Can anybody explain extactly compression is? How you get the compression ratio? Why the compression on my 400ex is high? How compression increase power? What things could be done to increase compression?

 

Well first of all compression is compression. The only way u can go higher is change the piston to a higher comp. piston. Usalley the stock piston is a flat top piston, u get higher compression when the top of the piston has a dom and the bigger the dom the more compression. My stock compression was 9:5:1 and i put a new 12:5:1 piston in it and could tell a huge difference. The reason higher cmpression produces more power is because that the top of the piston is bigger which compresse's the fuel more and makes a bigger and stronger combustion in the cylinder which drives everything else faster whick produces more power. [img]i/expressions/face-icon-small-smile.gif[/img] Lance

 

what is the highest compression ratio i can get and still run 93 or 94 octane gas? I own a 400ex. Also, why is the 400ex compression high already?

 

I am not sure on a 400ex what the stoc compression is but i would say 11:5:1 would be the highest u should go if u still want to run pump gas. Otherwise any higher and u will have to run race gas 105 octane and that stuff is $5 a gallon average. [img]i/expressions/face-icon-small-smile.gif[/img] Lance

 

compression is the amount of volume being squished, so it is like putting people in a room or car say 8-1 would be 8 people in on vw bug well go to 11-1 and you get the picture, very ruff explaination but it has to do with squishing the air fuel mixture into a smaller space.

 

Theres something you guys are missing, or maybe I am looking too deap into it. He asked about Compression and Compression ratios. Two different things. They are related, but independant. Compression ratio is the ratio between the volume of air in the cylinder with the piston down, verse the volume when the piston is up. Roughly speaking(for example), on a 250cc motor, when the piston is down it holds roughly 250cc(not including the head space) of air, when it is at TDC, lets say that air is compressed to 25cc. that is a 10:1 compression ratio. Piston dome sizes and head dome size will change this. More piston dome, or a shaved head will raise compression and compression ratio. Compresson is the ability to squeeze the air without leaking. So a high compression ratio will usually create a lot of compression, but if the rings wear out, Compression can go down, but the ratio is still the same. I.E.-A 12:1 compression ratio can create 175 psi, or 100psi of compression.
Hope this clears some stuff up. I used nice round numbers for the effect of simplicity.

 

the stock ratio on my 400 is 9.1:1 i thought that was high compared to other quads. why is it so high, compared to others? what is the highest compression i can run on my 400ex. i want to stay with pumped gas, also will i see a big differnence?

 

seems about normal

 

yfz 450 comes with a 11.9:1 compression setup from the factory. 9:1 is about average for a stock atv.

 

the yzf does run on pumped gas right? Is the 11.9:1 biggest it can be before need to run race gas? Also if i put a differnt cam in that wont change compression will it?