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Technical Mechanical fuel pump pressure

Discussion in 'The Hokey Ass Message Board' started by seb fontana, Feb 6, 2016.

  1. seb fontana
    Joined: Sep 1, 2005
    Posts: 6,325

    seb fontana
    Member
    from ct

    Holley 12-833 for sbf..If the max shut off pressure is 7.5 shouldn't it be there when the engine is idling? I am getting 6 psi at idle and Jeg's tech says thats where it should be..I don't understand...
     
  2. Glenn Thoreson
    Joined: Aug 13, 2010
    Posts: 78

    Glenn Thoreson
    Member
    from SW Wyoming

    Think about it a bit: 7.5 is the max, not the minimum. You probably have nothing to worry about.
     
  3. KoolKat-57
    Joined: Feb 22, 2010
    Posts: 2,930

    KoolKat-57
    Member
    from Dublin, OH

    7.5 is the most it will put out, 6 psi at idle is still a lot for most applications.
    KK
     
  4. Mike VV
    Joined: Sep 28, 2010
    Posts: 1,801

    Mike VV
    Member
    from SoCal

    You DON'T want a ton of pressure on a street engine. All it makes you do is to retune the carburetor. Higher pressure equals more fuel flowing thru the carb. quicker. This means putting smaller jets in, messing with the power valve.
    Best to run the 6 psi and tune from there. Unless you are going to 8000rpm a lot...6psi will do the job just fine.

    Mike
     
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  5. seb fontana
    Joined: Sep 1, 2005
    Posts: 6,325

    seb fontana
    Member
    from ct

    I'll expand some..Previous pump of the same part number and ratings would do 8 pounds at Idle..I thought as has been stated that for driving around and mild beatings 5 lbs would be enough so I put a regulator in and set it at 5 lbs...It did work fine till a outlet valve came loose then "0" pressure, I re-seated the valve but felt I couldn't trust the pump so replaced it...In my curiousity to see if the new pump matched the old one is when I found the 6 lbs and it won't go higher even with the electric pump on too; it would with the previous pump..
     
  6. Budget36
    Joined: Nov 29, 2014
    Posts: 3,220

    Budget36
    Member

    I could see pressure pertaining to tuning issues with fuel injection, but don't see how it pertains to a carb?

    Assuming the float needle isn't being over run, what's the difference between 5 psi and 50 psi?
     
  7. Ebbsspeed
    Joined: Nov 11, 2005
    Posts: 4,937

    Ebbsspeed
    ALLIANCE MEMBER

    Like Budget36 said above, the pressure has nothing to do with jets and power valves. As long as your float and needle/seat are doing their job there's no impact on anything else in the carb.
     
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  8. seb fontana
    Joined: Sep 1, 2005
    Posts: 6,325

    seb fontana
    Member
    from ct

    I don't argue the previous posts about only needing 6 psi, I am comfortable with that.. My old pump put out 7.5 psi at idle [I put a regulator to get down to 5 psi] ; since new pump is also rated for 7.5 psi I expect it too should put out 7.5 at idle..When else would a mechanical pump put out its max pressure but at idle when low fuel consumption? At 6000 rpm wot in 4th I would expect the pressure to drop to 5 psi or so do to volume of fuel being used..I put a pressure gauge in the out put of my old pump and pumped it by hand and got 7.5 psi..
     
  9. 2OLD2FAST
    Joined: Feb 3, 2010
    Posts: 2,157

    2OLD2FAST
    Member
    from illinois

    pressure increases w/rpm generally..bladder in pump works like a piston . the faster it cycles, the more fuel it moves..
    dave
     
  10. jimmy six
    Joined: Mar 21, 2006
    Posts: 5,870

    jimmy six
    Member

    We have a 374" 1/2 mile dirt car running a single 750 Holley and required mechanical pump and were told never to run it over 5psi. It has a -8 feed and a -8 to the reg. then -6's.... Good Luck.
     
  11. seb fontana
    Joined: Sep 1, 2005
    Posts: 6,325

    seb fontana
    Member
    from ct

    I am not looking to run over 6 psi I thought I wrote that I put a regulator in set at 5 psi..What is the pump pressure before the regulator? ..
     
