Compression ratio question

[erik6767]

ok couple questions

say when guys mod their 4 strokes, theyll put a 12:1 piston in or something.. stock banshees is 6.5:1, how(if possible) is that converted into psi? when talkin about different domes, we talk abotu psi's, whats the difference?
like why the 2 measurments for compression... is there a reason, or are they not the same ? why the 6.5:1 and 130psi *whatyever stock is?

[zillaguy89]

Well...I'm gonna try and answer this one for ya eric....I hope I can get what your after here...

that "compression ratio" is the relationship between the volume of air entering the cylinder, and its volume after the piston compresses the mixture. so your 4 stroke at 12:1 means that your volume of air is 12 times smaller after compressing it than that same intake charge was before compressing it. On a 2 stroke there really are 2 different compression ratios, because a 2 stroke starts to compress the mixture before it gets into the "combustion chamber". the 6.5:1 ration for the banshee is what they call "static compression ratio" on a 2 stroke.

I dont think its possible to take a PSI number, and come up with your comprssion ratio, or vice versa. THat is because the density of the air has a LOT to do with a true PSI number. Explanation: Take your banshee, hook up a compression gauge, go to Miami and take a compression test....lets say you get 145 PSI. Now dont even start your bike, but toss it in the truck and drive to Denver and test your compression again, you'll end up with a smaller number, say 130 PSI. The compression ratio of your bike didn't change, but the air in denver is less dense than in Miami, so effectively you need more of the "denver air" to get back up to 145 PSI, but you cant get more air because you can't suck in any more volume.

I probably just confused the holy hell out of everyone....haha but I hope this helps, and if I'm worng, someone please correct me!!!

[Ed Gein]

6.5:1 is the compression ratio, 130 PSI is the compression in PSI. Compression ratio: Volume of cylinder at BDC to Volume at TDC ( Bottom Dead Center, Top Dead Center) With a smaller dome, the mixture has less of an area to compress, making the pressure greater, which will make the combustion more volatile, meaning more power to the crank.......Make sense now?

[MysticMotorsports]

6.5:1 on a banshee refers to the "uncorrected" compression ratio and has NO effect on what octane fuel should be used! a 4 stroke does not have a uncorrected compression ratio becuase it does not have ports in the side of the cylinder. The more important reading is "corrected compression" which is volume at TDC vs. BDC! I could take a engine and raise the exh. port and the cranking psi WILL go down but the comp. ratio will stay the same.

[erik6767]

zillaguy- thanks for dumbing it down a bit, that definatley made sense, i get it now

thanks for the responses ed and mystic too, ive always wondered about that