Gas > Vapor

[racer951]

I've searched a few times looking for info on when gas turns to vapor (what temp) but haven't had any luck. When pulling my fan shroud during the last race, there was a dense vapor swirling slowly from the carb (for visual pruposes it looked like dry ice). I assume this was the gas in the carb turning to vapor and I'd like to get an approximate temp on that. Definitely time for new rings at the least...

[ZTFab]

Found this on a Firefighting website...best info I was able to come up with ...hope it helps.....

Flash point The temperature at which a particular flammable liquid gives off vapors (vaporizes) and therefore can ignite. The flash point differs for each type of flammable liquid. Kerosene has a flash point of 110 degrees Fahrenheit. Gasoline has a flash point of -40 degrees. This means that at 110 degrees or higher kerosene gives off flammable vapors and can ignite. However, gasoline requires a temperature of only 40 degrees to vaporize to cause an explosion or fire. This means that when the temperature is freezing, gasoline still vaporizes and can cause an explosion and / or fire. At the same temperature, kerosene cannot ignite. Liquids such as gasoline with a flash point below 100 degrees are called flammable liquids. Kerosene and other liquids with a flash point above 100 degrees are referred to as combustible liquids.

Flammable range refers to the percentage of a flammable liquid, in its gaseous state to air to create an explosive mixture. This varies with different flammable liquids. Gasoline has a flammability range of 1.4 to 7.6 percent. This means it will ignite when there is 1.4 parts of gasoline mixed with 100 parts air. With this in mind, 1.4 percent is known as the lower flammable limit and 7.6 is the upper flammable limit of the flammable range. A product mixed with air below the low end of its flammable range is too lean to burn. A flammable liquid, which exceeds its upper flammable limit, is to rich to ignite. Ethylene oxide is extremely flammable. It has a flammable range of 3.6 to 100 percent. This means it can burn even if there is no air.

Gasoline has a narrow flammable range and is metered precisely in a vehicle's carburetor to obtain the desired flammable range. A vehicle will have trouble operating if the carburetor meters too much gasoline. This is referred to as a rich mixture, which is too concentrated for ignition by the spark plugs. Too little gasoline in a vehicle's carburetor is called a lean mixture, which is too diluted for ignition.

[racer951]

So in other words there's no way to guess at a temp from the visible vapors? The alternator was ripping the fan shroud off and I had dropped the fan belt just before and it was HOT, just don't know how hot.

Thanks for that info, although I've never seen it do that at any lower temp.

[ChuckH]

I don't think you need new rings, i have seen the same thing coming from carbs before while tuning a engine, it happens after you turn the engine off and open the butterfly's also, rings leak air naturally and you just saw them doing it and pushing out some air.

[racer951]

The reason I'm going to put in new rings is running without a fan belt for a time, I know it got hot. Thank's for the info though, seeing it for the first time in that situation made me thought it was a telltale sign of a certain temp or situation. Good to hear its normal.

[ChuckH]

Do a compression test to see if they need to be changed, overheating it wouldn't hurt the rings before anything else, things like a main bearing would go out 1st.

Overheating is more likely to warp a aluminum head before it hurts anything else.

my guess is that your engine is fine

[mike_hinson]

Cory,
A compression test is good, but you should consider a leak down test to check the rings. There happens to be an article in the latest Hot VW's mag on leak down testing.
Mike Hinson

[Ramsey_ElWardani]

With a VW Air Cooled; if you got it that hot, I'd be more concerned with the heads and pistons than the rings.

[5racer]

if anything you mite have losein a valve seat that is disaster if it comes loose

[rubberboy]

A compression test is what should be used. Leak down tests are used more for valve sealing issues.

[ZTFab]

A compression test is what should be used. Leak down tests are used more for valve sealing issues.

I'd have to disagree with that...a leak down test will indicate where the most compression will be lost whether it's a valve or the rings.....the most prominent leak will appear first and bad rings will definently cause excessive leak down into the crankcase......

[ntsqd]

Plus, with a leak-down test you can listen for where the air is escaping. Good, easy clue as to where the problem is.

[baja_driver]

The reason I'm going to put in new rings is running without a fan belt for a time, I know it got hot. Thank's for the info though, seeing it for the first time in that situation made me thought it was a telltale sign of a certain temp or situation. Good to hear its normal.


I would say every motor does this...you usually do not see it because of the air cleaner. but when you run a race motor..they are gonna get hot, and when you turn it off..one of the intake valves will be open some what...and knowing that heat rises....the left over exhaust in the chamber is gonna go some where if the exhaust valve is closed..and thats the way it comes out...a normal thing.

[rubberboy]

I know a leak down test can be used and will give you more accurate results all im saying is it would be easier and just as effective to use a compression test IMO.

[ZTFab]

I know a leak down test can be used and will give you more accurate results all im saying is it would be easier and just as effective to use a compression test IMO.


I can agree with that somewhat.....it is easier to test the engine with a simple compression test to check for uniformity and proper psi...but a compression test will only tell you what psi the cylinders are capable of creating, it will not tell you WHAT is causing low compression....ideally a compression test is performed first and if low readings are obtained than a leak down test is done to let you know what the cause is....

[rubberboy]

ideally a compression test is performed first and if low readings are obtained than a leak down test is done to let you know what the cause is....

Thats exactly why im saying a leak down test is unecessary in this case. I should have been more clear when i mentioned valve sealing issues i was referring to sealing issues in general but i was tired and also most of the problems i have come across have been with valves.