Weight of the pistons?

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mew905

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I was curious about what the 48cc piston weighs, because they're balanced for the cranks, so if I can lower the 66cc piston weight down to the 48cc, then I'll be golden, however the 66cc piston AFTER I cut it all to ****, weighs 75.4 grams.
 

mew905

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Because the crank is already balanced for the 48cc piston, and a lighter reciprocating load will give it a bigger 'sweet spot' and less vibrations throughout the whole range. The only way to perfectly balance a motor at all rpms is to have equal weights at equal distances on both sides of the crank, i can balance the crank for a 66cc at a target rpm but because the counterweights are closer to the center of the crank than the piston is, it will weigh less than the piston at high speeds.
 

moonerdizzle

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you cant perfectly balance a single cylinder motor, it will always be for a certain rpm range unless you use a counter shaft balancer. but your not going to get a piston light enough and have enough meat left to have it equal out to the same weight as a 48,
 

mew905

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Yeah, i discovered that, even my useless 36mm long piston weighed 75.4 grams, however the less weight at the end of that rod, the less forces need to be countered
 

mew905

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Maybe, though jag has plans for an expansion pipe to optimize performance throughout the whole RPM range, i want to buy some off of him. My issue is reliability, and vibration reduction should boost that substantially
 

mew905

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yeah, I found an SKF dealer nearby so I plan on grabbing 4 6202 bearings and a 10x14x13 needle bearing (all rated to 35,000 RPM), if I end up selling more motors, that'll be included in the "basic tune-up package" (along with porting, lapping, and the likes)
 

48ccbiker

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the forces that need to counterbalance each other increase equally with increased RPM. The only force that changes is the engine compression due to different timing as the RPM changes. If you can set the timing to maintain the same pressure against the piston as it rises then the balance of the engine will be the same at all RPM.
 

Theon

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I found lightening the piston by 12g gave me an extra 500 RPM or more before serious vibs kicked in on what was otherwise a smooth motor to 9000.
So then smooth to 9500, without noticeable change elsewhere in the revs.
Also found some pistons and pins substantially heavier than others.
 

maniac57

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Oct 8, 2011
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the forces that need to counterbalance each other increase equally with increased RPM. The only force that changes is the engine compression due to different timing as the RPM changes. If you can set the timing to maintain the same pressure against the piston as it rises then the balance of the engine will be the same at all RPM.
There is so much wrong with this I simply cannot let it pass.
The compression of an engine has NOTHING whatsoever to do with balancing it.
Balancing any single cylinder engine is a compromise for a desired RPM range.
The forces on a rotating crank are directly proportional to the RPM and piston speed. Compression does not affect vibration in any way, the weight of the rotating assembly is the ONLY factor aside from RPM (and ignition timing which can be far enough off to induce vibration and break parts with huge rod loads from detonation) that affects balance.
It's a matter of physics. Loads increase with speed, and reducing weight reduces loads just as reducing speed does. Less weight = more RPM, More weight = less RPM for a given amount of vibration.
The compression may have a small affect on vibration in high compression race motors,but mostly it's the SPEED that changes vibration of any given weight of piston.
 
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rustycase

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May 26, 2011
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ALL reciprocating engines will have a primary harmonic, then a couple other lesser ones...
Proper engineers do their best to coax that event to an rpm range where it's not noticed.

A tuned pipe will provide power boost at a specified rpm.
Only compression ratio will produce more power across the operating range.

Well, unless you want to get rid of that pesky vacuum intake tract. :)

Have fun!
rc
 

Theon

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I have been told that compression ratio will affect crank balance by someone who has put a lot of time and money into producing a smoother motor. Dino at 'Screaming Roo'.
However I have not noticed any real difference in my motors with varying compression.
 

biknut

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Sep 28, 2010
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Usually vibration increases with compression, but if that's the only difference it's minor.

The biggest problem with china girl motors is cranks that aren't true. Plus I haven't seen any evidence that the factory's make any effort to balance the cranks. They use the same cranks in 50cc motors and 80cc motors even though the bigger 80 pistons weigh a lot more.

I think the balance is biased towards the 50s, because they seem smoother running to me.
 

maniac57

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Usually vibration increases with compression, but if that's the only difference it's minor.

The biggest problem with china girl motors is cranks that aren't true. Plus I haven't seen any evidence that the factory's make any effort to balance the cranks. They use the same cranks in 50cc motors and 80cc motors even though the bigger 80 pistons weigh a lot more.

I think the balance is biased towards the 50s, because they seem smoother running to me.
Okay I'll agree that very high compression can increase vibration somewhat, but nothing like the piston weight does. Lighter reciprocating parts have always been a good way to get more from less and these engines are no different.
The 49cc engines ARE smoother for the simple reason the 49cc piston and rod are lighter than the 66cc with the same crankweights. The give up a bit of low end but rev a bit more and run smoother.
So it follows getting your 66cc parts down to the 49cc weight will give you the exact same benefits without giving up the extra torque.
The hard part is finding stuff that fits properly.
 

mew905

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a chopped piston with a titanium ring should come pretty close to the right weight. I cut an intake window into my piston and drilled two 1/4" holes next to the wrist pin to reduce weight and got it down to about 97g from 106g if I remember right. With a titanium wrist pin, that should drop right down to 89g, but I havent got that far yet, even now with the heavy pin I'm seeing far less vibrations than stock. You're right, they are balanced for the 50cc engines and thats what I was aiming for with this thread, but as far as I can see, noone has the actual weight. I imagine if I can match it, it'll run damn smooth.

Though if I get the hoca piston, iron rings, titanium pin and a silver race bearing, I should see the same 97g assembly weight, and crank more power and efficiency. I still have a long way to go with this motor, particularly because a lot of it was just pissin around learning how it works, just waiting on the money for the replacement parts and the *proper* aftermarket parts, to get this thing hauling. Though I have been thinking about case induction since I saw arrow's race motor had it, and now that someone's actually doing it with a nearly step-by-step guide, I may go that route later on. I still have some more measurements to do so I know my limits, because I want to improve transfer efficiency as well, the transfers in these engines are garbage, and most even let fresh mix straight out the exhaust (terrible shaping).