Compression Ratio Theory

[Gusto]

I have heard that with higher alt (>3500ft) that you loose your compression ratio. Say you run a 11:1, at about 4k your compression ratio is now 10:1?

How true is this?

And does someone know how/when the ratio does changes?

[YZROOSTINYA]

i can see how this would be, higher altitude the air becomes less dense. that why planes have superchargers.

less air in the cylinder to compress means less compression. dont know if this is true. but i can see how it would be

[mysticfalcon]

rocky mountain has a compresion guide for banshees at different altitudes so id have to say thet yes the elevation does lower you compresion. its on the page with the cool heads if anybodys curious

[VegasEx'r]

You are right that less air=less power, so at higher altitudes an engine produces lower HP, but I don't think that the CR decreases as you go up in elevation. CR=cylinder volume + combustion chamber volume/combustion chamber volume, so it can't change. But since less air is placed into the cylinder, less power is made.

[Burden7]

I agree with Vegas. The compression ratio doesn't change, but the air/fuel ratio does. It just feels like you are loosing power at higher altitudes because the air/fuel mixture has changed. Just like you have to rejet for temp changes, you also have to rejet for altitude changes as well.

[CAIN730]

vegas is correct but a jetting change will not get back all of the power.

[Burden7]

Cain, so you're saying that all of the people that happen to live in higher altitudes are SOL when it comes to getting maximum power out of their quads? Please elaborate on why jetting for higher altitudes will only bring back some of the power.

[Texan32]

the higher the altitude, the less dense the oxygen gets. Oxygen is what the engine needs to produce power. People in higher altitudes must (well, not must )run a leaner jetting than those closer to sea level. Yes, correcting the jetting will "bring back some of the power". Max power is produced my optimum jetting. If your jetted too rich, you not making the full potential power.

Gusto, i think you are mistaking two similar things. Compression ratio and compression. The ratio will NOT change unless you physically alter the motor (thinner gaskets, biggert bore, piston,...). Compression is what is measured with a guage that screws into the spark plug hole. It measures PSI (pounds per square inch). Compression CAN change. If the desnity of the air changes, compression changes. Thick air makes more compression & thin air makes less compression. (why do you think the pro 2 stroke riders carry several differnt size domes for their heads....different weather conditions brings differnt air)

Burden, YES. Those living at higher altitudes are pretty much out of luck. Unless you can somehow rig up a O2 tank (i guess sorta like NO2) to pump oxygen into the intake.

[frozenh2o]

If you don't rejet, you'll loose at least 2% of your power for every 1000 ft of altitude. Doesn't sound like a lot, but think how much you have to spend to get a few more percent power with engine mods. If you rejet correctly, you can cut that power loss in half. To do it right, you should reduce your jet size by 1% for about every 1200ft of altitude.

[josh]

i think that theory is pretty good, but on the other side the compression comes from the piston and rings moving that air tight into the head, so i would say you do loose a little compression even if you do jet because of the thinner air at higher altitudes, when we go camping once a year in a certain spot we go from around 1,800 ft to 7500 ft, big diff, but we have stock bikes and we don't jet them and i would say there is a little power loss, maybe slightly noticeable, but not overly, so i guess in theory you are right about the thinner air and compression lossage, but i am not exactly sure about facts, it would be a good thing to look into tho