Post by rudycon on Jul 18, 2017 15:58:37 GMT -5
Reading is for sucks. Gimme the summary:
Today's fuels have a vapor point that is so low that it doesn't take much heat to get them to boil. Altitude makes about a 20% in 4000' difference in vapor pressure. ethanol/non-ethanol makes about a 10% difference in vapor pressure. Temperature of fuel from 60-180 degrees is a 133% difference in vapor pressure. Easiest thing to change is to run non-ethanol fuel. Biggest bang for buck is to figure out how to keep fuel from soaking up engine heat. It isn't the old carb or the "weak" fuel pump. It is the fuel TEMP that matters the most.
I love science and math with real world examples. Gimme the details:
I don't remember my Grandpa telling tales of woe about the time he vapor locked on Cadillac Hill.
Driving carbureted rigs in these ethanol enhanced fuel days is tougher than in the old days. Why?
First of all, what is the effect on vapor pressure for alcohol/gasoline blends?
Did you know there is a 1 point Reid Vapor Pressure (RVP) exemption for 10% and now 15% ethanaol fuel? ref:
www.ethanolrfa.org/wp-content/uploads/2015/09/RVP-Effects-Memo_03_26_12_Final.pdf
So ethanol in gasoline increases vapor pressure over good old motor gasoline until you get above about 50% ethanol. Don't ask me why it goes down better'n straight gasoline above 50%. Find a chemist. I'm just a Jeep guy.
Denver area has a summertime RVP maximum of 7.8 psi (good old Pounds per Square Inch, scew the metric system) to reduce pollution from evaporating gasoline in the summer. I THINK the 10% ethanol 1 point is tacked onto this so the vapor pressure could be as high as 8.8 PSI. Find a lawyer. I'm just a jeep guy.
Great. So ethanol laced fuel boils off sooner. What does this mean to me?
Check out this motor gasoline conversion chart to show actual PSIa of motor gasoline based on RVP over temp. Focus on the 8 and 9 RVP lines. The hotter the fuel, the higher the actual vapor pressure.
www.jmcampbell.com/tip-of-the-month/2016/02/correlations-for-conversion-between-true-and-reid-vapor-pressures-tvp-and-rvp/
What is PSIa? "a" Is for one "atmosphere" of pressure. Sea level. Great for Florida. Not so great for Colorado high country. We'll make an adjustment for that in a minute.
Use the RVP 9 line to get an idea that in the cool of the garage, summer ethanol colorado fuel has a vapor pressure of only 6 psi, but get it up to 180F where your coolant is running and it is about 30 PSI!
Great. So hot fuel boils at lower pressures. No duh.
Ever run your car on "fumes" and coast into the gas station. yeah. kinda like that. (Lucky you. I had to push it the last 100 yards to the pump!) If the vapor pressure exceeds the atmospheric pressure in our open fuel systems, the fuel will boil into vapor. Fumes. So can we calculate if/when we will vapor lock? Well sure! Now I'm talkin' basic back-of-the napkin calculations here. You braniacs can debate the cooling capacity of the venturi on a W.O. and it's ability to keep the fuel in the bowl a liquid at trail and highway speeds and other "Bill Nye Science Guy" stuff. I'm just a jeep guy.
Thirsty Dirt ranch is about 8000'. My house is about 9,000 feet. Lots of trails are above 6,000 feet. Atmospheric air pressure is a function of how much air you have above you pressing on you. The higher you go, the less pressing from above, but it also tends to get colder which increases air density a little bit too which "narrows the gap" of the change as we drive up Mt.Princeton to about 12,000 feet. Here is a link to a calculator to figure out the air pressure at various ambient temps and altitudes. You can set it to psi too. Well there ya go!
www.mide.com/pages/air-pressure-at-altitude-calculator
At the Thirsty Dirt Ranch, the air pressure on a nice hot 80 degree day is 11.5psi. Good news for our gasoline that boils at 110 degrees F at Mikes' place. No problems, eh?
What if we drive up Mt. Priceton? First of all, it is colder up there. Call it 60F and you can't drive to the top. Say you stop at around 12,000 feet. The air pressure in psi is now only 9.36psi. Whoa. Getting pretty close to that RVP number, eh? back to the actual psi at temp for motor gasoline chart...Let's see... OK, so the boiling point is lower. but not much. Now fuel boils at just 95 degrees. What if we "get the good stuff" non-ethanol fuel down in Nathrop? Takes an RVP point off. I don't actually know the RVP of the good gas in Nathrop. They are in an EPA attainment area and might be able to run higher RVP fuel than the 7.8 we run the front range in summer. But let's just say it is the same 7.8 RVP so the depot doesn't have to stock different blends. That would kick the boiling point back up a little bit on the chart. yep. maybe 10 degrees hotter. Back up to 105 degrees.
