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I have a 65 FI car with all internals in the engine to original specs. I was driving yesterday and it was hot here in the Denver area. The combination of elevation and temperature makes starting an FI car a little tricky. i've been using standard premium gasoline and was considering using some race gas that I have from my vintage racer. My logic is that the higher octane fuel may have a higher vapor pressure and therefore less subject to the heat soak problems we get into with FI cars. The car is not pinging. The question is does higher Octane give better vapor pressure properties? I've also considered using aviation gas for the same reason. With the simple logic that if it works and is stable in an airplane at 10,000 feet then it ought to be stable here at 6000 feet. Any of you motor reseach experts out there have an opinion? Jerry
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Interesting subject
Jerry, I think what you actually want is gasoline with a low Reid vapor pressure. I believe aviation gas is limited to a maximum of something less than 7 psi. Some state laws also limit automotive gasoline to a max of 7, but not here in Alabama. I've read figures for the pump gas we get as high as 13 psi! But I don't think getting low vapor pressure gas will make the difference you want. I've tried the 4 psi racing gas and couldn't detect a difference.
Let me explain. I've done some experimenting here in Mobile to see if gasoline vapor pressure makes a difference with FI heat soak (percolation) problems. In theory, it should be a critical factor. In my experiments, it is not.
I haven't found a good cure for FI heat soak. None of the thick gaskets, insulators, fuels, additives, etc., that I've tried have eliminated percolation during hot restarts when the temperature is over 90 at sea level. The aluminum fuel bowl just absorbs too much engine heat after shut down. I haven't noticed any difficulty starting the cars, just very rough running for 10 to 20 seconds until cool gas fills the bowl. -
Interesting subject
Jerry, I think what you actually want is gasoline with a low Reid vapor pressure. I believe aviation gas is limited to a maximum of something less than 7 psi. Some state laws also limit automotive gasoline to a max of 7, but not here in Alabama. I've read figures for the pump gas we get as high as 13 psi! But I don't think getting low vapor pressure gas will make the difference you want. I've tried the 4 psi racing gas and couldn't detect a difference.
Let me explain. I've done some experimenting here in Mobile to see if gasoline vapor pressure makes a difference with FI heat soak (percolation) problems. In theory, it should be a critical factor. In my experiments, it is not.
I haven't found a good cure for FI heat soak. None of the thick gaskets, insulators, fuels, additives, etc., that I've tried have eliminated percolation during hot restarts when the temperature is over 90 at sea level. The aluminum fuel bowl just absorbs too much engine heat after shut down. I haven't noticed any difficulty starting the cars, just very rough running for 10 to 20 seconds until cool gas fills the bowl.Comment
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Re: Interesting subject
I have noticed a significant difference in different brands of gas with my fueler. On my way back from a convention a few years ago, I sopeed for Gas and lunch, did gas first and then lunch, and darn near couldn't get the thing out of the parking lot! changed back to usual brand and reduced the problem back to "normal" levels.Bill Clupper #618Comment
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Re: Interesting subject
I have noticed a significant difference in different brands of gas with my fueler. On my way back from a convention a few years ago, I sopeed for Gas and lunch, did gas first and then lunch, and darn near couldn't get the thing out of the parking lot! changed back to usual brand and reduced the problem back to "normal" levels.Bill Clupper #618Comment
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Re:Thats exactly the problem I've had.
It's the engineer in me looking for logical solutions. Your right, the Reid vapor pressure is what i'm trying to minimize. Don't laugh but I have tried mixing one pint of diesel per 10 gallons of gasoline and it really seems to help.( maybe it's placebo)Comment
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Re:Thats exactly the problem I've had.
It's the engineer in me looking for logical solutions. Your right, the Reid vapor pressure is what i'm trying to minimize. Don't laugh but I have tried mixing one pint of diesel per 10 gallons of gasoline and it really seems to help.( maybe it's placebo)Comment
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Re:Thats exactly the problem I've had.
Jerry's experiment jibes with reality. Modern gasoline blends have lower vapor pressure than sixties blends, and modern fuels are very similar regardless of octane number. The motivation for low vapor pressure is reduction of evaporative emissions.
Modern cars have no problem starting with lower vapor pressure fuels since the injectors create good atomization - much better, of course than a choked carburetor - for good cold starting.
The FI hot start problem has been blamed on too high vapor pressure, but since today's fuel's have lower vapor pressure than when they were new, I believe the primary cause must be something else.
DukeComment
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Re:Thats exactly the problem I've had.
Jerry's experiment jibes with reality. Modern gasoline blends have lower vapor pressure than sixties blends, and modern fuels are very similar regardless of octane number. The motivation for low vapor pressure is reduction of evaporative emissions.
Modern cars have no problem starting with lower vapor pressure fuels since the injectors create good atomization - much better, of course than a choked carburetor - for good cold starting.
The FI hot start problem has been blamed on too high vapor pressure, but since today's fuel's have lower vapor pressure than when they were new, I believe the primary cause must be something else.
DukeComment
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Re:Interesting... any suggestions on what *NM*
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Re:Interesting... any suggestions on what *NM*
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Re relationship between vapor p & boiling.
i wonder if you can have a low Reid vapor presure and a low boiling point. The problem we are trying to solve is fuel boiling after heat soak in the fuel meter. What can we do to increase or elevate the boiling point?Comment
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Re relationship between vapor p & boiling.
i wonder if you can have a low Reid vapor presure and a low boiling point. The problem we are trying to solve is fuel boiling after heat soak in the fuel meter. What can we do to increase or elevate the boiling point?Comment
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Re: Re relationship between vapor p & boiling.
There is no simple answer to your question. Gasolines are a mixture of hundreds of hydrocarbon species and their boiling temperatures vary from about 90 to 400 F. Reid vapor pressure represents and average for the blend.
Gasolines are characterized by their distillation fraction curves, which can be plotted or tabulated as the percent that reach their boiling points at a given temperature. For example, the ten percent point might be about 120, the fifty percent point 200 and the 90 percent point 300. Winter blends usually have a lower ten to fifty percent point than summer blends to aid starting, and the "low end" high volatility components must be limited in summer to meet Reid vapor pressure requirements.
Given the good fuel atomization of modern EFI engines, less low boiling temperature components are needed for good starting, particularly in summer temperatures. Other than distilling your own gas (the late Dale Pearman claimed he did this) to boil off the low end components, I don't have a solution to the problem. At the same time, I don't understand how it could be worse today than in the sixties, since sixties vintage fuels had higher Reid vapor and probably a lower temperature distillation curve at the 10 and 20 percent points.
DukeComment
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Re: Re relationship between vapor p & boiling.
There is no simple answer to your question. Gasolines are a mixture of hundreds of hydrocarbon species and their boiling temperatures vary from about 90 to 400 F. Reid vapor pressure represents and average for the blend.
Gasolines are characterized by their distillation fraction curves, which can be plotted or tabulated as the percent that reach their boiling points at a given temperature. For example, the ten percent point might be about 120, the fifty percent point 200 and the 90 percent point 300. Winter blends usually have a lower ten to fifty percent point than summer blends to aid starting, and the "low end" high volatility components must be limited in summer to meet Reid vapor pressure requirements.
Given the good fuel atomization of modern EFI engines, less low boiling temperature components are needed for good starting, particularly in summer temperatures. Other than distilling your own gas (the late Dale Pearman claimed he did this) to boil off the low end components, I don't have a solution to the problem. At the same time, I don't understand how it could be worse today than in the sixties, since sixties vintage fuels had higher Reid vapor and probably a lower temperature distillation curve at the 10 and 20 percent points.
DukeComment
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