Register now to get rid of these ads!

Technical SBC 8.2 mpg ?

Discussion in 'Traditional Hot Rods' started by AGELE55, Aug 20, 2021.

  1. AGELE55
    Joined: Jan 4, 2018
    Posts: 394

    AGELE55
    Member

    I’m swapping carbs today on the 55. Prior to pulling it apart, I topped off the tank and crunched the numbers, suspecting my mileage was pretty awful. It was not awful, it was holy crap awful. 8.2 mpg.
    So the new motor (350cui, 300 hp, GM crate engine) , with less than 70 miles on it, is until today, running an Edlebrock 750, (transferred from the old engine). I’m installing a new 650 cfm now.
    Odd thing is it seemed to run pretty good, but the gas gauge was in a death dive. So other than being over carbureted, I also see my in-line fuel pressure gauge is bouncing floating around 8-10 psi at idle. Edlebrock recommends 6psi max and suggests 5.5. I’m surprised my stock mechanical pump is pushing that much pressure. Could the excess pressure contribute to the suck gas mileage? Maybe blow past the float needle?
    I was in Autozone earlier and they had a pressure regulator in stock, but it said adjustable between 1–4 psi. Hmmm. So too low with a regulator, and too high without it.
    New carb should be on later today, so curious to see if the mileage is still atrocious. I don’t think the excess pressure will hurt the carb…
     
  2. harpo1313
    Joined: Jan 4, 2008
    Posts: 2,329

    harpo1313
    Member
    from wareham,ma

    Trans and rear ratio?
     
  3. Budget36
    Joined: Nov 29, 2014
    Posts: 7,491

    Budget36
    Member

    What’s the oil smell like? Are the plugs black? I’d guess if the fuel was dribbling in it and through you’d not have much or any air bleed adjustment?
     
    Truck64 likes this.
  4. olscrounger
    Joined: Feb 23, 2008
    Posts: 4,398

    olscrounger
    ALLIANCE MEMBER

    About 5 lbs for Edelbrock carb. Have found that the 290hP crate is not good on mileage-may be due to low compression-not sure. Put smaller chamber heads on one with a Crane 266/440 cam and it woke it up a good bit--don't know if mileage improved though.
     

  5. iagsxr
    Joined: Aug 26, 2008
    Posts: 159

    iagsxr
    Member

    Yes, too much fuel pressure.

    The pump off the old engine?
     
    onetrickpony and olscrounger like this.
  6. trikejunkie
    Joined: Dec 2, 2011
    Posts: 102

    trikejunkie
    Member
    from Scotland

    That is a strange post to see - I was asking myself that yesterday! International Harevster truck -327 SBC ,350 auto trans,3.38 ish rear with tall tyres .I think I am similar MPG!!! I have a Weber carb 500cfm with a regulator running at 4psi . I think the main jetting may be to large-smells a bit rich so a jet kit and some meddling may be in order
     
    pitman and Deuces like this.
  7. Jack E/NJ
    Joined: Mar 5, 2011
    Posts: 685

    Jack E/NJ
    Member
    from NJ

    >>>I topped off the tank and crunched the numbers>>>

    Gas pump was probably set to liters. So you actually got 31 miles per gallon. Pretty good.
     
  8. b-body-bob
    Joined: Apr 23, 2011
    Posts: 514

    b-body-bob
    Member

    That's not bad compared to some 440s I've had in the past
     
    Frankie47, Deuces and Desoto291Hemi like this.
  9. squirrel
    Joined: Sep 23, 2004
    Posts: 50,173

    squirrel
    Member

    tell us about the gauge. Does it read Zero when there's no pressure?

    Tell us about the pump. Does it have 2 or 3 fittings on it? if 3, what's connected to the return (small) fitting?
     
  10. AGELE55
    Joined: Jan 4, 2018
    Posts: 394

    AGELE55
    Member

    Muncie with a 3.27 rear gear
     
  11. AGELE55
    Joined: Jan 4, 2018
    Posts: 394

    AGELE55
    Member

    Yep. I do believe I was noticing bad mileage on the old engine before it ate itself.
     
  12. AGELE55
    Joined: Jan 4, 2018
    Posts: 394

    AGELE55
    Member

    Standard GM mechanical pump. Inlet from the tank and a single output.
    Running, the gauge pulses along with the diaphragm usually between 8-10 psi. When I shut the engine off, it holds randomly between 8-10 lbs, depending I guess where the pump diaphragm ended up. If I bleed off pressure by loosening a line, it drops to zero.
     
    squirrel likes this.
  13. squirrel
    Joined: Sep 23, 2004
    Posts: 50,173

    squirrel
    Member

    Desoto291Hemi and olscrounger like this.
  14. AGELE55
    Joined: Jan 4, 2018
    Posts: 394

    AGELE55
    Member

    “Not good” and God awful are two differ thangs..:rolleyes:
     
    trikejunkie and Desoto291Hemi like this.
  15. squirrel
    Joined: Sep 23, 2004
    Posts: 50,173

    squirrel
    Member

    I would expect 8 to 10 around town with "spirited" driving. 12-15 on the road. So it's not really out of whack.

