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Post by xyshannen on Oct 12, 2014 16:09:09 GMT -5
I've been talking about it, but thought I would make a thread regarding this. I would like to get some expert feedback.
OK Here's what's going on, as most of you know I'm in the process of installing the 59mm (actually 58.5) BBK from Scrappy and I have their Ruima Taiwanese quality cast cylinder head and I will also be adding their 4mm stroker crank. Now I wanted their Big Bore Head but alas it's no longer available. But because of this I'm planing on shaving a few thousandths of an inch off of the cylinder deck or the head it'self (still not decided). The ultimate goal is to increase my compression. I would like a max of 12-1. as I know that's as high as you can go and still run pump gas with some octane boost. But my questions to the GY6 gods is..
Will I have tolerance issues between the valves and the piston that is supplied with the 59mm kit?
Will the lower end of my QMJ be able to handle this much compression?
If you don't think it will take it, how much can I increase my compression before having issues?
Is there an easier way to increase my compression such as a dome shaped high compression piston? If so could you please provide a link as a source for this?
Any insight you can provide is appreciated. Even if you think I'm a fool for wanting to increase my compression that much.
edit: Also this thread will serve as documentation of my progress and results. Good or bad.
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Post by alleyoop on Oct 12, 2014 18:56:18 GMT -5
I don't think you will have to worry about not getting a compression around 180psi which is approx 12.+ ratio. 150psi is your OEM standard 10.? ratio. My motor came in at 200psi with a 62mm and a +2 stroker. you will probably have to add a extra gasket so you can turn the motor over. You will not be able to use your kick start due to the high compression if you do the first kick start attempt you will probably break it. You will need a very strong battery with CCA's over 200 to turn that puppy over.
Alleyoop
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Post by xyshannen on Oct 12, 2014 19:42:32 GMT -5
Sweet!
Actually the stock QMJ compression is pretty low at 8-1.
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Post by alleyoop on Oct 12, 2014 20:12:30 GMT -5
Sweet! Actually the stock QMJ compression is pretty low at 8-1. NOrmally not a new one unless it's just a bad assembly or valves are tight you may be confusing it with the HP which is a little over 8. Alleyoop |
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Post by phssthpok on Oct 12, 2014 20:58:43 GMT -5
Bear in mind that any shaving of the deck (cylinder) or head will result in altered cam timing. It may not be a lot (or even enough to seriously affect anything). On the flip side it could be severe enough to cause run-ability issues, or even result in valve/piston contact. Either way, it WILL be there. Just something to factor into your plans.  |
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Post by xyshannen on Oct 12, 2014 21:28:05 GMT -5
Sweet! Actually the stock QMJ compression is pretty low at 8-1. NOrmally not a new one unless it's just a bad assembly or valves are tight you may be confusing it with the HP which is a little over 8. Alleyoop I got that number from a post scooter posted here. 157QMJ 149.6 cc 8.6 hp (6.4 kW) at 7,000 rpm 57.4 mm × 57.8 mm (2.26 in × 2.28 in) 8.8:1 Read more: itistheride.boards.net/thread/6854/thar-blows#ixzz3FzIaFluRI trusted this post because I've seen these numbers multiple times from multiple locations, just for the life of me I can't remember where. It's good to know they are wrong, so thank you for clarifying that. I know I've seen other on here post that their compression test was 150-160 PSI supporting what you are saying. I do have the equipment to test my compression before I install the motor pod so I'll be able to tell where I'm at. phssthpok That's a good point I hadn't considered, and thankfully from the looks of things just adding the stroker crank will get me the compression I'm wanting. So I can hold off on the milling of the deck or head. |
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Post by xyshannen on Oct 12, 2014 21:31:31 GMT -5
alleyoop Oh yeah Alley, in regards to my kick start... I got rid of the piece of _____ a long time ago! I can't believe they actually put such a piece of crap on a GY6 in the first place. I've had mopeds with better kickstarts.
