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?
1999 Mitsubishi 3000GT SL
-PMP's Intake Kit W/K&N Filter
-Borla Cat Back Exhaust System
-Custom Test pipe
-Custom Leather interior
1994 Mitsubishi 3000GT DOHC - TOTALED AS OF 1/27/04
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.
The Mitsubishi ECU in our cars automatically advances timing by about 5º 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.
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.
The AutoGuide.com network consists of the largest network of enthusiast-owned enthusiast-operated automotive communities.
AutoGuide.com provides the latest car reviews, auto show coverage, new car prices, and automotive news. The AutoGuide network operates more than 100 automotive forums where our users consult peers for shopping information and advice, and share opinions as a community.