Relative Humidity and Mixture Control.

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"I put in new Champion W16Y plugs last week before going on tour with the club. The car ran fine , the gav was closed, most times we drove between 40 & 50 mph. When I got home 300 mi. later I pulled the plugs and the porcelains were still white. The old plugs would usually show a bit of soot. A buddy suggested that maybe gasohol burns cleaner. Your thoughts?"

So I read this and started thinking...the guys running lean and driving fast. Burning gasohol which is nothing more than water absorbing alcohol put into gasoline to stretch out the Nation's gas supply as a lobbied concession to corn and other plant growers that does nothing for a low compression engine like our 'A's. It goes in as absorbed water and comes out as water vapor. A normal part of combustion. For every 1,000,000 btu's burned about 100 lb of water vapor is exhausted. No big deal.

What about Relative Humidity and the adjustable mixture controls on the carburetor? Starting with one standard cubic foot of air at 70 deg F. that weighs 1.087 lb per cu ft and dry.

Now lets say the relative humidity is 85% outside. Not that unusual for all the rain lately. That means that only 15% of that one cubic foot of air is available to mix with gasoline at a ratio of 14.7:1 by weight. And 85% of that one cubic foot of air is water vapor. Water vapor doesn't burn. It goes in as water and comes out as water. Water vapor does nothing toward making power in the engine but water vapor does displace gasoline in air.

Typically in 85% relativity humidity air the engine may stumble, bubble, spit, a bit and usually the driver twists open the adjustable gas control valve to the carburetor to compensate. And maybe even diddles with the advancement lever. BUT the mixture should be leaned out because only 15% of that one cubic foot of air is available to burn gasoline. The rest of that cubic foot is water vapor.

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[krswen]

Actually, 85% relative humidity doesn't mean that the incoming air to the engine is 85% water vapor. It means that the air is holding 85% of what it is capable of holding at that temperature and pressure. Typically the total amount of water vapor that is displacing usable oxygen is so small we don't even account for it in engine testing. It IS important for "normalizing" NOx emissions measurements however (not a concern with older "grandfathered" cars).

Ken

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"It means that the air is holding 85% of what it is capable of holding at that temperature and pressure. "

So I thought I said that by saying 'That means that only 15% of that one cubic foot of air is available to mix with gasoline at a ratio of 14.7:1 by weight. And 85% of that one cubic foot of air is water vapor."...and not available to carry fuel.

I gotta talk with a old Airforce guy who teaches flying. I think that's what he told me the other day at the aeroprot.

[krswen]

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At sea level and 15 degrees C (60 degrees F) fully saturated air only contains about 1.3% water vapor. At 85% humidity it would be 1.1%. The average Model A carb probably doesn't have the jets that close to optimum.

Ken