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Old 08-03-2010, 10:45 AM
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Rzeppa Rzeppa is offline
Cruise Moab Chairman
Join Date: Aug 2005
Location: Kittredge CO, USA
Posts: 6,668

Originally Posted by MDH33 View Post
A great hypothesis was brought up regarding why so many rigs vapor locked. Could it be the high Ethanol content of the fuel we have today? Possible, I think. Probably vaporizes at a lower temp than "real" fuel would.
First Martin, thank you for posting those photos!

Second, I decided to do a little investigating about the EtOH theory. Turns out that the hypothesis is supported by the chemistry. From Wiki, here is a graph of EtOH vapor pressure versus temperature. Remember, that the definition of boiling is when the vapor pressure of a liquid is equal to the ambient pressure.

From that graph, we can see that the temperature at which the vapor pressure equals one atmosphere (760 mmHg, or nominal sea level), is about 75C. Obviously, we were much higher than that, I would guess around 500 mmHg. For reference, the atmospheric pressure in Denver is about 620 mmHg.

Heptane is a component of gasoline so I looked it up. It is a much larger molecule, and thus has a much lower vapor pressure. At sea level it boils at a little over 98C.

Here is a good link to the Wiki page on vapor lock. The explanation is spot-on.
Jeff Zepp
Kittredge CO USA
1971 & 1976 FJ40s, 1978 FJ45, 1987 FJ60
Rising Sun 4WD Club, TLCA #4063
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