Let’s Talk Fuels
The problem at this time and altitude in Colorado.

Here in Colorado at high altitude, we can experience several types of fuel-related problems in carbureted engines. The higher the altitude, the worse the problems are. One is vapor lock where gasoline vaporizes from heat in the fuel line, thereby starving the engine for fuel. The other is the heat soak/ hot start/run problem.

When a hot engine is turned off, heat builds up in the engine compartment. This causes fuel to rise in the carburetor bowl and drip down from the venturi boosters into the engine. The engine is now flooded and is hard to start.

The next is what we call “geysering” and happens on a running engine. The elevated temperature of the pressurized fuel in the line entering the carb bowl past the needle and seat causes the fuel to boil violently. The drop in pressure from the line to atmospheric in the bowl causes this. The fuel “geysers” up out of the bowl vent tube.

There are two times of the year when these problems are greatest for our carbureted engines. One is in the late fall and the other is in the early spring. These are periods where winter fuels are in use but summer temperatures can be present. This is because winter fuels boil at lower temperatures than summer fuels. The problem is worsened as the vehicle goes up in altitude, because the boiling temperature of gasoline, just like water, goes down as the altitude is increased.

Unlike injected engines, carbureted engines have the fuel at ambient pressure in the carb bowl before entering the engine. Remember, the higher the fuel pressure, the higher the boiling temperature. The fuel is under pressure up to the carb. But, once the fuel passes the needle & seat into the bowl, it is at ambient pressure. Here is where we experience violent vaporization or boiling. We even see it geyser up out of the bowl vent tube. But if we cool the bowl with a damp cool rag the problem stops immediately. The fuel bowl doesn’t even feel hot when this problem occurs. This tells us that the boiling temperature at this altitude for the fuel is just a little above ambient. So if we subject it to summer temperatures, the problem occurs. It all has to do with fuel volatility.

Volatility is a measure of the gasoline’s tendency to vaporize under a given set of conditions. Reid Vapor Pressure (RVP) is a commonly used indicator of gasoline volatility. Boiling is a violent version of vaporizing. The higher the RVP, the greater the tendency to vaporize or vapor lock.

We don’t have this problem with our race gas on the hottest days in the middle of the summer. So let’s look at RVP. Our 110-octane race gas that we blend down has a RVP of 6.6pounds per square inch (psi). Our summer unleaded 91-octane gas that blend with race gas to lower the octane of the race gas with has an RVP of about 9psi. That is if all the remaining winter gas has been used up from the gas station’s tank.

This is because the RVP changes during the year. The summer gas delivered from the refinery has the 9psi RVP from mid April to mid September. After September, it starts to rise monthly till it gets to a max in December & January of about 15psi. Then it starts coming down again till April. The higher the RVP, the lower the boiling temperature. The addition of ethanol can also lower the boiling temperature.

So why does the gas from some stations boil worse than from others? If the April gas is in the tank truck but there is still some March gas in the station tank, the gas you get will have a RVP higher than the 9psi for April through summer.

If you don’t run a fuel with a low RVP, the best thing to do if the vehicle has been running hard and is shut down for a while is to open the hood to let the hot air out of the engine compartment. If the vehicle is prone to geyser while running, like at high altitude climbing up to the Eisenhower Tunnel, a simple method, that may sound rinky-dink, but really works, is to carry a cold wet towel and put it around the carb bowls when the problem occurs. This works every time on our dyno at 7500-foot elevation. Following is a chart of RVP classes and which is in use at different times of the year. These apply for Colorado.

Class RVP Max 10% Boiling Temp F 50% Boiling Temp F
Class A 9 158 170 to 250
Class B 10 149 170 to 250
Class C 11.5 140 170 to 250
Class D 13.5 131 150 to 250
Class E 15 122 150 to 245

Ethanol raises the RVP about 1 psi.

Blend by month: Jan E Feb E-D Mar D-C April C-A May thru Sept15 A Oct B-C Nov C-D Dec D-E

A worse case would be a very warm day in January when the fuel starts to boil at 122deg or lower with ethanol.

Insulating lines and carburetor will help buy time till the problem occurs and may solve the problem in marginal cases. Replacing a mechanical fuel pump on the engine with an electrical pump close to the fuel tank can reduce the fuel line vapor lock problem. The mechanical pump acts as a heat source. Hot fuel with vapor can be pushed much easier than pulled as an up front pump does.

In a nutshell, cool the fuel or raise it’s boiling temperature.

Jan Feb Mar D-C April May thru Sept 15 Oct Nov Dec
Happy motoring.

Walt Hane
Tech Tips-5

Bullet 1 Suspension
Bullet 2 Dry Nitrogen
Bullet 3 The Panhard Bar
Bullet 4 Loss of Oil Pressure
Bullet 5 Inlet Fuel/Air Systems
Bullet 6 Colorado Hot Fuel Problem