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return line too small???

PostPosted: October 12th, 2014, 7:44 am
by grumpyvette
Rod S wrote:I just installed a Mallory 140 fuel pump and a Mallory 4309 regulator. The instructions said to back the lock nut off and take the set screw out. If, upon priming, I had over three pounds of fuel pressure, my return line was too small. I just finished plumbing a 3/8 return line, the same size as my feed line. Upon priming, I had about 5 lbs. of fuel pressure. With the engine running I can't get it much below 6 lbs. I know, the obvious answer is that I have to plumb a 1/2 inch return line. I don't think there is any way around this. Is it best to get a roll of steel tubing and start bending, or is it best to get some of the teflon core stainless steel wrapped 1/2 in stuff and go with 8an fittings? Thanks in advance, Rod

the first logical thing to do is test if the problem IS THE 3/8" RETURN LINE SIZE BEING A RESTRICTION, causing the issue your having BEFORE you go investing time and effort in changing the current set up, hook a short section of lets say 12-16 feet of 1/2 line on the out side of the fuel pressure regulator, too semi, duplicate the larger and longe , less restrictive to flow,return line too the fuel tank,that youll have once its installed in the car, so its pouring the return fuel in a 5 gallon bucket ,as a quick and easy test, then test if you can reach the desired pressure levels, indicating the increase in cross sectional area on the return line actually cures that problem, rather than assuming and guessing., IF that cures the issue then proceed with swapping out the return line in the car knowing that will if properly installed, cure your problem, rather than go all thru the work only to spend the time and cash to potentially find it might not be or was not the issue.
if you find out that installing and allowing the larger and less restrictive ,temporary return line too drain in the bucket, has no measurably effect you will obviously have proven that is not the cause of the problem.if it cures your problem then you obviously go ahead with the install, and once its completed, if you no longer get the desired results youll know that a restrictive fitting or the routing is likely the cause.
a return line will be by definition low pressure, measure carefully, and you can have a local hydraulic supply custom fabricate teflon flex hose of the correct size type and with the correct fittings







Re: return line too small???

PostPosted: October 12th, 2014, 5:30 pm
by grumpyvette
while testing I,d point out a very commonly over looked component and factor is the internal passage size and cross sectional area of the fittings.while the NPT ,female thread size in your fuel pump and fuel pressure regulator will normally be either 3/8-NPT in most standard performance versions or 1/2-NPT in the larger higher out put versions,the barb length, diameter, and inside passage diameter will vary by manufacturer a great deal, easily varying by over 150% in cross sectional area