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Octane levels and vapor pressure

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  • Bill B.
    Very Frequent User
    • June 30, 1999
    • 182

    #16
    Re:Thats exactly the problem I've had.

    I would like to ask a add on question in response to Dukes statement that "modern fuels are very similar regardless of octane number". Does this mean that I am wasting my money using 91 octane fuel when I fill up when the 87 octane would be alright to use? The valve seats have not been reworked and I never lug down the engine when driving. The engine is a 327/300 HP in a 65 coupe. Thank you in advance for any responses.

    Bill Bonnichsen

    Comment

    • Bill B.
      Very Frequent User
      • June 30, 1999
      • 182

      #17
      Re:Thats exactly the problem I've had.

      I would like to ask a add on question in response to Dukes statement that "modern fuels are very similar regardless of octane number". Does this mean that I am wasting my money using 91 octane fuel when I fill up when the 87 octane would be alright to use? The valve seats have not been reworked and I never lug down the engine when driving. The engine is a 327/300 HP in a 65 coupe. Thank you in advance for any responses.

      Bill Bonnichsen

      Comment

      • Jerry G.
        Extremely Frequent Poster
        • April 1, 1985
        • 1022

        #18
        Re: Re relationship between vapor p & boiling.

        I went to a racing fuel site for Sunnoco and noticed some of the fuels have a higher boiling oint than others. There doesn't seem to be a correlation with RVp and boiling points.See cosbyoils.com So I think what I;m looking for is a higher boiling point not a lower RVP. Any fuel suppliers out there? Does adding diesel raise the boiling point?

        Comment

        • Jerry G.
          Extremely Frequent Poster
          • April 1, 1985
          • 1022

          #19
          Re: Re relationship between vapor p & boiling.

          I went to a racing fuel site for Sunnoco and noticed some of the fuels have a higher boiling oint than others. There doesn't seem to be a correlation with RVp and boiling points.See cosbyoils.com So I think what I;m looking for is a higher boiling point not a lower RVP. Any fuel suppliers out there? Does adding diesel raise the boiling point?

          Comment

          • Duke W.
            Beyond Control Poster
            • January 1, 1993
            • 15657

            #20
            Re:Thats exactly the problem I've had.

            The primary difference in modern fuels is octane rating, which is an expression of a fuel's detonation resistance.

            The 327/300 probably needs at least today's 91 octane premium to operate without significant detonation without excessively retarding the ignition timing.

            The 327/300 has an advertised compression ratio of 10.5:1. When GM "detuned" all engines for 1971 to operate on 91 RON unleaded fuel - essentially the same as today's 87 PON fuel, they dropped the CR to 8.5:1!

            Duke

            Comment

            • Duke W.
              Beyond Control Poster
              • January 1, 1993
              • 15657

              #21
              Re:Thats exactly the problem I've had.

              The primary difference in modern fuels is octane rating, which is an expression of a fuel's detonation resistance.

              The 327/300 probably needs at least today's 91 octane premium to operate without significant detonation without excessively retarding the ignition timing.

              The 327/300 has an advertised compression ratio of 10.5:1. When GM "detuned" all engines for 1971 to operate on 91 RON unleaded fuel - essentially the same as today's 87 PON fuel, they dropped the CR to 8.5:1!

              Duke

              Comment

              • Duke W.
                Beyond Control Poster
                • January 1, 1993
                • 15657

                #22
                Re: Re relationship between vapor p & boiling.

                As I previously explained, gasoline does not have a single boiling point, and any attempt to list a single number - median, mean - whatever - without explaining how it was derived from the fractional distillation curve is hokus pokus.

                RVP is the best single number measurement of a fuel's volatility. If you want to go deeper than that you have to analyse the fractional distillation curve.

                Duke

                Comment

                • Duke W.
                  Beyond Control Poster
                  • January 1, 1993
                  • 15657

                  #23
                  Re: Re relationship between vapor p & boiling.

                  As I previously explained, gasoline does not have a single boiling point, and any attempt to list a single number - median, mean - whatever - without explaining how it was derived from the fractional distillation curve is hokus pokus.

                  RVP is the best single number measurement of a fuel's volatility. If you want to go deeper than that you have to analyse the fractional distillation curve.

                  Duke

                  Comment

                  • Ed Jennings

                    #24
                    Re: Re relationship between vapor p & boiling.

                    I noticed on the FI car I used to have that the heat soak problem was very minimal in the summer and a real pain in the winter. I attributed the difference to "summer" and "winter" gas. In our area, the summer gas is "EPA spec" fuel, supposedly blended to minimize evaporative emissions. Presume that means a higher boiling point.

