ettev

OK! Is it true that an in-line metal canister filter PLACED BEFORE THE FUEL PUMP and INSTALLED HORIZONTALLY is a double no-no? Would a horizontal install really increase the chances of vapor lock or other air-in-the-line related headaches? Why is suction side placement frowned upon? If I want to use the correct metal fuel line from the pump to the Q-jet where would be the best place to install an in-line filter? Also, if an in-line filter is used in conjunction with the paper element in the carb inlet is too much flow restriction produced, or will the inlet spring compensate? I can't imagine an additional in-line filter (from an '82-'84 F.I. system) creating that much more restriction.

One other item. Is anybody using an electric fuel pump in conjunction with the regular fuel pump? I was thinking of mounting one on the rear of the frame. Any suggestions on a manufacturer, operating psi, power tap location, benefits of doing this, etc?


[Modified by ettev, 4:23 PM 4/14/2004]

Fevre

It may be a :nono: but I have run in the configuration for 3 years with no problems thus far. I spliced a pint sized one back near the tank. Why not filter the gunk BEFORE it gets to the pump? :confused: Have heard guys run a larger micron filter before the pump and smaller micron after.

ettev

:iagree: My thoughts exactly. Wouldn't a filter before the pump save the internals of the pump from the nasties being pulled from the tank?

zwede

Any filter placed before a pump has to be very low restriction. A mechanical pump already has a big handicap trying to suck fuel the length of the car. Any added restriction is bad. The sock in the tank should be enough of a filter. Dirt making it through the sock is not big enough to hurt a pump. The pressure side of the pump can live with a (reasonable) restriction so that is where the low-micron filter goes.

ettev

Why is the "pressure" side different? Isn't restriction, restriction? I'm not trying to be a smart___, just trying to learn why one side of the pump is better suited for the filter than the other side? What mechanical components inside the pump will "pulling" fuel thru a filter affect as oppsed to "pushing" through a filter afterward? Won't it be working just as hard, if not harder because of the filter restriction being on the uphill vertical?

zwede

Think of it this way.... Atmospheric pressure at sea level is 406" of water. This means that if you placed a tube in a lake and applied a vacuum in the tube the water would rise 406" inside the tube.

So the best pump in the world placed 407" above water level will never see even a drop of water. The water in the tube will rise no closer than an inch from the pump.

Now place this great pump under water. How high up could you pump it? Theoretically with enough pressure you can pump it into space!

So all pumps, no matter what they pump, are much better at pumping than sucking, because on the inlet side the atmosphere pushes fluids into the pump, and our atmospheric pressure isn't all that high.

lars

What zwede is saying is absolutely correct. In addition, there are 2 more factors that make a filter on the suction side undesirable:

First, a mechanical fuel pump is not a positive displacement pump. Because of this, it does a very poor job of "sucking." The slightest restriction on the suction side of a diaphragm pump grossly decreases its volume and output pressure. A diaphragm pump cannot "suck" a full 1 atmosphere of vacuum, so you have nowhere close to 29 inches of Mercury pressure potential - not even close.

Secondly, the negative pressure on the suction side is directly related to vapor pressure and boiling point of the fuel. If you "suck" harder, you increase the vaporization rate of the fuel, and directly promote and contribute to vapor lock on the suction side. If you put a filter in the suction side and lower suction side pressure (due to the slightest restriction through the filter) on a hot day, you stand a very good chance of fuel starvation from vaporization of the fuel in the fuel line. This is especially true with today's gasolines containing ethanol (very high vapor pressure and evaporation rate).

The reason the modern FI cars have the filter in-line prior to the engine is because they have a fuel tank-mounted high pressure fuel pump. This "pushes" the fuel through the filter instead of "sucking" it through. No OEM system has a "suck-through" filter design because of the problems associated with "pulling" a liquid rather than "pushing" it. You're asking for problems by putting filters and restrictions on the suction side.

Note also that a diaphragm mechanical pump really doesn't care about impurites in the fuel: you can shove marbles through a diaphragm pump without hurting it, so a filter prior to the pump serves no purpose.

If you really want a filter prior to the pump you should install a small electric boost pump back at the tank to keep the fuel line pressurized through the filter. Such an electric pump should be tied into an oil pressure switch so the pump will shut off automatically when the engine is not running (like if you get into an accident so you don't become a marshmellow).

Fevre

Come on Lars, a marble won't fit through the pump inlet. :jester I Was running a big o frame filter and did not like the clutter so I decided to try splicing a filter in where it is not visible. So far so good.

BTW Big carbs are not supposed to used on small motors either. :p:

lars

...we have small marbles here in Denver...

The success you have with a suction side filter depends on a lot of things. Most importantly is the vapor pressure of the fuel in your geographical area. Here in the metro Denver area, the politicians force us to use "oxygenated fuel." This is nothing more than gasoline with a bunch of poor quality ethanol added to it (10%). This makes the fuel extremely prone to vapor lock and boiling, so a suction side filter simply won't work on a hot day when running at WOT or when going uphill. Many other areas of the country suffer the same problems. But some areas use good fuels with little or no alcohol. If you have good fuel, and your fuel flow rquirements are low-to-moderate, a good suction side filter can be used with no adverse effects.