  12. jimmy six
    Joined: Mar 21, 2006
    Posts: 5,870

    jimmy six
    Member

    Enough to flood the carb at idle which we did when adjusting the pump. The mechanical pump is a high pressure/volume pump. I believe an Edlebrock. We first used one of the double ender pumps but it developed a leak we could not repair. We were instructed by AeroSpace Industries (regulator) told us to back it completly off, start the engine and the ease up to what ever is recommended by your carb guy and ours Sean Murphy. Any adjustment is always back the reg all the way off and ease back to 5. When ran on the dyno the 5 psi remained constant on a GoPro and the dyno readout. Good luck...
     
  13. chargin03
    Joined: Jan 8, 2013
    Posts: 414

    chargin03
    Member

    Maybe the old pump was defective causing the higher pres.
     
  14. Truck64
    Joined: Oct 18, 2015
    Posts: 3,967

    Truck64
    Member
    from Ioway

    Anytime a mechanical fuel pump is changed, the carburetor float should at least be checked, and tweak the the mixture screws. I think production variance or when a pump gets old they may get weaker. So a given float setting should be matched to the pump.

    The minimum volume spec for a V8 since Eisenhower was a road guard is 1 pint in 30 seconds (or 15 gallons an hour). I was taught a fuel pump (mechanical) must be able to provide this much fuel at all times under all conditions or the engine will have trouble at highway cruise. Of course a stock V8 won't (hopefully) actually burn 15 gallons per hour.

    So the needle seat has to hold back a considerable amount of volume and 5 or 7 PSI
     
  15. seb fontana
    Joined: Sep 1, 2005
    Posts: 6,325

    seb fontana
    Member
    from ct

    No..Old pump is 12-833 Holley and the write up [summitt or jegs] states 7.5 shut off and a regulator is not needed..and it did make 7.5 psi just as the write up claimed..I put a regulator in set to 5 psi as I was happy with the carb.....
     
  16. seb fontana
    Joined: Sep 1, 2005
    Posts: 6,325

    seb fontana
    Member
    from ct

    Quick search at jegs and found Eldebrock 1711; 130gph and 10# psi is probably the pump you have....Edelbrock 1711k1 kit comes with reulator...So you are right where you should be..
     
  17. exactly, the carb has a float bowl for a buffer and draws from there.

    Pumps are actually normally rated in pounds or gallons per hour. As in pounds of fuel or gallons of fuel that one will free flow in an hour. It is the volume of fuel that is important and not the pressure that it is supplied at. If I was racing I would much rather have a pump that flowed 110 gallons @ 3 pounds than one that flowed 70 gallons @ 6 pounds.

    @seb fontana perhaps the pump pressure comes up with RPM? On a mechanical pump given that it is flowing though an orifice that has not changed (come open after being closed for example) the flow should increase with RPM and thus the supply pressure. I could see you registering a pressure drop if the fuel level dropped when the throttle was opened and the pump was then working to fill the fuel bowl as opposed to the pump pushing against a closed needle at idle while you were reading the gauge. As long as the pump is keeping the carb full I don't think I would worry about it.
     
  18. seb fontana
    Joined: Sep 1, 2005
    Posts: 6,325

    seb fontana
    Member
    from ct

    Just got off the phone with Holley Tech..He said be happy at 6 psi..:rolleyes:...and that I should never see 7.5 psi thats why the write up states no regulator needed..Never said why its rated at 7.5 psi shut off when they do 6 psi..He said at wot at end of 1/4 pressure may be at 4 psi, still enough for my 300 hp..So I said if the pump really had 7.5 psi and I regulated it to 6 psi then at wot at end of 1/4 it would be 5.5 psi..He sorta got irritated and said I should return and by another brand! He said my previous pump must have been defective [ note couple posts up] if it did 7.5 psi; I said what did I know it did what the write up said? I think he got irritated again..Maybe I should tone down my comprehensive reading skills..Thanks guys..
     
  19. Ebbsspeed
    Joined: Nov 11, 2005
    Posts: 4,937

    Ebbsspeed
    ALLIANCE MEMBER

    Also keep in mind that it is a spring in the fuel pump pushing on the diaphragm that provides the pressurized fuel to the carb. The lever arm on the fuel pump merely "resets" the mechanism so the spring is compressed against the diaphragm. If you want to lower the pressure that a mechanical fuel pump puts out, you put in a weaker spring. If you have a pump where the diaphragm is clamped between two parts that are held together with screws it's real easy to do. If it's one of the NOS style where the pump diaphragm housing is crimped together, you're pretty much stuck.
     

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