mrtexas

I'm mostly a lurker here these days.

Has anyone determined the design feature of early Corvette mechanical
fuel injection systems that leads to percolation?

Ol Blue

Go here for the best information to-date.
https://www.corvetteforum.com/forums...rovements.html
Gary

jim lockwood

Oh, yes, it's a very well understood problem.

At idle, the system fuel pressure in Rochester FI is lower than the vapor pressure of the warm/hot fuel. As a consequence, fuel begins to vaporize in the lines to the nozzles and that leads to extremely lean engine operation. This happens only when the engine is idling. As soon as the engine is placed under a load, system fuel pressure rises above the fuel vapor pressure and perc stops instantly.

I've instrumented FI units with thermocouples. These revealed that with summer blend Kalifornia RFG, the vaporization happens at line temperatures around 135F to 140F. With winter blend Kalifornia RFG, nozzle line temperatures in the range of 105F - 110F are enough to promote fuel vaporization.

'63 - '65 FI units recirculate fuel to the hub of the fuel distribution spider and back to the bowl. This isn't a solution but it does keep the fuel in the nozzle lines cooler than in '62 and older FI units and '63 - '65 FI is less prone to fuel perc.

More than you ever wanted to know can be found in posting #7 in this thread:

combatting fuel perc

mrtexas

Interesting. IMHO the only significant difference in today's fuel and 1957-1965 fuels is the 10% alcohol content, less butane today which would help with percolation and no lead in gasoline today which doesn't affect percolation. Refinery processes have not changed much since the 1960/50s so mostly same the processes are making gasoline components for the last 60 years or so. Each refinery is unique but gasoline is made mostly today and in the past with fractions in order of volume from 1) Fluid Catalytic Crackers(gas oil feedstock), 2) Catalytic Reformers(naphtha feedstock), 3) Alkylation(iso butane and butylene feedstock) 4) Isomerization(low octane gasoline feedstock). Up to 8 or so fractions are blended together to make the finished product.

The refinery I worked at as a chemical engineer from 1978 to 2010 made around 6 million gallons/day of gasoline:

300k(thousand)bd/day crude unit makes a small amount of low octane gasoline
102kpd Fluid Catalytic Cracker makes low octane gasoline
90kbd Catalytic Reformer makes high octane gasoline(which in the past has been called "platformate" or "techron" to confuse consumers)
15kbd Alkylation makes iso octane
30kpd Isomerization which makes low octane gasoline

mrtexas

Did mechanical FI units have percolation problems before 10% alcohol in gasoline circa 2000?

BTW RFG means reformulated gasoline which is achieved by the 10% alcohol these days
and is not limited to California. All of US has RFG.

68hemi

It has been nearly 40 years since I owned a 1958 factory FI car but I never had a problem with the gas we had available back then. I thing the entire problem is with the ethanol. I know it is with carbed cars from my recent experience.

jim lockwood

Absolutely. I experienced fuel perc without knowing what was happening as far back as 1979.

RFG may now be a semi-generic term but I'm positive Kalifornia RFG motor fuel is unique in all the world.

silver837

We are just lucky Jim.