Fuel filters & vapor lock

[rons49]

In a passing conversation with some local car guys, someone stated the larger type " see through" style fuel filters, such as the Fram G-2, are a contributing factor to vapor lock, or at least fuel starvation. Same is true with the 50s type glass bowl AC and Carter as was stated. He claimed that the only filters that should be used are the small thimble style. Is there any truth to this? Any ideas? Ron

[nickelroadster]

I can't see how a see through bowl would have anything to do with vapor lock. It seem to me that he more important thing would be the routing of the fuel; lines and how close they are to sources of heat. I think I would like some cause and effect information or evidence before putting any credence to this theory. Just because someone says it doesn't mean that it is true.

[Dave@Moon]

Perhaps they were referring to the expansion of the liquid as it passes into the larger chamber from the fuel line, which could briefly result in a reduced pressure and therefore a lower vaporization point for a brief moment until the fuel is drawn into the line beyond the filter. Given the relatively low pressures carbureted systems work under, this effect is probably extremely small. Compared to the effect of heat sources within the engine compartment, I would think it must be insubstantial at best.

[rons49]

I frankly did not place much credence in the remark, but I thought that I would ask the experts.

[5219]

I have a training manual published in the mid forties by Cadillac for dealership mechanics. In the section on vapor lock, it advises servicemen to remove any non-standard fuel filters as they contibute to the problem.

The only filter on my LaSalle flathead is the strainer in the glass bowl on top of the fuel pump. Evidently, it was believed that the fuel, when under pressure in the line, was more likely to boil when it hit the wide-open spaces of a fuel filter.

[Coley]

The open space in a clear type filter is under pressure as well.

How else would the clear glass show a level at all.

The filter would be full with no air showing. or am I missing something??

[5219]

Gasoline, being a liquid, is not compressible. In the absence of air, it remains at a fixed pressure. The "empty" part of the fuel filter is full of air, which is compressible.

When the incompressible liquid under pressure meets compressible air, the air compresses. The liquid, if it is hot, releases vapor that it could not release when it was compressed. In effect, it boils.

Think about what happens when you remove a pressure cap from a hot radiator. If the liquid was not boiling, it will boil when it hits the air.

I think that this is related to something called Boyle's Law, which I kind of remember from high school chemistry.

[rons49]

The conversation was regarding adding an electric fuel pump. The small Airtex E-8011 or 8012 come with a small metal filter threaded to the inlet side. I have been told to lose those filters since they leak ( true to my experience) and clog too quickly. I replaced that filter with a clear Fram G-4. I can see if dirt accumulates. Location of the E-pump is by the gas tank even with the seam. An AC glass globe filter is mounted at the carb. Haven't had a problem with starvation since Auto Parts Cellar rebuilt my fuel pump 2 years ago. The issue was that fuel filters must be as small as possible to avoid air bubbles. Warranty for the Airtex is voided if no filter is used. Interesting annalogy. Ron

[hchris]

Boyles law - well done 5219 !!

The overall effect of fuel pressure loss and temperature rise are the reasons vapor lock occur, added to these laws of physics are the current properties of modern fuels; which by and large, are designed to run with high pressure fuel injection systems.

So best results are achieved by keeping the fuel as cool as possible (careful routing of fuel lines) and positive pressure in the feed lines, be it electric pumps and pressure regulators if needs be.

I doubt whether fuel filters on there own will cause vapor locks to occur but they may be the last straw cause if the rest of the supply line system is struggling due to low pressure / high temps.

Another thought is to use a higher octane rated fuel, as the vapor pressure rating will be also be higher lessening the probability of vapor lock.

[NTX5467]

If the particular style of filter was a contributor to vapor lock, then almost every Chrysler product V-8 since the 1960s would certainly have vapor lock issues . . . which they didn't. It was also mounted somewhat vertically, and fully exposed to underhood heated air coming through the radiator. By some accounts, that should be "a double whammy effect" and make things more prone to vapor lock.

On our '66 Newport, I was always amazed that the clear/opaque filter would not be full of fuel when the engine was running, but with varying amounts from "empty" to "3/4 full". Never missed a beat, even in the hottest weather.

I suspect that vapor lock can also be a result of what happens UNDER the car. Where the heat from the pavement radiates upward to the bottom of the car and the fuel tank (one reason to keep the bottom of the fuel tank "silver"?).

Just some suspicions . . .

NTX5467