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

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On a two stroke an expansion chamber is an exhaust pipe that is acoustically tuned like a musical instrument to use the reflected wave of the exhaust pulse to create a rarefaction or negative pressure in the head pipe of the expansion chamber during the period where the transfer ports and the exhaust port are both open at the same time. This helps scavenge the cylinder. Right before the exhaust port closes, another reflected wave creates positive pressure to push excess fuel air mixture that was drawn into the head pipe back into the cylinder as well. An expansion chamber can greatly boost the power of a two stroke engine in the rpm period that it’s tuned to work.

Leo Vince Aprilia SR50 exhaust
The Leo Vince exhaust on the bike.  If you don’t like chrome, the pipe is also available in a raw moto finish which looks cool but also must be oiled frequently so it doesn’t rust.

When changing the pipe, we switched the weights in the CVT transmission variator from the stock 11 grams to 6.8 grams. A lighter weight in the variator slows upshifting so the bike can accelerate faster and the engine will stay in its powerband better. This complements the slightly higher and narrower powerband created by the tuned Leo Vince pipe.

Aprilia SR50 stock intake silencer
This is the stock intake restrictor, silencer, snorkus is an elaborate part over two feet long.  We thought we could gain some power for sure by removing it because it looks really restrictive but it didn’t make much difference other than an improvement in throttle response.  The intake noise is slightly louder.  Removing this probably makes a bigger gain on a more highly modified engine.  The snorkus is removed by simply pulling it hard out of the airbox.
Arilia SR50 air box porting
The intake hole in the stock airbox  was pretty small looking when we removed the snorkus.  We ported it out but it didn’t do much either.

The combination of the Leo Vince pipe and the lighter variator roller weights made a big difference in the SR50’s driveabilty. Our little Aprilia could now zoom to 50 mph repeatedly and was peppy enough to cut through traffic on major streets. However we still wanted more and to see what other bolt on mods could do. Our next step was to modify the stock air box by removing the intake silencer and opening up the opening of the air box. Some people on Aprilia forums had had luck doing this freebie mod so we tried it but we didn’t feel much difference in performance.

Malossi high flow air filter
The Malossi high flow air filter didn’t seem to help much at this stage of modification

Next we added a replacement free flowing air filter element from Malossi, a high quality European scooter tuner.  AF1 carries this part. Think of Malossi as the HKS of the scooter world. We got this part mail order from Scooter Center in Europe. With cheap international shipping and online stores, it is now really easy to source parts from around the world. Like the airbox mod, we could not easily discern much difference in performance. Looking at other stuff that people on the forum tried, we next opened up our throttle body. The stock throttle body has a restrictor of sorts in the intake side of the throttle body. This restrictor is around 17mm in diameter. We simply bored this out to match the rest of the throttle bodie’s 20mm internal diameter. With our stock 50cc engine, we didn’t think that this did much either although the throttle response got snappier. However on a built big bore motor, these mods probably help.

Aprilia SR50 throttle body
big bore aprilia SR50 throttle body
The larger bell mouthed side of the SR50 throttle body as shown in the upper picture is the inlet.  The outlet side is the lower picture.  The 17mm inlet restrictor is right after the throttle valve and we bored it out on a drill press to 20mm, the same size as the inlet.  Although it appears to be a huge restriction, we didn’t notice much gain with the stock motor.  Sorry no before pictures, we forgot to bring the camera to the shop that day!  We think that this mod probably helps a more extensively built engine for sure.

 

4 comments

  1. This is awesome, I’ve been wanting to own one since 2010, and I just had a chance to own one, now looking at your build and reading about it, wants me to do that to my sr50… thanks for the right up

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