Chinese Scooter Club

Brighton Russell · Fri Jul 04, 2008 8:34 pm anyone modded a std 4 stroke exhaust?

I want to modify mine. to make the outlet pipe at least as big as the carb diameter. so not restricting. anyone done this or modded a std 4 stroke exhaust?. I dont want it loud as the main reason I got mine, is I cant stand loads of exhaust noise. I just want a more free flowing exhaust :wink: .

Fox · Fri Jul 04, 2008 9:50 pm

Two strokes need that restriction/back pressure.

Brighton Russell · Sat Jul 05, 2008 6:50 pm

Fox wrote: Two strokes need that restriction/back pressure.
I wasnt clear so have now edited. mine is a 4 stroke.

Fox · Sat Jul 05, 2008 7:13 pm

Good luck. It's probably not going to gain any speed, but it'll be louder.

larry8 · Sun Jul 06, 2008 1:32 am

Fox is right. I put a Buel muffler (straight thru) on my 125 and it made a lot of noise and the scoot didn't accelerate or gain any top speed.
Looked nice though.

px166bajaj · Sun Jul 06, 2008 6:52 am

I had a Honda 50 years ago with a sawn in half exhaust

It didnt make any difference to the top speed. :roll:

Yellow Scooter · Sun Jul 06, 2008 2:55 pm Re: anyone modded a std 4 stroke exhaust?

Brighton Russell wrote: I want to modify mine. to make the outlet pipe at least as big as the carb diameter. so not restricting. anyone done this or modded a std 4 stroke exhaust?. I dont want it loud as the main reason I got mine, is I cant stand loads of exhaust noise. I just want a more free flowing exhaust :wink: .
A quiet freeflow exhaust, especially on a scooter, is a contradictory statment. :lol:
Hacking the stock pipe or drilling holes will not make it any faster nor will it make it run any cooler. It's gonna take a group of performance parts like a freeflow air filter cone, in conjunction with a carb rejet, and a freeflow exhaust. And theres no way possible to add quiet horsepower with a scooter. The least offensive sounding exhaust I found sofar has been Leo Vince, or an MRP stainless.
However, you can change the rollers in the transmission to a lighter set making it accelerate at a higher rpm, closer to the peak power of the motor. That will be the best seat-of-the-pants mod you can do. For a 50cc the magic number seems to be 4.5g Any lighter, and you may end up compromising the top speed. Most scooters make max power at 7000rpm. So that's the number you'd be shooting for the CVT to hold the engine at while accelerating. ;)

DaveS · Sun Jul 06, 2008 5:32 pm Re: anyone modded a std 4 stroke exhaust?

Yellow Scooter wrote: For a 50cc the magic number seems to be 4.5g Any lighter, and you may end up compromising the top speed. Most scooters make max power at 7000rpm. So that's the number you'd be shooting for the CVT to hold the engine at while accelerating. ;)

Do you have an solid source for for that 7000 RPM number? :? I am not trying to be a nit-picker or anything, but since I have been doing a bunch of variator tuning experiments using an accurate TinyTach tachometer, I would really like to find a cast in stone specification for the maximum horsepower RPM. :?: For instance, the user manual for my YY50QT lists 7500 RPM for maximum horsepower, but in doing Google searches on the web I have seen maximum horsepower RPM numbers from below 7000 RPM to as high as 8000RPM. So far, I have been unable to find torque and horsepower versus RPM graphs for the 139QMB engine anywhere on the net, so tuning the variator is turning into a very seat of the pants operation. FWIW the 4.5 gram value you gave seems to be very good, and tends to run my engine at around 7400 RPM, which close to the center of the range of horsepower/RPM numbers I have found on the net. In my case 4 grams does not lower my top speed any because my CDI is not RPM limited, but it does cause the engine to run around 8000 RPM on a full throttle climb, and boy does it climb! I think that I remember Fox posting that he does/or has run 4 gram weights??? I hope that explains why I am asking about the source of peak horsepower RPM numbers that I read here???? :oops:

larry8 · Sun Jul 06, 2008 5:45 pm

You may be able to trace the HP and torque rpm's to the old Hondas back in the 60's or 70's.
But almost all the GY6 engines are slightly different even from the same manufacturer that publishing those figures would be meaningless.
One can take the scoot to a testing lab and put it on a dyno but the results would only be accurate for the scoot tested.

DaveS · Sun Jul 06, 2008 6:44 pm

larry8 wrote:
One can take the scoot to a testing lab and put it on a dyno but the results would only be accurate for the scoot tested.

Thanks Larry!! That is what I was afraid was the case. So far I get great acceleration and hill climbing results with 4 gram weights, but I am a bit nervous running 8000 RPM in doing so. Thanks to an error I made installing Dr. Pully non-round rollers, I know that my engine pulls strongly right up to 10,000 RPM, but I definately do not want to go there again! :shock: I guess that getting an accurate red line number is also going to be a pipe dream too? :( Right now, I am thinking of defining my own red line at a bit over 8000 RPM, but if I am wrong in that experiment it is going to be a costly experimentation error. :?