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Post by alleyoop on Oct 12, 2014 21:39:04 GMT -5
Yea, On my Trike they put on a pull start, problem with it was found on the first pull(HAHA). It loosened the Variator nut and ruined my Variator fan(HAHA). So like you I pulled the pull start off and left a nice big access hole for extra cooling. And here is another Specs for the 157QMJ motor: Alleyoop Click on Product details: www.icebearatv.com/product/pst150c |
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Post by scooter on Oct 12, 2014 22:35:16 GMT -5
That's from wikipedia, xyshannen. en.wikipedia.org/wiki/GY6_engineThey also list compression for two other gy6 engine sizes. They put their sources, such as Haynes Manuals, at the bottom of the article if you want to see how they got their numbers. How do you use psi to determine compression ratio? Is that like 150psi / 15psi = 10, where 15psi is normal air pressure? |
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Post by xyshannen on Oct 12, 2014 22:39:50 GMT -5
Yeah wiki is always right  lol These numbers are tossed around allot on various sources. What I do know is for a fact other members here have reported a compression test reading of 150-160 PSI.. Does anyone know the calculation involved in converting PSI to 8-1 or 10-1 compression? In the end it doesn't really matter...What matters is what kind of compression am I going to get once everything is done. |
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Post by scooter on Oct 12, 2014 22:50:08 GMT -5
Yeah wiki is always right lol These numbers are tossed around allot on various sources. What I do know is for a fact other members here have reported a compression test reading of 150-160 PSI.. Does anyone know the calculation involved in converting PSI to 8-1 or 10-1 compression? In the end it doesn't really matter...What matters is what kind of compression am I going to get once everything is done. I'm not supporting or defending wikipedia but I have used their sources for research papers and they generally have quality information based on quality source materials. I'd probably believe a Haynes manual over a scooter sales website but I tend to check several sources anyway when I need to be sure, so I'll definitely be looking out for more sources to find out which one is more accurate. As for the psi thing, I'd trust actual measurements like that over either source, but I suppose that could differ from engine to engine and manufacturer to manufacturer. But yeah, if there's a way to actually measure it, that's better than taking someone's word for it in my opinion. Physically you can fill the head with water and measure how much water it held, and you could do the same for the cylinder but the math for a cylinder is really easy, so if you have the parts in hand, you can calculate what your compression ratio will be that way. |
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Post by xyshannen on Oct 12, 2014 22:59:32 GMT -5
Yeah wiki is always right lol These numbers are tossed around allot on various sources. What I do know is for a fact other members here have reported a compression test reading of 150-160 PSI.. Does anyone know the calculation involved in converting PSI to 8-1 or 10-1 compression? In the end it doesn't really matter...What matters is what kind of compression am I going to get once everything is done. Physically you can fill the head with water and measure how much water it held, and you could do the same for the cylinder but the math for a cylinder is really easy, so if you have the parts in hand, you can calculate what your compression ratio will be that way.HUH?!? could you elaborate a little on that? I was just joking about the wiki thing. The trouble with wiki is anyone can go in and edit any of the info on it. In the past I was (well still am) seriously into rare plecos (fish) and it was pretty common not to trust info on wiki. Jackarses who thought they know everything would often edit the info pages with seriously wrong info. |
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Post by alleyoop on Oct 12, 2014 23:01:40 GMT -5
Take a compression test on the motor and divide by 14.7 to get your approx ratio. For example if your compression comes out to be 150psi divided by 14.7=10.2 ratio, 180 psi divided by 14.7=12.2.
Alleyoop
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Post by scooter on Oct 12, 2014 23:11:01 GMT -5
Physically you can fill the head with water and measure how much water it held, and you could do the same for the cylinder but the math for a cylinder is really easy, so if you have the parts in hand, you can calculate what your compression ratio will be that way.HUH?!? could you elaborate a little on that? I was just joking about the wiki thing. The trouble with wiki is anyone can go in and edit any of the info on it. In the past I was (well still am) seriously into rare plecos (fish) and it was pretty common not to trust info on wiki. Jackarses who thought they know everything would often edit the info pages with seriously wrong info. Sadly, people can edit it, but a lot of other people tend to find bad info and unedit it. I once posted a link to one of my websites and within 24 hours some angry person let me know via email that .coms can't post on there to pages that sell stuff, that my edit had been removed, and not to do it again. lol The key to using wikipedia seriously is to go to the sources they used to get their information. Those are not on wikipedia and cannot be randomly edited. To clarify, compression ratio, as I understand it, is the ratio of the volume of air when the piston starts to compress the air in it, and the final volume of compressed air. So if your piston held 100cc's of air as the valves close, and compressed it to a volume of 10cc's, that would be a 10:1 compression ratio. Correct me if I am wrong. I am no mechanic. and of course it's never that simple. also on wikipedia is a bunch of brain bashing math and science about it, like this little tidbit for example: "The absolute cylinder pressure is the result of an exponent of the dynamic compression ratio. This exponent is a polytropic value for the ratio of variable heats for air and similar gases at the temperatures present. This compensates for the temperature rise caused by compression, as well as heat lost to the cylinder. Under ideal (adiabatic) conditions, the exponent would be 1.4, but a lower value, generally between 1.2 and 1.3 is used, since the amount of heat lost will vary among engines based on design, size and materials used, but provides useful results for purposes of comparison. For example, if the static compression ratio is 10:1, and the dynamic compression ratio is 7.5:1, a useful value for cylinder pressure would be (7.5)^1.3 × atmospheric pressure, or 13.7 bar. (× 14.7 psi at sea level = 201.8 psi. The pressure shown on a gauge would be the absolute pressure less atmospheric pressure, or 187.1 psi.)" en.wikipedia.org/wiki/Compression_ratio |
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Post by xyshannen on Oct 12, 2014 23:36:53 GMT -5
I just went cross eyed reading that! Could also be that it's almost 1 AM here. It will probably ,make more sense in the morning after my coffee.  |
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..BAD TO THE BONE..
