Torque, or RPM? (Moped exhaust on 66cc)

Street Ryderz

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Oct 14, 2016
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I worked at a KymCo, TGB, Genuine, Adly and SYM scooter dealer from 2007 tp 2010. Part of my duties besides assembling and tuning scooters was installing aftermarket Leo-Vince and Genuine Scooter hop up kits on our bikes we sold. Most of these bike when uncorked or derestricted were close to 5 h.p. and that was with the stock exhaust.

Bikes I personally converted to performance kits were the Genuine Black-Cat, Buddy, Rattler 50 and 110 with a 125cc kit, SYM JetEuro 50 and the Adly ThunderJet 50.

The HT engines have two transfer ports and they're small. your lucky to get about 2.5 to 3 HP out of a well fettled 66cc HT engine. You can't really cut the port open much bigger without weakening the cylinder. And theres not much can be done to adding a third transfer
All of the scooters I discussed have 2 to 3 big transfer ports and case reeds. They can easily flow enough fuel-air to rev upward to 8~9k stock, even more with an aftermarket reed booster cage and bigger carb. Most run carbs in the VM18mm size stock

In every case with the scooters the adding of the expansion chamber exhaust took the wide, stock power band and moved it about 1500 ~ 3000 rpm higher, weakened the bottom end torque so you had to change the pulley weights and rear pulley spring and shoes to allow the engine to operate, (come onto the 'pipe',) at a higher rpm. And any bike with CVT's work with a pretty narrow RPM when they're tuned for a pipe, at least until the CVT is in it's tallest ratio. That's why you change the pulley weights so the engine starts to close the front sheaves at a higher rpm, and why there's a spring and flyweight kit added to the rear pulley, so the bike launches at about 4~5K instead of 2.5K.

So yeah I've got a few years professionally hopping up scooter engines, in addition to owning and running 2 cycle motorbikes from 1973 to 1990. Stock exhausts are always set up to create a smooth, wide powerband.

Mike is asking about improving torque. You can either have a wide powerband that work over a wide rpm range or you can change your exhaust to one that boosts the HP and RPM from stock but will take the wide torque range and squish it into a 3~4K range at a higher RPM with a slight boost in torque in that narrow range.

Effectively making the engine bog severely under 3500.

Several people here gave Mike decent advice cleaning up the ports, add a long intake runner, cut the internal exhaust pipe in the cap a few inches. Cannonball2 said that installing the Yamaha PW50 carb gave his bike a better transition from idle to mid throttle. It's all little changes like the PW carb which little smaller than the NT carb, but because of it's cleaner castings and better precision in metering, it's more precise in it's overall mixture, = engine runs smoother from idle on up to WOT. These are easily done minor tunes that can improve the HT engine's already wide power band.

You're not going to give the HT engine a big boost in overall torque by adding an expansion chamber only. That increase only comes by increasing the displacement, and some serious changes in the engine's intake like case reeds or a windowed piston-reed box. The scooter exhaust is made for engines running much more volume and bigger transfer ports.
The 50cc minarelli engines,puch,and yamaha witch are really also minarelli based,make 5-8hp stock and mild modded make 8-10 hp with fully modded making 16 hp at 70cc,that displacement has a pumping efficaintcy very simular to ours the 50cc does not no matter how you want to look at it!Our engines when case matched and the transfers corrected with port area's enlarged and matched pump the same volume and more due to larger case volume just at a lower pressure but this lower pressure works to our advantage in the lowend and as was stated before if the pipe can carry for 3-4 k rpm vs 2k from a dirtbike pipe it just makes more sense to stage it at around 4k and they provide plenty of usable torque.Look at this graph showing the pipes out put with a fixed gear,no variator these are extactly the range we run it!
 

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Vikingimike01

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Jul 1, 2018
Messages
222
Hey Lewie! Thanks for the help.

I'm not sure if I said this before, but later on I'll be windowing the piston, installing a DIO reed kit (If you know any better ones, please tell me!)

Also, for some reason, my engine kit came with widened, and enlarged transfer ports by default. Had to cut away a bit of the skirt on the bottom of the cylinder though.
 

Vikingimike01

Member
Joined
Jul 1, 2018
Messages
222
Hey guys! So I'm actually working on a battery mount right now. Practicing metal cutting, welding. I got myself an 1000 lumen, 50W halogen light.
Also got myself a 12v, 5ah battery, its not a small one. About 10cm tall, long, and 7cm wide. I'll be making a mount for the battery, and a holder. Pieces alredy cut, just need welding.

I'll be working on this project first. I decided that I'll get help from a professional with the exhaust. The mechanic came back today from vacation, but I don't want to bother him today. I'll be asking him tomorrow!
 
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