compresion ratio

[One_Bad_400]

me and my dad were talking... and we cant figure out a way that they change the compression ratio with only changing out anything else?? how do they bump the compression ratio up from like 11.0:1 to like 13.0:1?????????

[JOEX]

Originally posted by One_Bad_400
me and my dad were talking... and we cant figure out a way that they change the compression ratio with only changing out anything else?? how do they bump the compression ratio up from like 11.0:1 to like 13.0:1?????????
Not sure exactly what you're asking but here are some ways to up the compression ratio.....

Domed piston
Thinner, and in some cases, no gasket
Shaved head
Decked cylinder

[mikeboone]

Originally posted by One_Bad_400
me and my dad were talking... and we cant figure out a way that they change the compression ratio with only changing out anything else?? how do they bump the compression ratio up from like 11.0:1 to like 13.0:1?????????

Most of the time with 4 strokes you install a high comp piston which pushes the air fuel mix into a smaller space. The smaller the space the higher PSI the motor has, the more power it will make, and require higher octane fuel as the mix gets hotter the 'tighter' you get it.

With 2 strokes you can easily just shave the head as their are no valves. 4 strokes have VERY limited amount of means for this and you remove from cylinder instead.

[2004exrider]

Compression ratio= cylinder volume + Deck height vol. + Head gasket vol. + combustion chamber vol. DIVIDED by Deck height vol. + Head gasket vol. + Combustion chamber vol. = Compression ratio

[One_Bad_400]

i know that it pushes the air into a smaller space or w/e... but how do they do that... it the piston it taller... its not getting as much air when its down then it would regular.... if you make the piston shorter its grabing more air going down but when it goes up its not compresing it into as small as a space??? i onow really how to explain all this?? LOL

[Mike_Rides_Red]

Yes you probably loose a couple of cc's going from a 11:1 to 13:1 but you have much more compression. When you have higher compression ratio you get a more violent explosion because all the air/fuel mixture is compressed into a little space. If you want to know why higher compression makes a more violent explosion go take high school chemistry...

[F-16Guy]

I think I understand what you're asking. I've thought about this too, and I'm not sure if I'm right either. What you're dealing with is the swept volume, not what's in the combustion chamber normally at top dead center. That air is really always there since it's already present when the piston begins it's down stroke, and cannot be expelled at the end of the up stroke. So by adding a dome to the piston or shaving the head to reduce that area, you've left the swept volume of air (what the piston moves) virtually unchanged, but made a smaller space for it to fit into at the end of the compression stroke. This raises the ratio of uncompressed space to compressed space. The reason that you need a higher octane rated fuel is that gasses heat up when they are compressed, and the less volatile fuel is required so that it doesn't spontaneously combust before it's done being compressed.

[2004exrider]

you dont loose any CC's doing that. Even though the piston is a little taller its not going to be that big of a difference to the eye. So pretty much like he said, it is pushing the fuel air mixture into a tighter spot thus creating more power.

Jimmy

[F-16Guy]

A dome doesn't make a big difference compared to the overall amount of air the piston is moving, but it makes a huge difference when compared with the volume of the combustion chamber.