OK great, Rudy get to the point on vapor lock. How do I beat it? I've been collecting wooden clothes pins from garage sales for 2 years and putting them on my fuel lines and I STILL vapor lock when we stop to take pictures and stuff in the mountains.
So altitude doesn't have as large of an effect as you might think. From 8000 to 12000 feet only changes the vapor pressure (Assuming some summer time "normal" temps") about 1.69 psi.
Just switching from ethanol laced fuel to pure gasoline almost makes up for 4000' of less air! What REALLY matters is how hot your fuel is. A good goal would be to try to keep your pump, lines, and carb under 100 degrees F. That way, you should be able to drive most of the places people drive. Some good news! Put your hand on your running carburetor. The venturi effect and the vaporizing fuel cools things off! Lucky us! Until we shut down and the engine heat soaks into the carb.
There are 5 places I can think of where you can drive above 13,000 feet in CO. Dead ends to Mt. Evans and Pike's peak summits. Plus Mosquito, Imogene and Argentine passes. If you want to take your carburetor car up there, use good gas and keep the fuel temps under 90 degrees. Fans, line routing, heat shields, heat shields in the form of wooden clothespins...whatever you want to try.
HOT STOP!
"We stopped at the top of the pass and now I can't get 'er restarted! I think my fuel pump is broken!" Hold up there hop-a-long. Relax. The heat from the non-running engine and no cooling fan has soaked into your carb and fuel pump pushing the fuel over it's vapor point = vapor lock! Soak a rag in water and wrap it around your fuel pump and carb. Get that fuel system cooled down and see if you "fixed it!"
Now you can use your clothes-pins to hold up your dainties on the line again.
Today's fuels have a vapor point that is so low that it doesn't take much heat to get them to boil. Altitude makes about a 20% in 4000' difference in vapor pressure. ethanol/non-ethanol makes about a 10% difference in vapor pressure. Temperature of fuel from 60-180 degrees is a 133% difference in vapor pressure. Easiest thing to change is to run non-ethanol fuel. Biggest bang for buck is to figure out how to keep fuel from soaking up engine heat. It isn't the old carb or the "weak" fuel pump. It is the fuel TEMP that matters the most.
I love science and math with real world examples. Gimme the details:
I don't remember my Grandpa telling tales of woe about the time he vapor locked on Cadillac Hill.
Driving carbureted rigs in these ethanol enhanced fuel days is tougher than in the old days. Why?
First of all, what is the effect on vapor pressure for alcohol/gasoline blends?
Did you know there is a 1 point Reid Vapor Pressure (RVP) exemption for 10% and now 15% ethanaol fuel? ref:
www.ethanolrfa.org/wp-content/uploads/2015/09/RVP-Effects-Memo_03_26_12_Final.pdf
So ethanol in gasoline increases vapor pressure over good old motor gasoline until you get above about 50% ethanol. Don't ask me why it goes down better'n straight gasoline above 50%. Find a chemist. I'm just a Jeep guy.
Denver area has a summertime RVP maximum of 7.8 psi (good old Pounds per Square Inch, scew the metric system) to reduce pollution from evaporating gasoline in the summer. I THINK the 10% ethanol 1 point is tacked onto this so the vapor pressure could be as high as 8.8 PSI. Find a lawyer. I'm just a jeep guy.
Great. So ethanol laced fuel boils off sooner. What does this mean to me?
Check out this motor gasoline conversion chart to show actual PSIa of motor gasoline based on RVP over temp. Focus on the 8 and 9 RVP lines. The hotter the fuel, the higher the actual vapor pressure.
www.jmcampbell.com/tip-of-the-month/2016/02/correlations-for-conversion-between-true-and-reid-vapor-pressures-tvp-and-rvp/
What is PSIa? "a" Is for one "atmosphere" of pressure. Sea level. Great for Florida. Not so great for Colorado high country. We'll make an adjustment for that in a minute.
Use the RVP 9 line to get an idea that in the cool of the garage, summer ethanol colorado fuel has a vapor pressure of only 6 psi, but get it up to 180F where your coolant is running and it is about 30 PSI!
