Air molecules (nitrogen, oxygen, etc.) at altitude are less compact (more spread-out) so, doesn't this effect the compression on our engines?

If I have a 12:1 compression piston, what will the compression be at 4000' elevation where I usually ride?

If it goes down by 12 % (4% / 1000 feet is common de-rate for a boiler burner) I am now at 11:1 (12X.88)

Does this math work? I am just guessing here.

Or, is compression a ratio to outside pressure and the only loss is the loss in the number of air (and specifically, Oxygen) molecules available for combustion?

Thanks for your input.

 

The compression ration is the total volume of the cylinder and head space when the piston is at bottom dead center, Divided by the total volume of the cylinder and head space when the piston is at top dead center. So the answer to your question is no. Altitude does not affect the compression ratio. What you are thinking about is effective compression. Effective compression is the amount of pressure that is actually made by the piston and cylinder during the compression stroke. The effective compression will decrease when you go to higher altidudes from lower ones. For the density reasons that you stated.

 

Mudrush:

Sounds good to me. Since I will have a lower effective compression will I still need to run race gas with a 12:1 piston? Does effective compression effect pre-detonation?

Thanks

 

Yes, I would keep using race gas unless i was willing to test it extensively and retune as needed. You may be able to go to a lower race gas octane rating and still be ok. If you are familar with doing this. Try a little pump premium mixed one to one with some 110 octane to get about 101 octane. See if it spark knocks, . Be careful and pay close attention, Ease into it and see if you can hear any noise like marbles in a tin can or anything else. Also feel the engine while you ride it. If it hesitates or skips or does not run right at high rpms it may be detonating. Don't do this much at all. Detonation will murder a head gasket and piston. I'm sure it would be risky to run a 12:1 compression below 100 octane. If your ignition timing is jacked up high you may not be able to use 100 octane. It'll take some testing to be sure. You'll have a richer mix anyway, which will also help prevent detonation.

Any change in the output of an engine can affect pre-ignition. I would say that a reduction in effective compression can lower the chance of pre-ignition but, most of the time pre-ignition is caused by a hot spot in the combustion chamber like a carbon chip or too hot of a spark plug tip. Effective compression does affect the point when detonation can occur. If you lower the compression you will lower the chance of detonation. I don't know what type of fuel you are using. If you are using 104 octane you may want to stick with it. or mix it at 2 gals of 104 to 1 gal. of 93 pump gas to get about 100 octane. Just my $.02 Here's a link that may help.

http://www.turbofast.com.au/racefuel10.html

 

Effective or dynamic compression is affected by altitude. It is hard to predict what octane you can get away with when you go up in altitude because it is a non linear relationship. Your dynamic compression will drop some and result in being capable of running a lower octane, but how much is difficult to calculate. Your probably better off staying on the safe side and running a high octane. Detonation is difficult to detect with a loud exhaust. The best way to check for detonation is with a sensor. Since this is hardly practical you can find a steep hill and run up in a high gear lugging the motor at a fairly low rpm and listening for the "marbles in the engine" sound.