how many cc would a 12:1 piston add to a trx250 it had a 9:1

 

I could be wrong, but I don't think increasing compression adds any CC's to your motor. I think the only way to increase CC's is by getting it bored or stroked. Again, I could be wrong, but from what I've heard on these forums, compression has nothing to do with displacement.

 

ATVer 14 is right.Think about it.A High compression piston produces higher compression by being made taller,it still travels the same distance up and down,and its still the same diameter.Meaning bore and stroke stay the same therefore displacement remains stock.

'88 250R many extras

 

A higher compression piston has more of a dome to the top, which allows the motor to take in the same ammount of air, but with less volume (space) in the combustion chamber, It compresses (squishes) the air/fuel at a higher rate on the compression stroke, causing a more forcefull explosion, which in turn makes the piston's downstroke faster and more powerful, translating into more low-end power, at the cost of higher engine stress, heat, and lower reliability. This is also assosiated with the need for race gas on a engine with 12:1 compression engine (depending on the elevation and cam) due to the lower octain, more volitile gas exploding too early due to the increased heat and pressure of a high-compression combustion chamber. This is called detonation, and usually occors when the engine is under an additional load of stress, such as hillclimbing, high heat level, flat tires, ect. Man, I have way too much time on my hands...

 

couldn't of said it better. Actually a few people on here that really know what there talking about.

 

250rampage is exactly right in his comments. You would actually lose just a smidge of displacement because of the dome piston, but that's just splitting hairs.
I will say this though, If your not really hardcore about racing or riding hard, I would stay away from that much compression. It can give you serious reliability issues, and get real expensive when those problems show their face.

 

Yea, I would stick with 11:1 and under for rec riding. For me, 11:1 is great, even for racing. Even though with my setup, I could easily get away with premium pump gas, I go with a 2:1 mix of pump gas (93) to racing gas (110) for extra security.

 

Actually, roadracer96, you wouldn't lose any displacement with the use of a domed piston. The amount of air/fuel mixture displaced remains constant regardless of the piston shape.

 

The swept volume and listed displacement is based upon the equation to figure for the volume of a true cylinder shape. The combustion pocket and it's associated trapped volume at TDC would be independent of volume of the cylinder itself. For example, it's a two stroke I know, but I now it fairly well, a 250R, it's a 246cc in a stock bore size. To alter the compression ratio you change the trapped volume, the volume above the piston at TDC, to get a 13:1 compression ratio you need a trapped volume of roughly 20.5 cc's, now if wanted to raise that to a 16:1 compression ratio the trapped volume would have to decrease to about 16.5cc's. Has that 4 cc's difference changed the swept volume of 246cc's. No. We're only talking about what is above the piston at TDC. So how does this apply to four strokers. Because of valves and head shape and so on, rather than reduce the trapped volume by milling the head, you use a piston that has a pop-up dome, a piston with more crown volume. The increased crown volume of the piston reduces the trapped volume at TDC, thereby raising the compression ratio. The swept volume remains the same.

 

I disagree, to raise the compression the volume has been made smaller, the only way to raise the compression without changing the bore and stroke or decking the head would be to decrease the volume in the cylinder. You would have to loose some displacement in the process. The bore and stroke are remainging constant no matter what the shape, if it was the shape of a bullit and had 400cc and you changed it to a flat top and it was still 400cc the compression ratio would be the same. If the volume remains the same than so will the compression ratio. The domed top decreases the displacement in the cylinder and thus yields higher compression.