Great. So hot fuel boils at lower pressures. No duh.
Ever run your car on "fumes" and coast into the gas station. yeah. kinda like that. (Lucky you. I had to push it the last 100 yards to the pump!) If the vapor pressure exceeds the atmospheric pressure in our open fuel systems, the fuel will boil into vapor. Fumes. So can we calculate if/when we will vapor lock? Well sure! Now I'm talkin' basic back-of-the napkin calculations here. You braniacs can debate the cooling capacity of the venturi on a W.O. and it's ability to keep the fuel in the bowl a liquid at trail and highway speeds and other "Bill Nye Science Guy" stuff. I'm just a jeep guy.
Thirsty Dirt ranch is about 8000'. My house is about 9,000 feet. Lots of trails are above 6,000 feet. Atmospheric air pressure is a function of how much air you have above you pressing on you. The higher you go, the less pressing from above, but it also tends to get colder which increases air density a little bit too which "narrows the gap" of the change as we drive up Mt.Princeton to about 12,000 feet. Here is a link to a calculator to figure out the air pressure at various ambient temps and altitudes. You can set it to psi too. Well there ya go!
www.mide.com/pages/air-pressure-at-altitude-calculator
At the Thirsty Dirt Ranch, the air pressure on a nice hot 80 degree day is 11.5psi. Good news for our gasoline that boils at 110 degrees F at Mikes' place. No problems, eh?
What if we drive up Mt. Priceton? First of all, it is colder up there. Call it 60F and you can't drive to the top. Say you stop at around 12,000 feet. The air pressure in psi is now only 9.36psi. Whoa. Getting pretty close to that RVP number, eh? back to the actual psi at temp for motor gasoline chart...Let's see... OK, so the boiling point is lower. but not much. Now fuel boils at just 95 degrees. What if we "get the good stuff" non-ethanol fuel down in Nathrop? Takes an RVP point off. I don't actually know the RVP of the good gas in Nathrop. They are in an EPA attainment area and might be able to run higher RVP fuel than the 7.8 we run the front range in summer. But let's just say it is the same 7.8 RVP so the depot doesn't have to stock different blends. That would kick the boiling point back up a little bit on the chart. yep. maybe 10 degrees hotter. Back up to 105 degrees.
OK great, Rudy get to the point on vapor lock. How do I beat it? I've been collecting wooden clothes pins from garage sales for 2 years and putting them on my fuel lines and I STILL vapor lock when we stop to take pictures and stuff in the mountains.
So altitude doesn't have as large of an effect as you might think. From 8000 to 12000 feet only changes the vapor pressure (Assuming some summer time "normal" temps") about 1.69 psi.
Just switching from ethanol laced fuel to pure gasoline almost makes up for 4000' of less air! What REALLY matters is how hot your fuel is. A good goal would be to try to keep your pump, lines, and carb under 100 degrees F. That way, you should be able to drive most of the places people drive. Some good news! Put your hand on your running carburetor. The venturi effect and the vaporizing fuel cools things off! Lucky us! Until we shut down and the engine heat soaks into the carb.
There are 5 places I can think of where you can drive above 13,000 feet in CO. Dead ends to Mt. Evans and Pike's peak summits. Plus Mosquito, Imogene and Argentine passes. If you want to take your carburetor car up there, use good gas and keep the fuel temps under 90 degrees. Fans, line routing, heat shields, heat shields in the form of wooden clothespins...whatever you want to try.
HOT STOP!
"We stopped at the top of the pass and now I can't get 'er restarted! I think my fuel pump is broken!" Hold up there hop-a-long. Relax. The heat from the non-running engine and no cooling fan has soaked into your carb and fuel pump pushing the fuel over it's vapor point = vapor lock! Soak a rag in water and wrap it around your fuel pump and carb. Get that fuel system cooled down and see if you "fixed it!"
Now you can use your clothes-pins to hold up your dainties on the line again.
...... I can't say I've never had vapor lock, perhaps I have and if I did it was a minimal issue. I have indeed seen rigs with the dreaded issue, in fact my dad used to tell of my grandfathers 39 Mercury on a hot day giving trouble and grandpa cutting an orange in half and smashing one half on top the fuel pump to quell the problem. At the 2015 FCT I watched in disbelief a gentleman empty a bottle of cold water on a fuel pump and it worked!.....
