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Post by usmcdoc on Feb 12, 2014 17:46:42 GMT -5
If its a slow piston and cylinder, cut it till its fast!  190mech..kewl..thanks for the input! 2strokd..cut what? Hey now..share this with us..  |
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Post by usmcdoc on Feb 13, 2014 10:19:21 GMT -5
Ok..thanks for the link..that is a short skirt. Here are 2 pics..first one is the Hoca kit with a skirt length of 21.92mm.. The second pic is the Athena with a skirt length of 14.82. Due to the shorter length of the Athena skirt I can see why the piston has no port..just a notch at the bottom..due to the length there would be no room for a piston port. @ 2strokd.. so is this what ya mean by shorter is faster? Would that mean the 2nd pic would have more power and top end?..   |
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Post by 2strokd on Feb 13, 2014 10:56:39 GMT -5
It looks like the Athena cylinder is made for more peak power yes. The skirt is (correct me if i am wrong) for keeping the piston stable or from rocking at speed. The shorter the skirt the less friction from piston/cylinder contact, single ring piston over double for the same reason. Less drag. When i say "cut it till its fast" I pretty much mean cut, shave, file any material off and away from the piston that isnt needed. Match the trans "scallops" to the cylinder. File the ex, and or transfer sides of the piston to get target duration numbers etc. A slow piston/cylinder can be modded to lay down more power than it was ever designed for. When you are actually checking the books to see if your piston can go that fast you know you are onto something Its always a trade off though. Single rings, more ports..Bigger, higher ports usually mean less lowend and a peaky engine with a narrow power band. You need a darn good dialed CVT or OR kit to get the engine to stay on target with these type builds. Take a look at a good running 20+HP 11,000+RPM screamer 2T piston and you will see the differences quick between it and our common cast tall eco slow pistons. Here is a link to a KTM50cc piston for comparison sake. Remember, this type of 2T is made for all out RPM and gear bangin race fun! www.ebay.com/itm/NEW-KTM-50-PISTON-2-KIT-SX-JR-SXS-MINI-2009-2014-45230007000-II-/231156041712?pt=Motorcycles_Parts_Accessories&hash=item35d1f947f0&vxp=mtrNow, our standard run of the mill, work hard, work slow, non fast scooter Minarelli scooter piston www.ebay.com/itm/NEW-Piston-Kit-LEM-Malaguti-Italjet-Minarelli-Morini-39-00-/261394598297?pt=UK_Motorcycle_Parts_13&hash=item3cdc550999Tall, low piston pin, crappy scallops, fat double rings... All meant for a slow, long life. But again, this can be modded for much better performance. |
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Post by 190mech on Feb 13, 2014 11:44:09 GMT -5
Well written Robert!Doc,if you place a cylinder in an open case half,you'll see how the cylinder and piston skirt hangs into the case reed channel.The cyl and piston would be too weak to hack it all off,so we 'trench' the cases to allow the mixture to flow around the cyl toward the transfer channels like this;  This is a 103cc setup,a 70cc will be different,but very similar.We widen the center notch to allow better flow also;  Here is a shot with the piston at BDC,note how the side of the piston blocks the transfer channel?Thats another mod that should be considered if looking for all out power;  |
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Post by 2strokd on Feb 13, 2014 11:54:02 GMT -5
Yeah buddy! Here are some more pics on the subject.. Piston getting the transfer area matched up   Skirt getting get out of the way. This may result in a shorter piston life?   |
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Post by 90GTVert on Feb 13, 2014 16:57:39 GMT -5
I thought it would be a good idea to open up the cylinder skirt a little as well to get more flow through the cases. Below are before and after shots of the cut.   I didn't want to leave all of these sharp edges, so I rounded them out.   Then I rounded out the other areas by the transfers.  At this point I learned a lesson about checking before cutting. It wasn't running well, maxing at 40MPH. Now, since I had the piston and cylinder out, I wanted to check out the possibility of an exhaust port coverage issue at TDC as palezu mentioned. I could see that it is fully covered, but I wondered just how close the port would be to the edge of the skirt. I used a Sharpie to mark the port's location on the piston skirt. It's covered, but not by a lot.  This side is really bad. The marker is smeared a bit, but it's still very close. This seems like it could be a problem.  I was really surprised that .5mm or so on each side of the port just to remove some imperfections there would push the limits so far. (I had also ported the cylinder.) I got out the new piston, same thing as this one, to see how that looked. Oh, it appears I took off way more than I realized when smoothing out the edges.  You can see the stock piston comes to a thin and jagged point. I suppose since it's so thin the material went faster than I noticed.  I replaced the piston and re-modded... but left the area by the exhaust alone that time. Went from 40MPH max with the overzealous piston work to over 50MPH max after some additional tweaking. (stock 49cc engine with mild porting, Leo Vince GP, Hoca variator) |
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Post by usmcdoc on Feb 13, 2014 21:00:29 GMT -5
