Fuji 170, hard break in , iridium spark plug, uni air filter, 130 main jet,main needle modded, Found Doing 69mph down rt 9
jake318 Guest
Re: stock head on a 61mm big bore « Reply #1 on Nov 18, 2008, 12:57am »
Chad do you have 157Qmj motor or 1P57QMJ motor? ( number is on left side bottom of motor ) If P motor the only store that sells big bore kits is AUTOTECH355 on Ebay 160$ with head and cam included( Thats Cheaper than blacktop)
Joined: Aug 2008 Gender: Male Posts: 393 Location: Lisle, IL
Re: stock head on a 61mm big bore « Reply #3 on Nov 18, 2008, 1:46am »
Could you run your stock head on a 61mm big bore? Yes
Should you run your stock head/cam/intake manifold/carb/exhaust on a 61mm big bore? Only if you are interested in creating a nice "stump-puller" that has great torque and runs out of gas at about 6,000 RPM.
A stock 150cc GY6 (with stock cam and head) is already in need of more carb and a 1" exhaust pipe; adding more ccs is not going to help top end performance unless you add the supporting pieces to increase flow into and out of the engine.
The stock head has small ports (compare the size of the inlet on a MRP or similar AL intake to the stock intake port!) and small valves. You can open up the ports and install larger 28/24mm valves, stronger valve springs and a bigger cam (A10, A12 are usually suggested for big bore kits) or you can buy a big bore kit that already includes a big valve/big port head with good valve springs and a A10/A12 cam. Carlos (AutoTech355 on Ebay) has these kits for about $200.
Another option (one which I'm pursuing) is to put a high compression 13:1 piston in your stock cylinder, install big valves & upgraded valve springs in your stock head, DIY port your stock head and install an A10 cam. This will cost less than the big bore kit, avoid having to machine the case and give you a very stout high revving 150cc motor. In any case, you still are going to need at least a 30mm CV carb and an aftermarket exhaust to make either combination work at high RPM.
Joined: Sept 2007 Gender: Male Posts: 253 Location: SC
Re: stock head on a 61mm big bore « Reply #5 on Nov 18, 2008, 6:29am »
Good info.Also note if you have the erg head or not.The earlier non erg head has more compression.The ports were a bit bigger to start with and openning them up was easy.
Joined: Jul 2008 Gender: Male Posts: 156 Location: New Jersey
Re: stock head on a 61mm big bore « Reply #6 on Nov 22, 2008, 7:10pm »
well after getting my new cylinder kit which I am actually glad I got after my disassemble.. due to the side of my piston missing and the cylinder wall being messed up. I went to go install my 61mm kit. Found out after measuring the diameter was 2.5 inches aka 63mm. So I started grinding and got it installed. with the stock head. But I am thinking my compression is too high now. that or its just too cold out for the battery. cuz I can only get my scooter started if I jump it off my car. any thoughts?
Joined: Sept 2008 Gender: Male Posts: 639 Location: S.I. New York
Re: stock head on a 61mm big bore « Reply #7 on Nov 22, 2008, 8:48pm »
Quote:
Another option (one which I'm pursuing) is to put a high compression 13:1 piston in your stock cylinder.
mx5tc,please tell us more of these pistons, I think you guys are using AKUNARS. Any recomendations on which one of these (ie: model #) fits stock GY6s, big bores, "P's" . What is piston choice based on? If you can break it down like I'm an 5 year old that would be cool .
Joined: Jul 2008 Gender: Male Posts: 156 Location: New Jersey
Re: stock head on a 61mm big bore « Reply #8 on Nov 22, 2008, 11:08pm »
its funny Cuz the sleeve of the old cylinder fit in the sleeve of the new cylinder. Talk about a size difference.. Still cant figure out if the battery doesnt have enough amps to crank the engine fast enough to get it started or if its just too cold out aka in the 20s/30s..
well after getting my new cylinder kit which I am actually glad I got after my disassemble.. due to the side of my piston missing and the cylinder wall being messed up. I went to go install my 61mm kit. Found out after measuring the diameter was 2.5 inches aka 63mm. So I started grinding and got it installed. with the stock head. But I am thinking my compression is too high now. that or its just too cold out for the battery. cuz I can only get my scooter started if I jump it off my car. any thoughts?
Hard to determine without more information and pictures but you may have significantly increased the quench area of the combustion chamber by using a stock cylinder head on a 63mm big bore piston/cylinder.
For instance, if you were to place the stock cylinder on top of the stock piston, you'd see small areas around the edge of the piston top where the piston actually touches the cylinder head (in an assembled engine, there is a nominal clearance provided by the head gasket and any negative deck height) This quench area is increased if you increase the bore size from 57.4 to 63mm.
