Max RPM 66cc

nitrofreak3

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Dec 20, 2014
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probably on here but I can't find a thread on it. What is the max rpm you guys are getting on your 80cc / 66cc motors. I'm just curious to see if I'm in the realm. I currently run 29 inch tire with the stock 44 tooth sprocket. I'm reading 9800 rpms at 40.6 miles an hour. Flat ground in South Florida at sea level. still pulls hard great bottom I think I can gear down to a 36/39 tooth.just wondering what you guys are getting..shft.
 

Davezilla

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Mar 15, 2014
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Mine is doing somewhere around 10k rpm also, it's a 26" wheel with a 44 tooth sprocket and my latest GPS reading was 38 mph. Somewhere in here there is a speed/rpm/gear ratio calculator and I need to find it again so I can better calculate my rpm.
 

biknut

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Sep 28, 2010
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If you want your bike to last very long don't cruise it over 7000. 6000 is pretty easy on it. Over 8000 rpm is asking for trouble, but you can get away with it for short bursts.

It's not just the engine you should be concerned about. High rpm vibration will kill the whole bike.
 

rogergendron1

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Stock is about 5000 rpm but a well modded bike can push 10,000 but dont expect a long life at 10,000 i have to swap jugs and pistons every 6 months lol that and every jug has to be custom ported by my hands for hours to get what i want from it

I would zay do some light porting and portmatching but do not alter to far from stock and you can cruise safley and reliably at 7,000
 

rogergendron1

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And you can ride at 10,000 for extended periods ..... i run my race bike at 8,000 to 10,000 wot the whole time i am on it and it lasts a good 6 months of hard hard trail riding in the woods before the rings are gone and the crome is flaking off. Some last longer though. The ones that last the longest at super high rpms are the piston port non reed motors that have large carbs and around 120deg intake timing and a wide exhaust port that is not overly tall to maintain blowdown. Running that setup at a mix of 25:1 to 32:1 keeps it well lubed up and no overheating or bearing damage. Those 100:1 mix ratios are not for super high rpms
 

rogergendron1

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Remember ... an engines only rpm limmiting factor is its maxximum piston acseleration and piston speed befor rimg flutter.

A shoter stroke means the piston has less distance to travel during one crank rotation vs its longer stroke counter part. This means that the 66cc vs 70cc at 10,000 rpms debate will ho to the 66 cc motor simply because at 10,000 rpm the piston has to move 2mm further on the 70cc per rotation making the piston move 20,000 mm more per minuet than the 66cc thats a lot of extra wear and tear on the motor that the 66 cc 38mm does not have to cope with ... it simply means hat and over square motor large bore short stroke will last a lot longer at high rpms than a long stroke motor will and be able to get up to those rpms faster to.

This is why i run the 38 mm stroke motors if i am building a high rpm bike. I know the pison is traveling 20,000 mm less tham the 40mm stroke would every minuet at 10,000 rpm and it will last that mutch longer
 

bowljoman

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Aug 7, 2010
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You'll get more juice with a shifter than with higher rpm. I run 40mph+ at 6500 rpm using a piston port Shandong Huasheng Zhongtian 49CC. 1.5 hp

A GP460R piston ported motor is good for 4.5 horse power in stock form @ around 11000 rpm. Peak torque at 8400 rpm. That with my shifter setup would approach 60 mph.
 

Davezilla

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You'll get more juice with a shifter than with higher rpm. I run 40mph+ at 6500 rpm using a piston port Shandong Huasheng Zhongtian 49CC. 1.5 hp

A GP460R piston ported motor is good for 4.5 horse power in stock form @ around 11000 rpm. Peak torque at 8400 rpm. That with my shifter setup would approach 60 mph.
We're talking China Doll engines here, any of us can get more by switching brands and/ or adding gears... The fun part here is the challenge to push the China Doll to it's limits, then see what we can do to keep them alive at those limits. I also got a Morini sitting on the shelf at my shop just waiting on the right project too, but that's too easy... bolt on speed pretty much.

Don't get me wrong... a KTM with gears is a LOT of fun and can go Really fast, but it's a lot more fun making a China Girl scream with only one speed, then make it fast as all get out and be able to take off from a dead stop without pedal assistance.

I found the rpm/speed calculator in another post and mine needs 8650 rpm to do 38 mph with a 44 tooth rear sprocket and 26" rear wheel, It also accelerates from a dead stop on it's own power and gets up to speed fairly quickly. It's been ported extensively with lots of attention to my dial indicator and degree wheels to set each port's timing exactly where I want it to be.
I did set the timing a bit higher than recommended but accompanied by the pipe I made for it (modified KTM 50, re routed and extended the expansion chamber by 4") I didn't lose anything on the bottom end but gained a lot up top.
I'll post the link to that thread in here for those who already know their speed and want to calculate rpm, or know rpm but want to calculate speed etc...

Of course, now that I got an engine that performs I may add a jackshaft to get more acceleration and speed, or I got a nice little CVT that I may add to the mix... but that's the fun part... After the engine is built and ya know it can perform without the "crutches" of gears, then ya add them in.
 

bowljoman

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The engines Im speaking of are commonly used on bicycle engine kits sold by staton. China gilrs dont own the kit market ;) , and it never hurts to inform others of the options they have when high performance is the goal.

These engines are a far cry from a KTM! why even bring that kind of bike up? Have you built a shifter bike? What shifter kit did you use?

If I can take a bone stock 49 CC motor without a pipe or intake mods, bolt it on my shifter and hit 40mph+ and have the setup last for thousands upon thousands of miles, then wouldnt you agree it a viable alternative to the destructive high RPMs?

