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big bore kit or high compression piston(stock bore)?

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Discussion Starter #1
Hello guys. I need your advice, I just ported my head and now i need to buy new cylinder and new piston for my 07 yfz. My quad is road legal so i'll ride mainly on street. I also have Yoshimura full system and FCI intake. I have 2 options: stay stock with 95mm CW Cylinder and 13.1 CP Piston (M1001) that i had before or try a new setup for me,big bore kit with Athena cylinder and CP piston (M1009) 12.5 compression. i'm from Europe and is kinda hard to find those kits here so i need something reliable,so is big bore worth it? it's going to be a big difference? Thanks :dude::smile
 

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at this point you may as well do the big bore. i am not sure you will feel it in the seat of your pants but maybe. i have an 06 with an fmf, no airbox, stock bore, and port and polish with a 52 rwhp dyno sheet. i have not had the bigbore stroker on the dyno but it should be about 60-62 rwhp. but it is a stroker, you wont get that out of just a big bore. as to reliability the most reliable that machine will ever be is as it left from the factory and every mod makes it a little more temperamental but you are already headed in that direction and a bigger jug shouldn't make it worse. in general the bigger the bore the thinner the cylinder wall and the more heat issues you have but as long as you do a good job with assembly and make sure your cooling system is good you will be fine. if not larger radiators are available but i don't think you need one. i have a 98mm bore,polished head, and 4mm stroke and only run in the sand and i have not had any heat issues.
as to the kit i dont know about international shipping but the are significantly less expensive (than athena) big bore kits available on ebay
tuning can be a little tough with a big bore and there is not a lot of good advice on the net but i have done several write ups on here about big bore specific jetting, search and read those. either way you are going to have do some jetting and tuning. let me know what way you go and i will help you get it dialed in
 

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Discussion Starter #3
You mean, just from porting the head i am losing some reliability? Did you feel big difference between the stock bore and the big bore you have now? Cuz i was pretty happy with mine when it was stock bore, so i am not sure if i really need the big bore kit. i just ride in the city...you know...maybe some wheelie and drag racing some friends, but the problem is i am doing lots of miles with it. do you think with good maintenance can a big bore last as a stock bore? Thanks for the info.
 

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yes. those machines are engineered to provide the best power balanced with reliability possible. every component on the machine is designed for the factory 36ish hp. when you start bumping that up, even more so for you riding on pavement (those machines rely on the slip of the dirt to keep from breaking things like axles) . every bit of power you add adds a stress somewhere and decreases reliability and then compound the issue with riding on pavement.
as to how it feels i have not had a big bore that was not polished in a yfz i had a stock warrior and a stock warrior with a big bore jug and it had a little more right off of the bottom but other than that not much difference. ported with a stock bore compared to bone stock is a noticeable difference but i have never had a ported bike with just a big bore. ported, polished, big bore and stroker crank it is not even the same machine.
you dont "need" the big bore kit. if you get it jetted correctly you will see a significant increase in power just from the porting. the bore itself will last every bit as long as the factory bore. the coating are the same thickness so wear is not the issue heat is the issue ( head gaskets, warping etc.) and as long as you keep an eye on that you will be fine. if it was me i would do the big bore, you will get a little more power for free since the big bore jug and piston is about the same price as stock and with the porting you will need to retune it anyway so might as well.
if you are using this thing to commute you may run into an issue with fuel, the porting alone is going to have an impact on how far you can go on a tank of fuel and the big bore will compound that.
you will need to keep a close eye on the chain, sprockets and axle, like i said theses things were not designed for pavement and depending on how well its tuned and the option you will be adding up to 15ish rwhp.
 

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Discussion Starter #5
i see. One of my friends has a stock head and he wants to buy a big bore kit aswell. Is it going to worth for him? just polished head, and slip on exhaust. is better to stay stock for him? he probably won't notice the extra power, unless he is going to do a port job, right? Thanks
 

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if he has to replace it anyway i would do the big bore but if he is just trying to get performance it is not worth doing without the porting
 
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