How does Altitude affect Compression?

I live at 5,600 feet elevation, here in Albuquerque, New Mexico. I was told that I loose a compression point due to that. Just wondering if that was true or not and why? Motor is 13.1, which up here if the above is true puts me at 12.1????

Neal

The compression ratio will still be the same, BUT, because of the less dense air at high elevation, the amount of air/fuel in the combustion chamber will be less, causing lower cylinder pressure.

Because of lower cylinder pressure your octane requirements will go down, so your 13:1 compression engine will "act" like an engine with lower compression even though the actual compression ratio (swept volume/combustion chamber volume) will still be the same.

[QUOTE=Z28COnrad] the amount of air/fuel in the combustion chamber will be less, QUOTE]

Good post, just wanted to make one correction to the "air/fuel" > oxygen/fuel ratio

maybe you'll need to go to 14 to 1! lol...

Hey Neal. No need to worry with that stroker of yours anyway. But back to your question, no, you don't lose compression. We just get killed by the lack of oxygen in the air cause we're so high up here. Especially you since your N/A. You put the spray on that thing yet? Oxygen in a bottle, so make up for the high altitude. Haha! I hang with Greg by the way (Blue turbo LX guy at the track from time to time). Later.

Ken R.

compression can not be changed. ever (unless you change engine parts like heads or pistons)

its just a ratio of volume at TDC : BDC

cylinder pressure is the thats ALWAYS changing (pretty much everything changes it. air temp, coolant temp, rpm, altitude, random air turb, air density, etc)

The compression ratio will still be the same, BUT, because of the less dense air at high elevation, the amount of air/fuel in the combustion chamber will be less, causing lower cylinder pressure.

Because of lower cylinder pressure your octane requirements will go down, so your 13:1 compression engine will "act" like an engine with lower compression even though the actual compression ratio (swept volume/combustion chamber volume) will still be the same.


Thanks. I run VP103 init when racing, but this weekend tried some VP110 leaded fuel and I saw a drop in MPH. 1 to 2mph on average. most of the time it runs 121 to 122 and I was down at 119 to 120. High altitude sucks period. I want to see what this thing runs at a sea level track.

Neal

Thanks. I run VP103 init when racing, but this weekend tried some VP110 leaded fuel and I saw a drop in MPH. 1 to 2mph on average. most of the time it runs 121 to 122 and I was down at 119 to 120. High altitude sucks period. I want to see what this thing runs at a sea level track.

Neal


Hope you dont run leaded fuel through a FUEL INJECTED engine with O2 sensors!

Hope you dont run leaded fuel through a FUEL INJECTED engine with O2 sensors!

The computer is FAST so I'm ok.

O2 sensors still hate Lead, FAST or Stock. And if im not mistaken I want to say injectors dont like lead either. Not positive on the injectors but O2s for sure dont like real leaded fuel.

I haven't seen any prolbems with injectors due to leaded fuel.... I've run a lot of C16 through my setup, and the injectors are still healthy. There are a lot of 1,000HP EFI engines running leaded fuels.

I haven't seen any prolbems with injectors due to leaded fuel.... I've run a lot of C16 through my setup, and the injectors are still healthy. There are a lot of 1,000HP EFI engines running leaded fuels.

Thanks Fred, wasnt sure on the injectors, thought maybe i heard that at one time, N/M.

Hope you dont run leaded fuel through a FUEL INJECTED engine with O2 sensors!

Nothing wrong with this if you’re using DENSO o2's as they designed too run in this environment and they tolerate it better than any stock sensor.
For LT1’s with stock ecm's give denso # 234-4012 a try ;)

How do the wideband NTK L1H1 sensors that the FAST system use hold up to leaded fuel? I used Leaded race fuel with the stock sensors, and they were junk within a few passes at the track. I'd hate to do that to an expensive ($300) NTK sensor

The Denso O2's are "better" than the stockers, but even Denso only claims "by a factor of 2" with regard to lead tolerance. One of the auto rags tested them, and the first sample failed immediatley on exposure to leaded fuel. The other sample fared better. I think a stock O2 sensor is rated at 50 hours lead exposure, and if you think about it, thats a lot of 1/4-mile passes.

I still have the stock O2 sensors in my exhaust, mainly to plug the bungs. They are monitored by the stock PCM, and one of them is still working in what seems to be a correct way, after a lot of leaded fuel has been run through the system. The other one seems to read on the "low side". I don't use the sensors, since the main engine control via the MoTeC ECU is open loop.

The wide-bands that I have seen are reported to have a life of 500 hours. In one heavy leaded fuel application (C16), the wide band sensor would not last a full racing season, and the car was not an "every weekend" car. Maybe 50-60 passes and dyno pulls.

When you think about it, a high end shop with an engine and/or chassis dyno is going to be doing a lot of pulls with leaded fuels... next time I'm in a shop, I'll ask how long their sensors last.