Honestly I think its a shame china girl users have not adapted the sturmey archer S3X hub as a inline jackshaft. Its a fixed-gear 3-speed that can drive in reverse!(allowing you to flip the sprocket to the other side of the bike) :)

Not wanting to start a pee contest, just trying to help others see the options.

btw, dont waste time on the cvt.
 

nitrofreak3

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Dec 20, 2014
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I ordered a 36t, gonna see how that pulls my 230 lbs azz..
Has a 40 now 0n a 29" Kent cruiser, 41 MPH all capped up, trying to keep it quite.
 

Davezilla

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Not wanting to start a pee contest, just trying to help others see the options.
btw, dont waste time on the cvt.
No pee pee contest needed... 66cc = China Doll, not other engines...

Shift kits are ok too, but some of us like the challenge to build something that can go fast by tuning the engine and not buying a bandaid fix... That can be done after everything else is in order... I've run a few CVT's in my past and I really like them, they work great once ya get them to upshift at a rate your engine's powerband can work with, some of them may need to be beefed up some or better bearings pressed in, but they definitely work.


this thread has both the math to figure it all out with as well as a cool calculator program The Aleman provided where you can just plug in the numbers to find your bike's speed, by reversing the formulas or putting in the desired numbers it's pretty easy to see which sprocket will give you the speed you're looking for, or if you know the speed and need to figure out the rpm it's not hard to go back and plug in different numbers until they match yours. http://motorbicycling.com/showthread.php?t=54576
 
And you can ride at 10,000 for extended periods ..... i run my race bike at 8,000 to 10,000 wot the whole time i am on it and it lasts a good 6 months of hard hard trail riding in the woods before the rings are gone and the crome is flaking off. Some last longer though. The ones that last the longest at super high rpms are the piston port non reed motors that have large carbs and around 120deg intake timing and a wide exhaust port that is not overly tall to maintain blowdown. Running that setup at a mix of 25:1 to 32:1 keeps it well lubed up and no overheating or bearing damage. Those 100:1 mix ratios are not for super high rpms
I run my Aussie built 'Roo Print 66' at around 8000 - 9000 rpm quite comfortably - it was designed for such a thing... I use Motol 800 Oil at a 25:1 ratio and the beast just keeps going and getting stronger...

I would recommend a 39 tooth sprocket though for country riding - a 36 for urban stop & start riding & a 44 tooth for open road cruising or pulling a trailer... And for a China Girl A80 - around 5000 rpm is it's best performance levels all around...

drn2
 

Theon

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Jan 20, 2014
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I have three china Girl motors that will all do over 10 000 RPM, all 40mm stroke.
If you want it to do over 10 000 and still have good bottom end, reeds.
I've never been game to push them past 10 500, but I think they will.
But unfortunately not all cranks were created equal.
And an unbalanced motor will probably not reach those revs due to power lost to vibs.
Agreed with Roger, that's why you need more than one MB.
 

Theon

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I find if you get your power band to kick in around 6000, It will push through to 10K in no time at all, giving a nice cruising revs of 5000 to 6000 and power on tap.
 
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nitrofreak3

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Dec 20, 2014
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I find if you get your power band to kick in around 6000, It will push through to 10K in no time at all, giving a nice cruising revs of 5000 to 6000 and power on tap.
What size carb are you running, I build 25-30 cc piston port motors for r/c boats and cars. The walboro carbs after mods are larger than the nt, and rt dax clone. We are turning 19,500 to 23,000 on a 29.5 cc piston port motor and making 8+ HP. @ 17,000 revs. Carbs are 13.7 mm you think a keihen 20mm is a good carb?
 
I have three china Girl motors that will all do over 10 000 RPM, all 40mm stroke.
If you want it to do over 10 000 and still have good bottom end, reeds.
I've never been game to push them past 10 500, but I think they will.
But unfortunately not all cranks were created equal.
And an unbalanced motor will probably not reach those revs due to power lost to vibs.
Agreed with Roger, that's why you need more than one MB.
So true - I have a personal collection of 6 bikes I am in various stages of building... Different bikes for different tasks...

I have not tried a reed system yet - nor will I use case induction on my bikes...

The 'Roo Print 66' engine I use is just a beast - non reeded - and 10,000 rpm is easy as for it with absolutely no drama... It is an Aussie engine - now available in the States actually - and ya should give one a try...

The engine is totally custom with computer balanced crank... Smooth...!!!

I am only new to the Motorized Bike thing - so I don't know a lot about them - but in my 47 years on earth I have built many 2 Stroke Road Racers, Harley Choppers, High Performance Hemi's, speed boats and even a bloody helicopter... But these are by far the most fun...

Question: Would a reversion bottle work well with a non-reeded engine at high rpm...? I do not use expansion chambers on my bikes...

drn2
 

Theon

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Jan 20, 2014
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In fact 2 of my 10 000 RPM motors are based on the Roo Crank, so technically maybe not China Girls?
But After modding my Roo with reeds, you don't know what your missing.
 
In fact 2 of my 10 000 RPM motors are based on the Roo Crank, so technically maybe not China Girls?
But After modding my Roo with reeds, you don't know what your missing.
I'm still only a noob with em... But I do have a couple of "Special" bikes I am building which will explore the reed and other go-fast bits you have in the States...

All built around the Roo Prints... And no - once you start injecting Roo cranks into A80 cases - match them with the good anti vibration bearings and the China Girl Syndrome goes out the window...

drn2