year, finally got it round the block last month. The most expensive single component was the $300 license plate.
Mike Kojima
Mike Kojimalink
Monday, January 11, 2010 5:45 PM
Looks sweet! We will have a 1978 RD400 project sometime this year.
Thursday, December 02, 2010 6:27 AM
Mike, great job on the Aprilia project. I was happy to see this because I plan to get a new SR50 next spring and I definitely want to upgrade it. Your project gives me valuable information to get the performance I want. Any other tips you have are appreciated. I am not a mechanic and will probably need help duplicating your effort. Would a local Aprilia dealer do this for me? Do I buy a manual and try to do this myself? Thanks!
Mike Kojima
Mike Kojimalink
Thursday, December 02, 2010 7:07 AM
Scooters are pretty simple but I would suggest joining www.apriliaforums.com They have a good bunch in the SR50 section that is helpful.
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