Success- Geared to 60 MPH: A 115mm Variator

[SEANIA]

Had to do modifications. There is no more starter at 115mm. Kickstart forever, or a drill.

The belt in the pic hasn't had the engine started/isn't tensioned.

Also swapped variator parts to let the belt ride further inward in the rear.






Had taken measurements and done the math for the belt size. Closest, within a few mm, is a Gates PL30302 belt. Is 757mm, from the wanted 755mm, up from the stock 743(? 745?) This is the size with the mod to let the belt ride in further. Without it a even larger belt would be needed, that, is a size is not made from what I can tell. "First Belt" makes a 777 16.5 30 which seems to work, but the short belt teeth mean you'll need your CVT ramp angle and spacing perfectly adjusted to not flip it.

All this should let the belt ride up to 113mm out of the 115mm. That's important though, as the variator face tops out at 113mm. This is up from the stock 85mm, combined with riding another few mm inward on the torque pulley, making the theoretical gearing limit 60mph.  Downside is the GY6 150 weights bottom out at 6 grams. Requiring a aftermarket bore to handle the rate it gears up at.

Next is to put a zuma minarelli horizontal bore on it (fits stock), and to modify a exhaust from one to tilt up 15° to compensate for that tilt on the Kymco bore direction. A 2Fast cylinder anyone? 20+hp and liquid cooling here we come.

[CROSSBOLT]

50 cc = 3 cubic inches. 20 hp / 3 inches = 6.666666 hp/cu.in. which is WAY beyond any carbed hot rod achievement to date.

[SEANIA]

50 cc = 3 cubic inches. 20 hp / 3 inches = 6.666666 hp/cu.in. which is WAY beyond any carbed hot rod achievement to date.

I don't know what a "carbed hot hod achievement" is, but google isn't returning anything usefull.

20hp out of a 50cc 2 stroke isn't unrealistic. even 20hp out of a 50cc 4 stroke is possible, just, much harder to pull off. 20hp-25hp-ish is the average range for race grade 50cc 2 stroke bikes. This isn't some unheard of mathematically impossible thing either. It's fairly common for anything designed for it. That's without using exotic tech too, and without a turbo.

If you want to use a turbo- put 15 pounds of boost onto a yamaha YZ125, and with no other modifications other then tuning the fuel intake to keep up with it- it will hit 50hp+.

"If it can why haven't I heard of larger engines doing that" because they're larger. The longer the stroke and wider the piston, the harder it's going to be to both hit the RPM needed to make that large of a displacement to power ratio, and the harder it will be to make the engine efficient at that RPM. F1 engines pull close to around 300hp to a liter of displacement, and a large part of the reason the only do that much, is because they're limited by the rules in ways that wont let them be built to output more.

**EDIT**
I'm not aiming to do it at 50. I'm aiming to do it at around 75cc-ish. Which you can buy off the shelf bolt on parts for. Either that, or swap it to a Zuma Minarelli horizontal (same mount), put a 100cc bore in it, and turbo that. As the engine case for them has a spot for a circle clip next to the crankshaft oil seal to reinforce them- meaning you can throw boost at it without blowing out said seals. I'd do it at 70ccon the current motor, but, a VZ21 needs more displacement then that at sub 10k RPM to make boost. They'll work off 50, but, you need to be hitting over 15,000 RPM to do that, and, that isn't easy to do.

but, yeah, back on point. 20hp outa 50cc is very possible.

[CROSSBOLT]

I'm not aiming to do it at 50. I'm aiming to do it at around 75cc-ish. Which you can buy off the shelf bolt on parts for. Either that, or swap it to a Zuma Minarelli horizontal (same mount), put a 100cc bore in it, and turbo that. As the engine case for them has a spot for a circle clip next to the crankshaft oil seal to reinforce them- meaning you can throw boost at it without blowing out said seals. I'd do it at 70ccon the current motor, but, a VZ21 needs more displacement then that at sub 10k RPM to make boost. They'll work off 50, but, you need to be hitting over 15,000 RPM to do that, and, that isn't easy to do.

That changes things a bit doesn't it! 100 cc is about 6 cubic inches so a tad over 1 hp/cu in which is real easy and realistic. I get the turbo part as well but it ain't 50 cc with a carb. The Downtown 300i is about 18 cubic inches and turns about 29 US horsepower, not quite 2 hp/cu in but close, with fuel injection. "Carbed hot rod achievement" is my term for current state of art with backyard bolt-on speed goodies. You probably won't find the term on google. I say go for it!

More stuff has been done to increase the output of car and motorcycle engines in backyards and garages than anywhere. Just be prepared to spend a lot of money and time. Honda had a real 50 cc Gran Prix racer in the 60's that did about 120 mph on the straights if I recall right. I saw it at the 1960 World's Fair Honda exhibit. It was built like Rolex! They spent cubic bucks on that and got phenomenal torque and power out of that tiny engine. You could do the same. Don't let anyone discourage you!

[SEANIA]

Cant do what Honda did.
They used a fully custom 50cc twin cylinder 4 stroke that would hit 20k RPM to get their 20hp.

On a 2 stroke 50 it's much more doable. Since, it's effectively displacing twice as much at the same RPM as a 50cc 4 stroke.

I think displacement shouldn't be looked at by it self to say if something is possible or not.

Comparitive power of engines should be looked  at with a combination of stable max RPM, power stroke count, and displacement per powerstroke- multiplied over a period of time.

The 300i may displace way more in each power stroke, but it has half the amount of power strokes, and does it at a lower rpm. So the effective total amount displaced over minute isnt as high as say a high revving 120cc two stroke.