                    Comment

                    • Ed Jennings

                      #25
                      Re: Re relationship between vapor p & boiling.

                      I noticed on the FI car I used to have that the heat soak problem was very minimal in the summer and a real pain in the winter. I attributed the difference to "summer" and "winter" gas. In our area, the summer gas is "EPA spec" fuel, supposedly blended to minimize evaporative emissions. Presume that means a higher boiling point.

                      Comment

                      • Duke W.
                        Beyond Control Poster
                        • January 1, 1993
                        • 15657

                        #26
                        Re: Re relationship between vapor p & boiling.

                        I've been following you discussions on this subject for some time, but I didn't realize until now that is was more of a problem in the winter than summmer.

                        Yes, most winter blends have lower Reid vapor pressure and a higher fraction of low boiling point components to aid starting in cold weather, but in your part of the country you can occasionally get summer tempertures during the winter season, which could definitely contrbute to hot starting problems due to vapor in the fuel spider.

                        Modern EFI cars operate at about 40 psi all the time, including cranking, and considerable excess fuel is pumped through the rail and bypassed back to the tank. The combination of higher rail pressure and rapid circulation of the fuel at cranking speed quickly purges hot fuel from the rail that could possibly generate vapor and poor starting. On the Rochester FI system, fuel pressure is a function of pump speed and only a few psi is available at cranking speed.

                        I guess I still don't understand how you could have parted with your '62 FI car!

                        Duke

                        Comment

                        • Duke W.
                          Beyond Control Poster
                          • January 1, 1993
                          • 15657

                          #27
                          Re: Re relationship between vapor p & boiling.

                          I've been following you discussions on this subject for some time, but I didn't realize until now that is was more of a problem in the winter than summmer.

                          Yes, most winter blends have lower Reid vapor pressure and a higher fraction of low boiling point components to aid starting in cold weather, but in your part of the country you can occasionally get summer tempertures during the winter season, which could definitely contrbute to hot starting problems due to vapor in the fuel spider.

                          Modern EFI cars operate at about 40 psi all the time, including cranking, and considerable excess fuel is pumped through the rail and bypassed back to the tank. The combination of higher rail pressure and rapid circulation of the fuel at cranking speed quickly purges hot fuel from the rail that could possibly generate vapor and poor starting. On the Rochester FI system, fuel pressure is a function of pump speed and only a few psi is available at cranking speed.

                          I guess I still don't understand how you could have parted with your '62 FI car!

                          Duke

                          Comment

                          • Ed Jennings

                            #28
                            Re: Re relationship between vapor p & boiling.

                            Duke,
                            The most severe hot start problems occured in the spring months, when temps can easily get into the 80's. The EPA mandated spec fuel doesn't go on sale here until about May 1 and lasts through September. I pretty much drove that car year around, and it could be a real aggrivation in the "winter". Some of the worst episodes of heat soak I encountered were in Jan/Feb. In summer, the worst heat soak conditions might produce a few seconds of rough idle, but usually had no problems at all.

                            As to selling the car, I will probably suffer from perodic seller's remorse for years to come. But, we wanted a car that was a bit more travel friendly and a bit more civilized. Those are the rationalizations. The actual need was to free up some capital for business purposes. I still have the 67 coupe, and don't plan on parting with it anytime soon. I am enjoying the 75 vert. The engine has been rebuilt to a little better than L82 specs, and all the pollution stuff is gone. Basically, all the ills of the 75 MY have been corrected. Also, this is one I can play with, without being too concerned with originality. There I go rationalizing again.

                            Comment

                            • Ed Jennings

                              #29
                              Re: Re relationship between vapor p & boiling.

                              Duke,
                              The most severe hot start problems occured in the spring months, when temps can easily get into the 80's. The EPA mandated spec fuel doesn't go on sale here until about May 1 and lasts through September. I pretty much drove that car year around, and it could be a real aggrivation in the "winter". Some of the worst episodes of heat soak I encountered were in Jan/Feb. In summer, the worst heat soak conditions might produce a few seconds of rough idle, but usually had no problems at all.

                              As to selling the car, I will probably suffer from perodic seller's remorse for years to come. But, we wanted a car that was a bit more travel friendly and a bit more civilized. Those are the rationalizations. The actual need was to free up some capital for business purposes. I still have the 67 coupe, and don't plan on parting with it anytime soon. I am enjoying the 75 vert. The engine has been rebuilt to a little better than L82 specs, and all the pollution stuff is gone. Basically, all the ills of the 75 MY have been corrected. Also, this is one I can play with, without being too concerned with originality. There I go rationalizing again.

                              Comment

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