You can calculate what the increase in compression is from going to a 57.4 bore to a 63mm bore but you need to know what the volume of the stock combustion chamber is as well as the compressed thickness of the head gasket (assuming the deck height of the piston is same on both the stock and big bore setup)
I suspect that the big bore heads supplied with the big bore cylinder kits have proportionally larger combustion chambers (both to reduce valve shrouding and keep compression closer to "stock")
mx5tc,please tell us more of these pistons, I think you guys are using AKUNARS. Any recomendations on which one of these (ie: model #) fits stock GY6s, big bores, "P's" . What is piston choice based on? If you can break it down like I'm an 5 year old that would be cool .
Tiger,
After exchanging a few emails with Akunar, I went with the 57.5mm Pro Piston (AKPRP010-B). Will not know what the exact compression ratio is until I can calculate deck height and combustion chamber volume etc. but Akunar estimates 12-13:1 ratio. 58, 58.5, & 59mm pistons are also available in this style.
According to Akunar, a GY6 (stock, big bore, P head etc) has 15mm diameter piston pin and a height from the top of the piston pin hole to piston deck (this a modified compression height; compression height is normally measured from the center of the piston pin to the deck) of 13mm. Any piston you put in a GYG needs to have a 15mm pin (to fit on the GY6 rod), a 13mm "compression height" (so it fits properly in a standard cylinder) and an appropriate bore size to match the cylinder.
FWIW, the valve reliefs aren't very large (Akunar advised checking valve clearance on a trial assembly) and will most likely need to be enlarged to ensure adequate clearance.
Joined: Jul 2008 Gender: Male Posts: 156 Location: New Jersey
Re: stock head on a 61mm big bore « Reply #11 on Nov 23, 2008, 1:19am »
well I can tell you its about 1/32 around so from what i read above and think . I do have way too much compression. To start with the stock battery. But like I have tested once started it runs great..I'll do a compression test as soon as I can..also gonna have to figure out how to get more amps to the starter..probably hook a battery up parallel.
well I can tell you its about 1/32 around so from what i read above and think . I do have way too much compression. To start with the stock battery. But like I have tested once started it runs great..I'll do a compression test as soon as I can..also gonna have to figure out how to get more amps to the starter..probably hook a battery up parallel.
If I get some time, I'll try and "guestimate" what I think your new compression ratio is (based on standard flat top piston with 9.0 compression in standard GY6). I wouldn't complain, compression is a good thing, standard rule of thumb is that every 1.0 increase in compression raises power and torque by approximately 4%. Just make sure that you are running good 93 octane premium and be ready to add a little octane booster if you hear it ping.
You also might want to try PMing Jake318 to see what he did to get his scoot to start after he increased his compression to 12.5 or 13.0 to 1.
Joined: Aug 2008 Gender: Male Posts: 393 Location: Lisle, IL
Compression Increase from big bore kit « Reply #13 on Nov 23, 2008, 8:57pm »
I made some simplifying assumptions and came up with the following:
Stock GY6 with 9.0:1 static compression ratio:
57.4mm bore 0.00" deck height with flat top piston .040" thick head gasket 16.0cc combustion chamber volume
Using the same parameters with a 63mm big bore kit:
63mm bore 0.00" deck height with flat top piston .040" thick head gasket 16.0cc combustion chamber volume
equals a 10.4:1 static compression ratio
Using the "standard" rule of thumb (1.0 points of compression = 4% power increase for NA engines), a 9hp 9.0:1 GY6 would put out about 9.5hp at 10.4:1 compression. Additionally, a 9.5hp GY6 makes about .0633hp/cc, your big bore kit adds approx 30cc so you are now "theoretically" making 11.4hp. Actual horsepower is dependent on the ability of your intake "system" (filter, carb, intake manifold, port, valve and cam timing) and exhaust "system" (valve, port, cam timing, header & muffler) to flow enough volume at high RPM to support the big bore kit (which they don't). Nonetheless, your low end and midrange torque should be quite impressive. Just my estimates, YMMV,
Joined: Aug 2008 Gender: Male Posts: 393 Location: Lisle, IL
Re: stock head on a 61mm big bore « Reply #15 on Nov 23, 2008, 10:40pm »
Tiger,
Thanks for the kudos! FWIW, I'd think long and hard about spending time and $ on ANY telecom certification. I've been in telecom since the start of "Divestiture" (1984) and it's been a declining industry (in terms of jobs) since late 2001 due to a combination of overcapacity and consolidation..pm me if you want more specifics.
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