Does Barometric Pressure and Humidity affect our cars more than other cars? - 3000GT/Stealth International Message Center
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Old 04-22-2003, 04:52 PM   #1 (permalink)
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Default Does Barometric Pressure and Humidity affect our cars more than other cars?

I was thinking about my problem the other day while talking to a body who has a highly modified 5.0 and he was talking about how the Humidity and Borometric Pressure (BP) affected his times. In my case I ran a 14.8 @ 96 when it was very low humidity and very high BP and about 40 degrees. Since I have been having problems it and when I made my best 15.6 @ 91 it has been very humid (90%) and the BP has been lower and the Temps have been up to 70 degrees. Will high humidity, low BP, and higher temps cause me to loose .8 on my ET and 5 MPH on my traps? Does the humidty and BP affect our cars more than other cars for some reason?

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Old 04-22-2003, 09:40 PM   #2 (permalink)
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I think it affects imports more than american made cars due to the smaller more compact engines and there high compression ratios. It does however have an affect on all cars no matter what make or model.
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Old 04-22-2003, 11:20 PM   #3 (permalink)
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to any tuners & real mechanics

urban myth: advancing timing one degree for every thousand feet elevation?
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Old 04-23-2003, 08:18 AM   #4 (permalink)
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The Mitsubishi ECU in our cars automatically advances timing by about 5 at higher altitude. No. Not a myth; read the service manual.

Humidity (water vapor in the air) does reduce the oxygen content but the effect, while real, is almost negligible. In fact, the cooling effect of the vapor on the intake charge probably compensates for the very slightly reduced O2 content.

Altitude always reduces the available oxygen per cubic foot of air (lower density, lower pressure). ALL engines are effected, regardless of country of origin, displacement, or compression ratio. Forced induction engines can compensate somewhat for the reduced O2 content by increasing boost.
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Old 04-23-2003, 08:30 AM   #5 (permalink)
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Thanks for all the info. It answered half of my question. But will the changes in Humidity, BP, and Temps make the difference that I had between my best time and what I am running now?

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Old 04-23-2003, 09:06 AM   #6 (permalink)
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Quote:
Originally posted by Jeff Lucius
Forced induction engines can compensate somewhat for the reduced O2 content by increasing boost.
Does this mean that you can run slightly higher boost in high altitudes without more knock, or will it knock at the same psi in both situations?
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Old 04-23-2003, 12:42 PM   #7 (permalink)
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Without forced induction, engines tend not to knock as readily at higher altitudes because the intake charge is less dense. That is why 93 octand gas is sold in Ohio (600' asl), 91 octane gas in Denver, CO (5-6000' asl) and 87 octane gase is sold in Keystone, CO (10,000' asl).

With forced induction, you have to consider what the "boost" really is. A 10 psig (psi gauge) boost setting means that the manifold pressure is 10 psi higher than the atmospheric pressure. At sea level, 10 psi of boost means a total manifold pressure of 14.7 psi plus 10 psi or 24.7 psia (psi absolute). In Denver, CO, where normal atmospheric pressure about 12 psi, 10 psi boost means a total manifold pressure of only 22 psia.

So if all else is equal, the same engine with the same boost settings at sea level and in Colorado, the tendency to knock should be less in Colorado because the intake charge is less dense (less real pressure so charge is less dense).

In Colorado, we need 2.7 psi of boost to just be at the same intake charge density as an engine with 0 boost at sea level. And that's what I mean about a forced induction engine can compensate for the thinner air by increasing boost. Normally aspirated engines just have to live with it.
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Old 04-23-2003, 03:59 PM   #8 (permalink)
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Well that seems like good news to me up here in Utah. I can run a few higher psi according to the gauge and not knock where as somone at sea level reading that much psi would be knocking.
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