yfzr compression & power

[adrenaline junky]

I have read on here that hi comp. pistons were not delivering great results on the R's. Some believe that it is probably due to the piston dome providing an inefficient combustion chamber shape.

After looking at the Gforce website someone found after running across one of their intakes on ebay, I took a look at their engine kit. A couple people who are familiar with the company said they built some of the fastest yfzr's in Canada. Well on their website G-Force Division , interestingly enough, their race engine mod list does not include a new piston, but does include machining the cylinder height. Maybe this is how they are raising compression instead of using domed pistons. Thoughts from any builders on this?

[Narmasil]

Of course there are high compression pistons without Dome. Machinic a stock piston for HIGHER compression i have never heard of really. but for lower its pretty common... I think that about 12.5:1 compression is ideal without dome. i HAD a 14:1 non domed on my bike but i had detonation problems and had to run on E85

[adrenaline junky]

I never said anything about machining the piston, they don't even mention the piston at all which is my whole point. They machine the cylinder, presumably to increase compression.

[funyun]

They don't touch the piston they are decking the head to increase compression." High octane junkie" on here has a factory LTR motor that they used a stock piston in and just decked the head for compression.

[Narmasil]

aaaaaaah nevermind i read wrong... but there isnt really much differance on a stocker piston than a aftermarket one. depends on weight then really and i've seen many aftermarket pistons are quite heavy

[Narmasil]

im not quite sure what "decking" is you mean carve off? I would assume that decking the top will give you a bit different combustion room and decking the cylinder doesnt really change anything except rising compression. Am i right or what? :)

[rjwz28]

Yeah decking the head reduces the combustion chamber volume and can make for less-than-optimal head characteristics. I can see the benefits to porting the head and decking the cylinder while leaving a stock piston in place.

[adrenaline junky]

Hey Mix, compression advantage aside, would your YFZ Venom piston have better combustion chamber characteristics than that of the stock piston?

[adrenaline junky]

Head milling doesnt affect squish clearance? I thought this was the volume of the space above the piston when at TDC. What is it then?

[adrenaline junky]

So if I wanted to increase my comp. ratio from 11.4:1 or whatever it is stock to 13:1, would the best way to do this be to mill down the cylinder height? What comp. ratio could you get to doing it this way with a stock piston before piston to valve clearance becomes an issue? I'm guessing 13:1 would not be attainable with this approach.