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Post by milly on Oct 13, 2019 15:39:07 GMT -5
As some of you maybe aware by my other post's on the forum I have picked up a brand new engine not even in the frame yet. Well I had a brand new 80cc barrel and piston hanging about from doing my son's scooter. I don't have a head as both have been used on his due to valve seating problems they only lasted a short time and to be fair the kits were ultra cheap about 30 US dollar's max. My question is can I use the 50cc head that came with the engine as I know compression maybe a tad higher. Or double up the base gasket or head one to lower compression slightly as I know a lot of you are wiser than me on this subject Thankyou
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Post by scooterted on Oct 13, 2019 15:46:32 GMT -5
People do it. Ive been wondering just how much it raises compression myself.
Im about to do it with the airsal set that has a DOMED piston. I expect it to work, but im unsure what my compression ratio will be and am concerened about detonation.
Ive run a compression check on my engine every change i made, from stock to 72cc. I intend to do it when i get the rings broken in with this upgrade. So ill at least have *something* to compare. Ill have my first word in a week or two.
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Post by GrumpyUnk on Oct 17, 2019 19:58:31 GMT -5
If you do use the 39mm head, be sure to use a 'fiber & metal' head gasket. That will add a bit of clearance, and cut the CR just a bit. I do not like the 'stamped steel' head gaskets for replacement work. If you look at the surface of a new head and new cylinder, they are very smooth, almost mirror in some cases. A stamped steel will work with that, but if you have any imperfection, it is harder for it to seal well. If you are using a head that has been pulled from a running engine, be sure to check with a straightedge that the surface is flat. Lay the gauge in multiple directions across the surface and look for 'dips' or 'gaps'. I found several to have clearance in the area between the cam chain chamber and the combustion chamber. Two have had gaps. I used AlOx paper on a flat, moving the head around with little down pressure until I got a continuous 'ground' surface. I have done several using 39mm heads, and I would recommend use of higher than 87 octane fuel. At least listen closely for pinging if you use 87. The 'premium' or 90 octane suggested in the doc and motor plates is not the "MOTOR" or "RESEARCH" rating used in USA.(actually 87 is the average between the two rating methods) Experience says the starter motor has no problem with the increased compression. I do not know of a local source for fiber head gasket sets, only aliexpress, and you must search for them if desired. tom You can get a 'bare' cylinder head designed for a 47mm bore, and swap in valves or get a 'loaded' head for not a lot of money. tom
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Post by milly on Oct 17, 2019 20:23:18 GMT -5
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Post by GrumpyUnk on Oct 26, 2019 9:35:41 GMT -5
FWIW, I have looked at '80cc heads' on the web. They sure look like standard heads with a relief around the circumference. The relief being a circle of some depth apparently at the 47mm diameter of the cylinder. It appears it would allow a bit more combustion chamber volume, but I don't know how much it would matter. If you look, you may be able to see the relief come very close to the outermost spot on the valve seat. IOW, they start grinding(I assume) a regular 39mm head just outside the edge of the valve seat(in the head), and grind for some distance, I expect to 47mm, which in theory would match the edge of the head gasket fire ring. tom
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Post by scooterted on Oct 26, 2019 10:35:52 GMT -5
FWIW, I have looked at '80cc heads' on the web. They sure look like standard heads with a relief around the circumference. The relief being a circle of some depth apparently at the 47mm diameter of the cylinder. It appears it would allow a bit more combustion chamber volume, but I don't know how much it would matter. If you look, you may be able to see the relief come very close to the outermost spot on the valve seat. IOW, they start grinding(I assume) a regular 39mm head just outside the edge of the valve seat(in the head), and grind for some distance, I expect to 47mm, which in theory would match the edge of the head gasket fire ring. tom That is exactly what they do. I would have compression test* data to prove a lower ratio right now, if i didnt screw up thinking i had 64 mm valves when i actually had 69mm. Oh well. I can wait. *im well aware that one cannot extrapolate compression ratio from psi, bit it is real world evidence.
