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  #1 (permalink)  
Old 04-06-2010, 07:16 AM
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Question on humidity and motorcycle running temperature

Ok two questions but I am just learning about air cooled engines.

In a differnt thread I read this;
Quote:
...Its a hair on the rich side, which is ok with me because the Boston humidity really sucks and the bike gets hotttttttt....
I had thought that the water in the air would help the engine to transfer heat better not worse ands the added H20 in the combustion would keep that a bit cooler as well......am I bass ackwards on this?

#2 what is the the ticking sound I hear when a hot engine is cooling?
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Old 04-06-2010, 07:56 AM
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Humidity is moisture in the air, which in turn takes up air space.
By taking up air space it makes for a richer condition in which the fuel helps cool the motor.
Cool dense air makes for good horsepower, but humidity absolutely kills it.

Make sense???

As far as the crackling and popping on cooldown, I would say this is due to the cooling and constricting of the growth of parts after heating.

I'm not positive of this cooling theory, so don't take it as the gospel.
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Old 04-06-2010, 09:28 AM
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Clear as mud. If the water in the air makes it harder to atomize the fuel then shouldn't the same volume of air hold less fuel and the humidity result in a leaner mixture?

And shouldn't the water aid in heat exchange at the colling fins and help keep the engine cooler?
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Old 04-06-2010, 09:41 AM
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Quote:
Originally Posted by vision View Post

I had thought that the water in the air would help the engine to transfer heat better not worse ands the added H20 in the combustion would keep that a bit cooler as well......am I bass ackwards on this?
Hot air, and humid air are less dense

Which means that there is less of it (air) in the A/F mixture

So (all other things equal) you're bike is gonna run richer on a hot day at the beach - than on a cold night in the desert

It's not the water in the air that's cooling - it's the absence of air that has been displaced by the water
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Old 04-06-2010, 09:46 AM
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OK got that now.

But then why would a guy in humidity want his engine running on the rich side is the humidty will just make it richer?
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Old 04-06-2010, 09:48 AM
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Quote:
Originally Posted by vision View Post
If the water in the air makes it harder to atomize the fuel then shouldn't the same volume of air hold less fuel and the humidity result in a leaner mixture?

And shouldn't the water aid in heat exchange at the colling fins and help keep the engine cooler?
The motor likes to run on Air/Fuel - not Water/Fuel

The more water (humidity) in the A/F mix - the less of a combustible charge is gonna be produced

It's not so much that the water is harder to atomize

It's more that the water is taking up valuable space in the A/F mix that could be better utilized by air
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Old 04-06-2010, 10:38 AM
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Fuel is a solid, humidity is a solid, (moisture in the air) and air is--------well, air is air.
If you look at humidity as a solid fuel, only non combustible, then you can understand how it takes up air space.
Normal air fuel ratio is just over 14-1. This means 14 parts air to 1 part fuel. (I strongly suggest you never try tuning to this ratio.) 12.8:1 to 13.4:1 is a good range.
Anyway, back to the subject.
As a fabricator you can probably relate this way;
Take your O/A torch and adjust the flame to get rid of the soot and have a consistent yellow flame. Hold the tip approximately 2 inches off a piece of 10 ga. sheet.
How long does it take to get cherry red?
This is what happens inside the engine when there is excess humidity in the air. Makes the air fuel ratio junk.
Now let's say we can magically take moisture out of the air. What you get here is more air space which in turn raises your A/F ratio.
To represent this, start adding more oxygen to your flame. Adjust the flame to a normal blue welding flame. Hold it 2 inches from your metal and see how long it takes to get cherry red.
This is what happens to an engine with excess humidity,
Hence, more humidity, less A/F ratio, equals cooler burn, equates to cooler engine temp.


I sure hope this make sense because transfering from brain to screen is a tough deal for me.
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Old 04-06-2010, 12:18 PM
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Quote:
Originally Posted by vision View Post

But then why would a guy in humidity want his engine running on the rich side is the humidty will just make it richer?
IMHO, intentionally tuning rich in the hopes of lower operating temps is a farce

Although technically valid from a physics stand point - Running fat is ONLY cooler in relation to running excessively lean - which is VERY hot



A properly tuned A/F mix is the best case scenario for your motor

And adding more fuel to that optimum mix is not gonna lower your operating temps appreciably

It's only gonna compromise the performance
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Last edited by Merlin Hawaii; 04-06-2010 at 01:00 PM..
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