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Post by 90GTVert on Feb 24, 2011 21:10:37 GMT -5
If you can do that, more power to you. I'd love to see it.
If it's going to have to run rich, with retarded timing, and restricted throttle.... why the hell are we going to all this trouble? This sort of high comp setup with race fuel might be just the ticket for Chuck to burn up the dragstrip... but I can't see it being good for your metropolitanization project if that stuff has to be done. It would end up being so much easier to mill a little bit off of the cylinder or head to achieve higher compression (within reason) and cut slightly deeper valve reliefs to compensate. My NCY 50mm piston's reliefs didn't line up with the valves in the smaller bore head so I cut new reliefs in. Plenty of room for it on that piston. If you are dealing with the 39mm, it should be the easiest of them all. The valves are gonna hit close to the edge of the piston, which tends to be where it's the thickest.
If you are really going to go for the super 50cc, you will probably need to start looking into cams. Maybe all of this compression would be good if you have a cam with more valve overlap that will keep cylinder pressures down. I dunno. There really is a lot of engineering and thinking that goes into making a truly great little engine.
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Post by rocketdog on Feb 25, 2011 7:38:57 GMT -5
"It would end up being so much easier to mill a little bit off of the cylinder or head to achieve higher compression (within reason) and cut slightly deeper valve reliefs to compensate".
That's what I would try. Get your local Machine Shop to take .020 off the head. It would probably bump the 50 up a whole point. Not speaking from experience here it's just a guess. And like you said, take a dremel and cut a little clearance into the 50 piston. It wouldn't take much.
As for a cam, well you know what the choices are in the 139 class. Slim.
RD
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Post by 90GTVert on Feb 25, 2011 8:47:57 GMT -5
Yeah, .020" would take .6cc out above the piston. That would take a 49cc with about 6cc about the piston to make 9.2:1 up to about 10.1:1. Most likely you'll be able to knock the 1/50 of an inch out of the piston without an issue if necessary to compensate. Of course check your clearances before getting anything cut or milled.
I know there aren't many choices for a 50. Just saying that there is a lot to be considered if you wanna turn the typical Chinese 50 into something that rivals or exceeds the others mentioned.
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Post by motorhead on Feb 25, 2011 9:06:19 GMT -5
If it's going to have to run rich, with retarded timing, and restricted throttle.... why the hell are we going to all this trouble? I only brought up those things in the section of my post about using such a piston and getting a little more life out of it. Damn straight I want to make one these and run it without restriction. This piston is exactly what I need. Once it's ready all that's needed is a calculation of the compression ratio and doing what's needed to bring the compression to 12:1. |
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Post by 90GTVert on Feb 25, 2011 9:19:50 GMT -5
Another thing to be on the lookout for is a CDI with an adjustable or preferably programmable timing curve. I think that might help you a great deal on your quest.
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Post by dude on Feb 25, 2011 9:40:27 GMT -5
I seen where the 139QMB 44mm piston is domed like the 50cc. Which would be better 44/dome vs 47/no dome for performance? I was looking at SD parts he list a 52mm BBK and its says Compression should not exceed 220psi after break in. It looks like the piston has a dome.
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Post by 90GTVert on Feb 25, 2011 10:03:12 GMT -5
Kinda hard to say if a 44 domed or 47 flat would be better. I would choose the 47mm probably just for the added displacement. Assuming both were using the same cylinder head, there's more volume to compress with a 47mm cylinder, so you're gonna see higher compression figures from a flat top 47mm vs a flat top 44mm.
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Post by scooterguy863 on Mar 6, 2011 17:52:18 GMT -5
i am set to pick up a new 2009 metro in the grey color this tuesday i test rode it and a new vino 50 the vino got up to speed faster but stayed around 38 mph where the metro reached 43 mph and since i already have a vino it made more sense to get the metro
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Post by motorhead on Nov 9, 2011 19:46:00 GMT -5
Finally, I can answer my own question after all this time. In terms of build quality no Chinese Scooter can come close to a Honda of equal displacement. I had the pleasure to work on a 2008 metropolitan and once I got the seat off... It was a thing of beauty. Solid molded aluminum frame. Electronic components wired in a quality similar to larger motorcycles. A very clever radiator mounting system. The carburetor on the metropolitan looks like a slightly smaller GY6 carb. The crank case doesn't split like a QMB does it splits on an axis running along the crank shaft. A fuse panel with multiple fuses. The Met looks like a Honda Reflex under all the plastic and if you can afford one, I say go for it. It is super quiet even at full throttle and at a stop i have to pump the throttle to see if the bike has stalled because its so smooth it feels like it shut off.
But! I took it out for a spin after completing the maintenance and in terms of speed and get up and go... Not even close. I am sure I rode a governed model since other have repored speeds of 40+, but the ride was crap. It took for ever to accelerate. Since starting this thread I have been on 39mm QMBs that I have rebuilt myself that hit 35mph and a few that hit 45mph. Not with gps but with my wife driving next to me. The faster ones were znen but any quinji or bashan bike still hits the same old 25mph. So on a long enough track the right stock QMB can eventually overtake a stock minarelli. I guess what's left for this thread if anything is to see if we can make one of these stay on the road for 20+ years.
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Post by motorhead on Nov 10, 2011 8:57:48 GMT -5
Since i work on scooters all day long now one thing i have noticed when it comes to chinese scooters is that no two 50cc heads are the same. most have an 18mm intake port but a few have a port 23mm wide. And i said in my slow cranking thread, one head i had made so much compression that the engine could not be turned over. Also the port casting varies widely. Some ports are rough all the way down and some look and feel smooth. the combustion chamber side of heads also are milled differently from factory to factory. My first scooter had a flat top piston and now I am seeying domed pistons coming from 4T 50cc. I guess when you buy a vespa honda yamaha etc... you get good molded parts from the start.
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Post by 90GTVert on Nov 10, 2011 9:25:01 GMT -5
Multiple factories, whatever parts they have on hand or are cheapest, and what slips by quality control can make a big difference in what's basically "the same" engine in all the 4T 50cc Chinese models.
We know that we can have different :
Bore diameters
Piston designs
Head designs
Cam designs
EGR/Non-EGR
Carb jetting
Roller weight
Stators
CDIs
... all in the same engine, and I'm sure I'm missing stuff there. Then add in multiple case lengths and imagine how many "combos" there are. If you buy a 2002 Honda, you get a 2002 Honda engine not a random collection of bits and pieces from any Honda ever made that might happen to fit. lol My SunL's flywheel was machined so it was aligned 31 degrees off of TDC. I can't imagine buying a Jap scoot and finding tat.
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Post by motorhead on Nov 14, 2011 15:20:33 GMT -5
here are some things for you guys to chew on Attachments:
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