Yeah, that 750DP on my 350 is WAY too big - can't understand how the car can even run with that thing... :lol: Now, to make things MUCH WORSE, I'm going to be installing a BIG single plane intake manifold and a solid lifter flat tappet cam that is WAY too big for a street car. I'm sure this thing is really going to run BAD... it'll have NO TORQUE. It will be FLAT at low rpm. In fact, it's going to run so bad that I'll be running for money between stoplights....


[Modified by lars, 11:24 AM 4/15/2004]

ettev

So what pump characteristics are appropriate (gpm, psi, etc)? Would something like this do the trick: http://cgi.ebay.com/ebaymotors/ws/eB...category=33550

lars

If you retain the mechanical pump, you don't need much - the pump you linked is more than enough. The main idea is to just keep the suction line under slight pressure so that the mechanical pump does not have to "suck" the fuel through the filter and up the line. That Holley pump will do the job nicely. Holley also makes an oil pressure switch so you can keep the electric pump safe.

Gage

I spliced 1 ft rubber hose that runs from my tank to frame line and put a clear plastic 2.99 purolator fuel filter in there. Since it hangs below the tank gravity alone keeps it filled up with gas so I don't think the pump has to work any harder to pull gas through it as it would to pull gas from the tank. My vapor lock issue that I get from time to time is due to the previous owner forgot to block off exhaust crossover on new intake.

lars

Gage -
"Vapor lock" does not occur because of the exhaust crossover. Vapor lock is a condition that occurs when the mechanical pump no longer receives liquid fuel from the tank - rather it gets gas vapor due to the fuel vaporizing in the suction line from the tank. Since the pump is not a positive displacement pump, the fuel pressure drops to zero the instant the pump receives a gulp of vaporized fuel instead of liquid fuel. Your in-line suction filter would contribute to this problem regardless of the filter being installed below the tank level. The crossover has nothing to do with vapor lock, although it can contribute to the fuel boiling in the carb bowl. This is not a vapor lock condition.

Bullshark

FYI - This is straight out of the Holley Commander 950 MPFI installation manual:

"1.15 Fuel Pump Inlet Filter
The function of this filter is to eliminate any impurities that might harm the fuel pump. In the in-line fuel pump type, this filter is
external to the fuel tank and is in a replaceable cartridge filter. In the in-tank fuel pumps, the fuel filter is in the form of a sock and
is directly attached to the pump in the "pump on a stick" version and attached to the fuel pump module in the module version.
These filters have a rating of 120-150 microns.
NOTE: This fuel filter is required to avoid fuel pump damage.
1.16 Main Fuel Filter
The function of this filter is to eliminate any contaminants after the fuel pump. These are either small enough to pass through the
fuel filter of the pump inlet or are generated by the fuel pump. This fuel filter is also of the cartridge type, but is designed to
sustain much higher fuel pressures than the fuel pump inlet filter. These filters have a rating of 10 microns.
NOTE: This fuel filter is required to avoid fuel injector damage."

:beatdeadhorse:

Bullshark

Gage

So Lars, I guess my sporadic problem may not be vapor lock, rather when I drive the car for a awhile, park it for several minutes (grocery store, fuel stop) then drive off again, it starts up fine, but when I go to accelerate moderately or better as I just pull onto the highway, it acts like it is out of gas, then in about 2-3 seconds, it picks up and runs like a champ. No other bogs until I park it again or get caught in rush hr traffic and try to hammer it after I get on the open road.

lars

Holley in-line fuel pumps are not diaphragm type pumps - they are positive displacment vane-type pumps. These pumps must have filtered fuel because of the critical vane-to-housing clearance. If you're using one of these pumps you must have a suction side filter - that is correct. The standard mechanical fuel pump that's installed on the engine is a diaphragm pump. It has no internal clearance issues, and particulate can pass right through the pump with no problem: you can pump swamp water through a diaphragm pump as long as the big chunks can get through the checkvalve. A diaphragm pump needs no filter other than the "sock" in the fuel tank to keep the big chunks out of the checkvalve.

Let me give this sucker a whack... :beatdeadhorse:

lars

Gage -
You're most likely boiling and vaporizing the fuel in the carb - this is different from vapor lock. However, a suction side filter won't do anything to help the problem. You can do several things to alleviate this: Add a reflective heat shield and use an insulated base gasket. Also, the use of a fuel return line greatly helps this problem by keeping cool fuel available to the carb at all times.

Gage

Lars, Thanks. I will try a 1/2" spacer until I get the intake off to block of the exhaust crossover. We don't need the extra heat here in Texas. And I am using the fuel return line which runs from the pump back to the tank.

lars

Gage -
The other thing you can do that is quite effective is to install a rear electric fuel boost pump back by the tank. This will do 2 things: First, it will keep your tank-to-pump fuel line lightly pressurized to assure that the mechanical pump is always receiving liquid fuel and no vapor. Second, it allows you to circulate cool fuel through your return line system prior to engine start-up. This eliminates the problem of the fuel in the engine compartment fuel lines from becoming "heat soaked" because you can "purge" the hot fuel out of the system by running the pump prior to engine start-up. For best effectiveness, the return line on a system like this should be "T"ed off at or near the carb inlet.

blunsford

Not to mention that gasoline under a vacuum (suction) has a much lower boiling point than gasoline under pressure.