I spent much of this weekend further hot-rodding my Aprilia RS50 scooter. Progress to date:

Back in April of this year, I replaced the cracked stock (iron 49cc) cylinder with an aftermarket aluminum 80cc cylinder, piston, pin, and head assembly. This added immense more power over the stock output; no longer did I need to feather the clutch when starting off - the stock engine died if I let the clutch out too quickly - and it gained torque and horsepower across the RPM spectrum. However, the new engine was limited by the geared-for-acceleration sprockets and teeny-tiny carburetor.

For your reference, here is how a high-performance two-stroke engine works:

Click the image to see the wiki article.

Here's what I did to my scoot this weekend:

• Replaced the sprockets. Stock is 12 teeth front and 47 teeth rear, resulting in a theoretical top speed of 62 mph at 12,000 RPM. My new sprockets are 15:43, with a theoretical top speed of 85 mph. Also, the rear sprocket is now aluminum, meaning shorter life but a lot less unsprung weight for quicker acceleration. I didn't replace the chain (it only has about 3000 miles, and the sprockets looked nearly new), but I did clean the grime out of it and thoroughly lube it.


• Replaced the stock carburetor. The stock carb has a tiny 12mm bore, and it also features a water-heated passageway to make it easier to start in cold weather while also reducing the intake air-mass. These features limit power in two ways, reducing flow and reducing the amount of fuel/air mix entering the engine. My new carb is a 24mm Mikuni unit. That's twice the diameter, resulting in something like four times the airflow. And sans heated intake, it moves a lot denser fuel/air charge.


• Clearly, the stock airbox with its tiny snorkel would have strangled this new carb, so I removed that and simply capped the Mikuni with a free-breathing, washable, pod-type filter.


• Next in line bringing fuel/air mix into the engine is the reed cage (that's the little rectangular flapper device between carb and engine in the graphic above). In a two-stroke engine, either a rotary valve (as in most Vespas) or a reed cage allows the intake charge into the engine at the right moment. My new cage uses bigger reeds that open further to let in much more intake charge. The reed petals are also made out of carbon fiber, which is far lighter than the stock metal petals, so they open and close much faster for better intake timing.


• I also did some other needed work, like replacing a cracked blinker lens and cleaning every part of the machine that is normally covered with body panels and the like. I must have spent several hours just cleaning and Armor-All-ing the plastic.

So, how did it work? Here are my top speeds so far:

Stock: In stock trim, this bike will do about 45 mph.
Stage One: Otherwise stock, the 80cc cylinder kit brought top speed to about 60 mph.
Stage Two: With new gearing, new carburetor, new reeds, and new air filter, I hit an indicated 76 mph at just under 11,000 RPM. Top safe RPM on this engine is 12,000 RPM, so the theoretical top speed of 85 mph is probably attainable with this new configuration.

Stage Three will include a new exhaust chamber (exhaust resonance makes a huge difference in two-stroke engine tuning). Stage Four: Back to 49cc cylinder kit (for the land-speed-record run) with nitrous injection! With the addition of a racing exhaust chamber and nitrous injection to the current intake and gearing setup, I hope to boost the little 49cc engine back up to its 80cc performance, meaning the 49cc stock-body/power-adder record should be mine!

However, I have to admit that the biggest barrier to higher speeds right now is me. I'm six feet tall and rather broad across the shoulders. With my helmet tucked behind the windscreen and resting on the gas tank, my elbows pressed against the middle of my thighs. Also, I weigh about 180-185 pounds, about as much as the scooter, itself. When it comes time for the real top-speed run in 49cc configuration, I think I'll ask my friend, Alex, to do the run for me. He is probably several inches and 40 pounds lighter, which means he'll get up to speed quicker and be able to reach a higher top speed. *sigh*

More news as I get it!

Best,
Chris

I've always wanted to participate in the land-speed-record runs - to have an excuse to watch 'em - but I don't really feel like spending huge amounts and running at ludicrous speeds. So I thought, "Huh, I wonder what the 50cc class looks like?" Turns out it's only 67mph.

P'shah! I can do better than that! My 50cc scooter (when it was only 49cc, that is) was able to run nearly 50mph in stock trim... what would a turbo (or nitrous) do for that? Well, nothing responds as well to turbocharging as a 2-stroke engine, usually boosting power by 50% (though absolutely ruining fuel economy... as if that's a concern for a land-speed-record scooter!).

So I've been looking at eBay, and it seems there are a lot of tiny turbos out there for under a hundred bucks. A turbo would need to have pressurized oil, something a 2-stroke does not supply, so I'd also need an electric oil pump to keep it spinning.

Alternately, I could pipe in a hit of nitrous using a teeny-tiny nozzle squirting into the intake manifold. Say, a 5-10 horsepower hit when the engine reaches top RPM. This would be a pretty simple thing to set up and less likely to explode the engine than turbocharging a high-compression engine (though doing so works pretty reliably on small-displacement engines like those in motorcycles). Nitrous has the added advantage that it's only "on" when turned on, as opposed to a turbo.

Sounds like fun, no? The nice thing about aiming for a 70mph speed record is that it's pretty simple to test it on public roads *g*

So, gearheads: Ideas?

Best,
Chris

So I have a motorcycle again! I didn't buy one, no; I spent the afternoon swapping out the exploded 49cc cylinder/piston/head unit of my Aprilia RS50 with an aftermarked 80cc unit. In essense, I now have a small motorcycle rather than a motorcycle-looking scooter. I just got back from a test-ride, and even taking it kind of easy during break-in, I learned a few things:

1) Wow, but does nearly doubling the displacement of an engine make a huge difference!
2) It's as fast as the cars around me off the line.
3) The low-end torque is the happiest discovery; I used to have to rev the engine over 6,000rpm to be able to take off without stalling. Now I can just let out the clutch at idle and it runs just fine. Wow!
4) Not sure of the top speed, but on a ride out along 31st St. (that's the one that goes out to Target, right?), I hit 50mph before I let off the throttle (saw a policeman). If I want to ride this bike on the highway, all I need is a new gearset, because it's going to over-rev before it runs out of speed. I bet this bike will go 80mph! Wow!

I wonder how much more I could get out of this thing with a nice exhaust, bigger carb, and better reeds (it's currently running all stock except for the new top end)....

Chris