Like oldgeek said..Good info..keep it coming! Regarding the skirt on the cylinder bottom..a few years back I bought my daughter a Yamaha PW80. The previous owner said he had trimmed and modded the skirt. That lil bike was a real powerhouse.. BUT..at high speed I heard an odd sound like a rattle or clack? Anyway when I pulled the top end..the skirt had been apparently cut too short cause the piston had severe signs of 'Piston Flop' We put a new top end on it and power was back down to what is was made for I suppose.. a 'Kiddie' Bike. Had planned on trimming the piston and cylinder some..but was not sure how much before it had piston slap again...and I was not up for buying yet another top end. So anyway..the Hoca kit I have with its window type ports does make good power and pretty evenly through the powerband. But with this Athena with its oddball notch in the piston skirt vice a window..and its half as long skirt on the cylinder had me wondering what type of power and powerband it would have. So what I am seeing in this thread is that it most likely will a high rpm screamer with a more narrow power band. Sure it would be a hoot to ride..but not sure I would want that kit on my daily driver. @ Brent..I opened an Ebay generic kit today and the flashing in the piston ports extended right at 1/3 the way across the 'windows' I was going to Dremel the flashing out..but decided to just get another piston..cause if their casting process is that incredibly poor..heaven knows what hidden internal flaws the metal has in it..  Ok now I gotta go back..read all the info you fine folks provided..take a closer look at the pics and get a better idea of what ya did. Oh..lastly in regards to what was mentioned about the trimming giving more and easier flow.. Have any of you seen an improvement from the 2-5mm spacers that go under the Reed Cage? Supposedly they decrease resistance and turbulence thus allowing the crank and cylinder to take a fresh charge better. Any input on those? Glenn |
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Post by 190mech on Feb 14, 2014 5:39:25 GMT -5
Reed spacers get the cage out of the rear transfer duct which helps flow.I made mine out of 1/4" aluminum plate,they used to sell them on ebay pretty cheap..
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Post by 90GTVert on Feb 14, 2014 8:31:18 GMT -5
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Post by 2strokd on Feb 15, 2014 13:59:03 GMT -5
I would take the two kits and compare them with whatever pipe you are using. Best match is the winner. I figured ide share more pics from my stroker build.  Cases trench cut. 190 helped me allot on this one by cutting for crank and cylinder clearance. As well as roughing in the case trenching, or flowpathing.  Everything is rough and still needs need the boost trans worked.  Everything is getting better now      A pic of a pretty matched up "trench cut" assembled top-end. You can see after the base gasket seated in the skirt was still blocking some.  |
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Post by 2strokd on Feb 15, 2014 14:03:01 GMT -5
First ride Good memories!  |
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Post by usmcdoc on Feb 15, 2014 14:37:47 GMT -5
I would take the two kits and compare them with whatever pipe you are using. Best match is the winner. I figured ide share more pics from my stroker build. Everything is getting better now Now that looks awesome! We tend to only think we have restrictions in the CVT..carb and exhaust. So folks spend lots of money on those parts for their engine.. When the REAL restrictions are the actual engine design itself. So for a little labor and no money spent..we can improve performance. Sure it's not a 'Bolt on n go' thing.. But I am sure the labor involved is more rewarding than just buying parts and hoping for the best.. |
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Post by 2strokd on Feb 15, 2014 15:09:31 GMT -5
It sure is doc!
For example, i mentioned this in another post but a guy i know dyno,d a stock Minarelli A/C 49cc scoot with a Leo GP exhaust through the CVT and it made 4.5HP.
I dont think he did any case mods aside from port matching the trans tunnels? But he did smooth out all the edges, and checked/changed port durations some. By the time he was done with it and still using the stock engine it made over 7HP at the wheel through the CVT! If thats not a in your face result for some port matching and really tuning an engine i dont know what is?
On that note i need to ask more about what exactly he did and if he did and internal case mods?
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Post by 2strokd on Feb 15, 2014 15:11:52 GMT -5
I gotta question!
Do you guys think i should have left more skirt on the intake side to help direct the transfer and boost charge more toward there tunnels?
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Post by usmcdoc on Feb 15, 2014 16:05:50 GMT -5
I gotta question! Do you guys think i should have left more skirt on the intake side to help direct the transfer and boost charge more toward there tunnels? 4.5hp up to 7 hp on 'stock' engine is incredible! best thing is..he had to buy no expensive often over rated parts. As for leaving more material on the skirt..Brent may know that answer. All I know from experience with that was with my daughters PW80 the guy modded. Removing material did add a fantastic gain..but he removed too much and the piston slapped and was not stabilized.. But now..Take that stock engine the guy ported from 4.5hp to 7 hp.. THEN add a BBK and other stuff. That would be a wild ride.. When I went from a non modded 70 BBK with all else stock..to adding just a tuned pipe and 5 gram sliders to that.. I was in shock as suddenly the power band went from 6k-7k..it was now 7k-10k and could be felt whereas with the stock pipe ..it was there..but more like.. "Ok..wow..woopty doo".. |
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