Lets talk octane versus boiling points

[747mopar]

I'm really curious to see what everyone has to say about this. I had vapor locking issues earlier this summer and was told to run high test (supposedly less prone to vapor locking), I insulated the fuel line, added a carb spacer and went back to the high test and am now having zero problems with vapor lock. I have been trying to find a FACTUAL chart, graph or any official study on boiling points versus octane simply because I want the facts. So does anyone have a link or some good sources for this because my searches show everything but that. Thanks

 

[texas69bee]

http://www.automotiveu.com/vaporlock.htm
http://www.automotiveu.com/SolvingOverheating.htm
http://www.madsci.org/posts/archives/aug99/934767548.Ch.r.html

 

[383man]

I can only tell you what I have seen. My 63 has a 440/493 in it I built and I use a dual plane Indy intake with an 850 DP. And up to 2 years ago I never had any problems. And the problems I have had were not vapor lock but it was fuel boiling in the carb bowls. I have seen it a good bit as my buddies 65 Coronet did it at Carlisle also. What happened as I was in his car also is that his car was not over heating but it was flooding. The fuel was boiling in the carb as it was boiling over into the eng out the carb bowl vents. I had to tell his son who was driving to keep the pedal on the floor to get it started and then to give it gas to keep it running until we got moving and got some air moving over the eng to cool the fuel. The car was only running 185 but it was hot under the hood as this cheap pump gas with all this ethanal will boil much easier then the older pump gas. I had to add a phenolic spacer and a heat shield on my 63 and then ran my gas line in the outer pass side wheelwell. I even added a cool can back on my 63 but have not had to use it yet. I have not had any vapor lock problems myself and as you know vapor lock is when you get no fuel at the carb as it is when the fuel wont flow and get to the carb because of heat and vapor in the fuel line. A good thing to help fight vapor lock is use an electric fuel pump back at the tank so fuel is being pushed up to the carb and not pulled under a vacum by a mech pump. Fuel under pressure being pushed by an electric pump wont go into vapor as easy as fuel being pulled under vacum will. Ron

 

[Meep-Meep]

The fuel today is designed to vaporize with a totally different method of fuel delivery (i.e. fuel injection). And I don't think it's the ethanol content per say that causes all the vaporizing issues but rather the fuel chemistry. There is no hot spot in the modern intake either and the intake itself is plastic. This is done to keep the inlet charge temp as consistent as possible, for not only emissions but efficiency. That said it's not carb friendly fuel. I have also noticed that high compression engines seem to be more prone to boiling the fuel out of the carbs than lower compression engines. My boat has absolutely no issues with vapor lock but the engine runs very cool on average and happens to be the only carbureted engine I have that works perfectly. If it rained more around here I'd commute in it because it runs so well.

I believe you want to look at Reid vapor pressure curves.