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Post by GrumpyUnk on Oct 28, 2019 12:10:57 GMT -5
I think if you had a 'perfect gas' and expensive equipment, and had no errors, you could extrapolate back and forth between compression ratio and psi. These engines were designed and primarily used in countries with very questionable fuel quality. Ditto their lubricants. They should tolerate a pretty poor fuel before detonation/pinging, so given the higher octane and even quality of fuel in other parts of the world, they should tolerate a good increase in compression ratio pretty well before having problems. Maybe. Perhaps. As I suggested, getting a thicker fiber/metal combination head gasket would help alleviate a bit of the increase in compression ratio when using a 47mm bore with a factory 39mm cylinder head. Use good fuel, run it, and see what happens. Go to a higher octane if needed, or grind away some of the cylinder head material to lower the CR. tom
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Post by scooterted on Oct 28, 2019 14:27:38 GMT -5
I did ride it with the domed piston and the stock head. Without detonation.
I did decide to go with the larger combustion chamber though. Im currently waiting on parts.
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Post by scooterted on Oct 31, 2019 16:46:25 GMT -5
I got the 50mm head install on my scoot done today. Compression was a wopping 255+psi cold. Unfortunatly, i dont think i have very good results to compare with like i hoped. I think when i tested the 50mm piston with the stock head i didnt get the tester on right (or i burned a valve on the stock head, dont see any damage as far as i can tell) I screwed the pooch on that but heres my results anyway. Stock* 180psi cold 47mm bore stock head* 205psi cold 50mm domed piston**, stock head 215psi cold 50mm domed piston*, stock head 195psi hot, 180psi cold??? I think i had the tester installed wrong here. 50mm domed piston*, 50mm head 255psi cold. *with broken in rings. **with new rings.
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Post by GrumpyUnk on Nov 1, 2019 10:13:10 GMT -5
FWIW, the volume pumped by these hummingbird pistons is sometimes not enough for lower-quality compression gauges to read properly. The 50mm domed with stock head... wonder what it would do with broken-in rings. I think MORE psi. The 47mm numbers make me more comfortable using the standard head with a 47mm bore. That 205 psi is pretty reasonable given the 180-190 you can get with a Ford Lima 2.3 4-banger, which happily runs on 87 octane unleaded. Not too much more, and seemingly not enough to be a threat to the connecting rod, bearings, or the head gasket. Good. I don't have a 'good enough' compression gauge to get reliable numbers, though the ones I have checked have been somewhat questionable as their rings/cylinders/pistons were suspect and that's why I was checking. Maybe I should try on one running well... One thing to to consider is that when checking car/truck engines, the 'pros' suggest opening the throttle wide open to allow for full air flow. If you check with the throttle closed, you cannot draw enough air in at cranking speeds to get a good check of compression. I think the results would be a bit skewed towards lower pressure. If you remove the valve that you wonder about, or fill the up-turned cylinder head with water, alcohol, or fuel(diesel/gas), a leaking valve will allow fuel to seep past the valve into the exhaust or intake port. No seep, no leak. A bit of carbon could allow lower than expected values. A too small valve clearance would not matter much as long as there was some clearance. It might get too close when heated to operating temperature, and cause leakage and hard re-start, but it should not affect cold cranking compression as the valve will still close. A close clearance will cause the valves, both, to open earlier and close later by a slight amount. Measurable? I dunno. tom
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Post by scooterted on Nov 1, 2019 12:01:45 GMT -5
Fwiw i actially tested all those with the carb disconnected completely. Just an open port.
Im 100% certain the domed piston/stock head combo would be even higher with broken in rings (I really wish i had thought to double check the way i installed the gauge.) Its (domed piston/stock head) a really tight fit in there, I was playing around with it before installing.
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Post by jackrides on Nov 1, 2019 23:49:02 GMT -5
Boom?
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Post by milly on Nov 3, 2019 22:45:49 GMT -5
Well a quick update, running with the 50cc head and a extra base gasket started and ran fine just got to tune the carb before I can road test it and awaiting a rectifier which decided to burn itself out and a rear light unit as poached mine for son's scooter. Should have all that this week hopefully. I shall report back in time.
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Post by jackrides on Nov 3, 2019 23:57:20 GMT -5
Do you have a good compression gauge? The kind with the valve (like a tire valve) close to the compression area?
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Post by scooterted on Nov 4, 2019 7:58:38 GMT -5
Do you have a good compression gauge? The kind with the valve (like a tire valve) close to the compression area? If youre talking to me. Yes i do. Thats what i used.
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