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Post by AtariGuy on Aug 19, 2018 15:53:08 GMT -5
I'm lucky enough to have an account (recently in fact) at the local airport so i can get 100LL avgas for my 2 stroke but otherwise i know with compressions above 180psi you should run 90+ (us rated, r/m method?) octane and 200+ you almost need an octane additive. I believe dexameth had about 200psi on his 50 build, not sure where it's at on his 52 build now. Mine (just checked today after 200 miles since installed) is reading 145 pounds. But thats a dished piston, 0.065" worth of gaskets, and an a9 cam.
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Post by dexameth on Aug 19, 2018 16:31:49 GMT -5
My 205psi was the 52mm stroker build that I have right now. I only run premium in all my scoots.
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Post by magoconnor on Aug 19, 2018 17:06:06 GMT -5
So i'd just avoid big valves unless you have a high compression piston, a stroker crank, or the big 52.4mm bore kit If I were to upgrade to a 50mm cylinder, I would buy the airsal kit which includes the sweet piston. Do you think it would okay to run it with the 47mm 22/19 head? Or I'm better of finding a similar head with stock valves? Maybe you could explain to me, why the bigger valves would hurt the performance?
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Post by dexameth on Aug 19, 2018 17:41:25 GMT -5
So i'd just avoid big valves unless you have a high compression piston, a stroker crank, or the big 52.4mm bore kit If I were to upgrade to a 50mm cylinder, I would buy the airsal kit which includes the sweet piston. Do you think it would okay to run it with the 47mm 22/19 head? Or I'm better of finding a similar head with stock valves? Maybe you could explain to me, why the bigger valves would hurt the performance? Airsal claims they test that cylinder and piston with a stock head... 😳
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Post by magoconnor on Aug 19, 2018 17:51:30 GMT -5
Do you guys think I will have trouple with the airsal dome-shaped piston, hitting my 21/19 valves?
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Post by AtariGuy on Aug 19, 2018 21:39:42 GMT -5
I'm not saying the valves are the cause of the performance problems - big valves theoretically kick ass. The problem in this case is - to make that big of valves fit that small of a head, they have to make the combustion chamber bigger - the bigger the combustion chamber, the lower the compression. The math comes out to roughly 7:1 compression ratio, down from 10.5:1. Now, considering the efforts they try to avoid engineering a whole new head (and casting molds, machining setups and changeovers, etc) they cast 2 types of heads. One with an 8cc combustion chamber to fit stock size valves, and one with a 13cc combustion chamber to fit big valves. Then they set the parts into their mills to turn out a squish band for the size head. All that crap aside - i would stick with a stock valve size head for the 8cc combustion chamber. Whether you use a 47mm head or a 50mm head is up to you, the squish band difference is roughly 1cc or less difference. Using an airsal piston though, i'd avise using a 50mm head just in case. An airsal piston in a 50mm bvh shouldnt interfere, but to play it safe, i highly reccommend getting some modelling dough (playdoh or a synthetic oil based clay from a craft store) and sticking a 2mm thick layer to the valve relief areas of the piston before fitting the head - once the engine is mocked up, turn the crank over by hand 6-8 times, then take the head back off and see how close the valves come to the piston. I believe 90GTVert has a youtube video on checking valve clearance like that if you need a visual idea what i mean. Once you're assured there wont be any interference, clean the piston of any dough/clay, oil everything back up and finish assembly.
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Post by AtariGuy on Aug 19, 2018 21:55:55 GMT -5
Nice and short - my advice is a small valve head, 50mm to 50mm, and get the airsal piston.
Or stick with your 47mm setup currently.
A big valve head will give you problems unless you can afford the solutions.
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Post by magoconnor on Aug 20, 2018 7:13:14 GMT -5
Thanks for the answear. I was looking over the speed list, and I found dexameth doing real good. He is also using a "bvh", so is there a another way to get the big valves, without enlarging the combustion chamber? His build seems to rock. You have to scroll a little down the page, to see the setup: 49ccscoot.proboards.com/thread/17708/peace-sports-83cc-bbk-52mph?page=2Humanshield appears also to be using one, he also talks a little about the combustion chamber here: 49ccscoot.proboards.com/thread/16593/50mm-bbk-thread-installed?page=5He says: A stock head's volume is 5cc's (Too much compression on a 50mm BBK) A 21mm (intake valve) head's volume is 8cc's (perfect for the 50mm BBK) A 23mm (intake valve) head's volume is 13cc (too low compression for a 50mm BBK without a stroker crank or domed piston
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Post by dexameth on Aug 20, 2018 9:28:50 GMT -5
Yeah back then I had that eBay kit. The head was a 23/20 BVH and it was tuned to work pretty good. It cracked between the valves and into the little spot behind the spark plug and had a terrible leak; it turned into scrap metal. Had around 10k miles on it.
That's when I upgraded to the Hoca head. The combustion chamber is filled slightly, not the typical dome chamber like most which I suspect is there to increase the combustion back to somewhat normal values.
You know what, I never did another speed run after the 52mm upgrade from that 50mm, nor did I when I swapped heads. Now I have a wheelie setup with the stroker build and it's maxing at 58ish. I should set it back up for speed and let 'er rip haha!
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Post by magoconnor on Aug 20, 2018 9:43:59 GMT -5
Cool man Then my 47mm Bvh with 21\19 valves should work, right? Or is there something I am missing. according to humamshield's information My Bvh's volume should only amount to 8ccs, which as he writes should be perfect with a 50mm cylinder. Also I have read you had some experience with the airsal piston using a Bvh, how did that work out? Did the valves clear the dome-shaped piston?
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Post by dexameth on Aug 20, 2018 10:18:08 GMT -5
Yeah, I'd say that should work. But, to be sure... do the clay test. I had no issues. Even after the upgraded 52mm domed piston I had no clearance issues.
The Hoca head has smaller valves than that BVH I was running. It's got 21/19 valves and a smaller combustion chamber but WAY bigger ports.
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Post by magoconnor on Aug 20, 2018 10:24:57 GMT -5
The hoca head from what I can tell, is exactly like mine. Was yours also 47mm ?
And those 21\19 valves worked with the airsal piston?
sorry for asking so much I just wanna be sorta sure, before purchasing.
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Post by dexameth on Aug 20, 2018 10:37:10 GMT -5
I should have clarified about the Hoca head: I never tried it with the 50mm, but it did bolt up fine to the 52mm domed piston till it broke the ring land. That was the whole reason to upgrade to a stroker. It has a 52mm combustion chamber.
I have a 21.5mm valve on my desk, as well as the 50mm Airsal piston. The valve reliefs look to measure 22mm so the diameter will clear, but again I am unsure of squish clearance but I'd suspect it to be fine.
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Post by dexameth on Aug 20, 2018 10:40:47 GMT -5
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Post by magoconnor on Aug 20, 2018 10:55:35 GMT -5
Thank you very much, I will go ahead and purchase the slightly expensive airsal kit. And hopefully soon be flying down the roads
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