    Also, going by one part tank fillup is not real accurate...averaging over several full tanks will give you a more accurate number.
     
  16. AGELE55
    Joined: Jan 4, 2018
    Posts: 394

    AGELE55
    Member

    Define
    a. Little Chevy
    b. Driving around town…lol
    I’ve been known to drive a “little” Chevy around town with a heavy foot.
     
    Deuces likes this.
  17. AGELE55
    Joined: Jan 4, 2018
    Posts: 394

    AGELE55
    Member

    I only traveled 64 miles and burned 7.8 gallons. Just initial test drive and then to the alignment shop. I’m dreaming of 16mpg…. :D
     
    kidcampbell71, trikejunkie and Deuces like this.
  18. alchemy
    Joined: Sep 27, 2002
    Posts: 17,440

    alchemy
    ALLIANCE MEMBER

    But it might burn your car down. I know someone who's over-pressurized carb overflowed out a vent and started a fire. Luckily only the carb needed replacing and the hood repainted.

    Then he's installing a regulator and gauge.
     
    Truck64, harpo1313, Deuces and 3 others like this.
  19. AGELE55
    Joined: Jan 4, 2018
    Posts: 394

    AGELE55
    Member

    Awesome. Looks exactly like the one at Autozone, but with a pressure range where I need it.
     
  20. olscrounger
    Joined: Feb 23, 2008
    Posts: 4,398

    olscrounger
    ALLIANCE MEMBER

    Regulator Squirrel shows is what we use. The little ones with the dial-no thx.
     
    dirty old man and AGELE55 like this.
  21. It's a damn hot rod ,did you build it for economy ????? Trade it in for a Honda
    LOL
     
  22. One of the perks of living in Canada , driving season is 5 months , the other 7 we save for fuel !!!!
     
  23. sdluck
    Joined: Sep 19, 2006
    Posts: 2,806

    sdluck
    Member

    I have tuned a couple of these and they all need a lot of timing initial and vacuum advance
     
  24. sdluck
    Joined: Sep 19, 2006
    Posts: 2,806

    sdluck
    Member

    Time it with a vacuum gauge.
     
  25. AGELE55
    Joined: Jan 4, 2018
    Posts: 394

    AGELE55
    Member

    Thinking of a nice Prius...LOL. ;)
     
    VANDENPLAS and wood remover like this.
  26. trollst
    Joined: Jan 27, 2012
    Posts: 2,069

    trollst
    ALLIANCE MEMBER

    No kidding, 327, 30 over, camel hump heads, flat top pistons, 273 gears, turbo 350, 20-25 mpg consistently, always has, and yes, it'll get out of its own way. Sees 90% hiway stuff. DSC07849.JPG DSC07873.JPG
     
  27. Tman
    Joined: Mar 2, 2001
    Posts: 35,128

    Tman
    ALLIANCE MEMBER

    I lot of folks do not realize there is a low pressure version of that regulator and a standard one.

    My similar 350 with a cheap/new 650 Holley got 18 to 20ish all the time. Rear was 3.23 with a T400
     
    pitman, rod1, wood remover and 2 others like this.
  28. That's what I use.
    Off the shelf replacement mechanical pumps can put out way too much pressure, so never assume just because it's a mechanical pump the pressure will be okay. You can waste a lot of time and money chasing engine issues and blaming carburetors if the pressure is to high in the first place.
     
  29. 6sally6
    Joined: Feb 16, 2014
    Posts: 1,783

    6sally6
    Member

    Check around the intake/carb base for a vacuum leak......(voice of experience with a Y-block engine.)
     
    wood remover and Deuces like this.
  30. Joe Travers
    Joined: Mar 21, 2021
    Posts: 485

    Joe Travers
    Member
    from Louisiana

    Air/fuel ratio much too rich, I presume. Do you read the plugs? I have never had to use a regulator on a carburetor in almost 50 years with mechanical pump. The regulator issue came up when Edelbrock came out with their 'tuning' perspectives. Holley's have never let me down when floats were set correctly at idle. Use a vacuum gauge and set timing and idle mixture for maximum vacuum. Carb size is not the issue, it's the jetting and most likely ignition timing.

    Joe
     
    pitman, deathrowdave, Truck64 and 3 others like this.

Share This Page

Register now to get rid of these ads!

Archive

Copyright © 1995-2021 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. Terms of Service. Privacy Policy.

Atomic Industry
Forum software by XenForo™ ©2010-2014 XenForo Ltd.