Aerostudent · Wed Jul 09, 2008 5:42 am

I will have to be the contradictory one here and say that an exhaust should add some performance to your scoot, but not much unless it is coupled with a freeflow filter and either an HP carb, or at least jetting to match the filter and exhaust.

If you are making an exhaust, I have read several places that 1" is the ideal size for a header on a GY6 engine. The can won't make much of a difference other than sound, it is the header pipes that kill these scooters, though I did gain about 3mph my drilling out the end of my can (freeflow filter and 38/120 jets as well), but seemed to lose a bit of acceleration off the line, though it could have been due to the carb needing readjusting.

Again, don't expect much unless you add a filter and jets, but be sure to use ~1" header pipe. Have fun!

Aero

Yellow Scooter · Wed Jul 09, 2008 1:06 pm

Yeah DaveS, All you need to do is look at the horsepower numbers under the engine specs. All of the GY6 engines list their peak power numbers. Example: HP 7@7000rpm.
The acutal power numbers themselves can and most often are inflated, but the rpms in which they produce peak power have been quite consistent across the board. I have no dyonmeter at my home nor have I ever strapped my scooter to one. So I suppose you can say its a well educated guess of mine, that if the motor is geared to accelerate at a constant 7000rpms, you'll be at peak power point for that motor, and be at your maximum acceleration potential. If you have it geared to hold just under the redline or past the peak power number, it will be counter-productive. You'll lose a good portion of your top end speed, and will be well past the peak power of your motor. In other words your acceleration will be slower, but considerably louder.
Of corse, I'm basing this on a stock GY6 fresh from Chinas production line. Things like camshaft upgrades head shaving/porting obviously change those figures, and power peaks at diffrent rpms etc....
The roller weights that govern the rpms in which the scoot accelerates at are as individual as the scooter driver is. It's accoring to your weight, the hill grades, altitude, hell, even the weather all factor in. I drive a 150cc. My bike is about the size of a Vento Phantom. For me 12g rollers hold my motor at 7000rpm. I'm not really sure about a 50cc. The 4.5g is purely speculative and based more on consumer input rather than science and tachometers. It might be 4g that hold it at the magic number for all I know. Nobody has ever posted anything about what weight held the motor at what RPM. If someone does, That would be great, and could be stickified somewhere in the tuning & mods section.
You wanna have a go at that since you got the 50? :|

DaveS · Wed Jul 09, 2008 2:10 pm

Yellow Scooter wrote: Yeah DaveS, All you need to do is look at the horsepower numbers under the engine specs. All of the GY6 engines list their peak power numbers. Example: HP 7@7000rpm.
The acutal power numbers themselves can and most often are inflated, but the rpms in which they produce peak power have been quite consistent across the board. I have no dyonmeter at my home nor have I ever strapped my scooter to one. So I suppose you can say its a well educated guess of mine, that if the motor is geared to accelerate at a constant 7000rpms, you'll be at peak power point for that motor, and be at your maximum acceleration potential. If you have it geared to hold just under the redline or past the peak power number, it will be counter-productive. You'll lose a good portion of your top end speed, and will be well past the peak power of your motor. In other words your acceleration will be slower, but considerably louder.
Of corse, I'm basing this on a stock GY6 fresh from Chinas production line. Things like camshaft upgrades head shaving/porting obviously change those figures, and power peaks at diffrent rpms etc....
The roller weights that govern the rpms in which the scoot accelerates at are as individual as the scooter driver is. It's accoring to your weight, the hill grades, altitude, hell, even the weather all factor in. I drive a 150cc. My bike is about the size of a Vento Phantom. For me 12g rollers hold my motor at 7000rpm. I'm not really sure about a 50cc. The 4.5g is purely speculative and based more on consumer input rather than science and tachometers. It might be 4g that hold it at the magic number for all I know. Nobody has ever posted anything about what weight held the motor at what RPM. If someone does, That would be great, and could be stickified somewhere in the tuning & mods section.
You wanna have a go at that since you got the 50? :|

Thanks Yellow Scooter! Actually I have already posted some values that I got with my 139QMB, a few different roller weights, and an accurate TinyTach tachometer. I have not seen many specification sheets listing peak horsepower at such a low RPM as 7000 RPM, but I have seen a lot that list 7500 RPM and a few that list 8000 RPM, such as this one:

FWIW, here are the numbers that I have right now.

7.5 grams (my original) = 6500 RPM
4.5 grams....................= 7400 RPM
4.0 grams....................= 8000 RPM

I get the strongest acceleration and best hill climbing with the 4 gram roller weights. I accidently installed some Dr Pully weights incorrectly once and the engine pulled strongly right up to 10,000 RPM with that setup. That is why I am tending to take all the specification sheets that I have read with large chunks of salt. They are just not accurate. For now, I intend to keep the 4 gram weights since they pull great and since the engine lived through my accidental 10,000 RPM I am considering 8000 RPM as a safe RPM number. That, and the fact the some sheets indicate 8000 RPM as the maximum power value.