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Spread Bore carbs for mileage?

Discussion in 'The Hokey Ass Message Board' started by blowby, Aug 2, 2013.

  1. blowby
    Joined: Dec 27, 2012
    Posts: 7,265

    blowby
    Member
    from Nicasio Ca

    Thinking about taking my Vette ('66 but doesn't the HAMB have a "like body" clause?) on a trip. 454 with lumpy cam and 850 DP Holley, TH400 and 3:08s. Last I checked it got about 9mpg. Anyone think mileage would improve noticeably if I plopped a spread bore Holley or Q-jet on it and kept my right foot under control? I saw some published figures for a 454 Impala at 8-10 mpg, maybe that's the best one could expect.
     

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  2. Ole don
    Joined: Dec 16, 2005
    Posts: 2,915

    Ole don
    Member

    I tried that with an adapter plate. I think you need to keep the four holes separate to keep air velocity up. The spread bore adapter has one big hole. My fuel mileage was worse, and throttle response really sucked. It came off right away.
     
  3. ANDEREGG TRIBUTE
    Joined: Jan 1, 2008
    Posts: 1,370

    ANDEREGG TRIBUTE
    Member
    from Bordertown

    IMHO 650 w vac secs. The lumpy cam is prob the real mpg killer.
     
  4. Stroker McGurk
    Joined: Feb 17, 2012
    Posts: 291

    Stroker McGurk
    Member
    from Canada

    Probably aint gonna be any better.....You could try a 2bbl with adapter......might get 9 1/2mpg....lol
     

  5. ANDEREGG TRIBUTE
    Joined: Jan 1, 2008
    Posts: 1,370

    ANDEREGG TRIBUTE
    Member
    from Bordertown

    I am real interested in the responses to this thread, and also curious if our old cars will ever get decent mileage with todays gasoline.

    It is a big block Vette, the anti-mileage car...lol
     
  6. Russco
    Joined: Nov 27, 2005
    Posts: 4,167

    Russco
    Member
    from Central IL

    Lumpy cam and 3.08 gears, probably won't make much difference what carb you run.
     
  7. Deuces
    Joined: Nov 3, 2009
    Posts: 19,606

    Deuces
    Member
    from Michigan

    The exhaust note will sound the same with a smaller carb....
    Go with a Holley List# 4777 650 cfm DP carb...
     

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  8. dreracecar
    Joined: Aug 27, 2009
    Posts: 3,110

    dreracecar
    Member
    from so-cal

    put it on a trailer and pull it around with a truck that gets 15mpg
     
  9. A motor like that will take what it needs or it won't run very good is the bottom line.
     
  10. blowby
    Joined: Dec 27, 2012
    Posts: 7,265

    blowby
    Member
    from Nicasio Ca

  11. mixedupamx
    Joined: Dec 2, 2006
    Posts: 513

    mixedupamx
    Member

    double pumpers are mileage killers but if you switch to a spread bore it may not help if the tune is off. I would get a fuel air ratio meter with a wide band oxygen sensor in the exhaust. I've looked into them and they can be bought for about $200.00 this would let you know what circuit in the carb is rich or lean. I've not gotten one yet but I've read about them several times (one article was by Jeff Smith in Car Craft) and they sound like a great tuning tool to really dial a carb in. also check out your dist. advance curve as that can make a huge diff. in drivability and MPG . the 3.08's should keep the rpm's down enough to allow some improvement, good luck
     
  12. 53sled
    Joined: Jul 5, 2005
    Posts: 5,818

    53sled
    Member
    from KCMO

    4.8L ls turbo motor and overdrive.
     
  13. Andy
    Joined: Nov 17, 2002
    Posts: 4,679

    Andy
    Member

    I had 73 Caprice wagon with the 454. It would get 15 mpg at 55. It weighed 7000 lbs. This was with the smog stuff still on it. If I had tuned it, I feel that I could have gotten near 20 on the highway. I know it always ran rich.
     
  14. dreracecar
    Joined: Aug 27, 2009
    Posts: 3,110

    dreracecar
    Member
    from so-cal

    Getting 9mpg @ $4 a gal will cost you 0.44 per mile. The same at 12mpg will cost 0.33 per mile so just the increase of 3 more miles per gallon will result in a total savings of $3.66 for 100 miles. So if you were to change out the carb and the new one cost $366.00 it would take you 1000 miles to break even before you see any savings.
     
  15. hoop98
    Joined: Jan 23, 2013
    Posts: 1,363

    hoop98
    Member
    from Texas

    You need a vacuum advance calibrated for that engines vacuum level and a Power Valve that doesn't move until you really accelerate. Also high gain boosters will help the primaries at part throttle.
     
  16. blowby
    Joined: Dec 27, 2012
    Posts: 7,265

    blowby
    Member
    from Nicasio Ca

    That's darn good!

    Hmm, I'm calculating more like $110 savings on a 1,000 mile trip, about the length I'm contemplating. Still, that's steaks instead of burgers for the whole trip.

    Interesting, the adjustable advance and power valve wouldn't cost much. Ported or engine vacuum? I've never understood the benefits of each, or why they switched.
     
  17. Well if you're going to get all logical about it, it sort of sucks all the fun out of it. ;)

    With an auto trans and 3.08s I think I'd lean toward vacuum secondaries instead of a double pumper. With a sharp tune-up and a lighter foot a spread bore should help your mileage too. And try driving with a manifold vacuum gauge installed where you can see it while driving. Try to keep the vacuum reading as high as possible while cruising.
     
  18. dreracecar
    Joined: Aug 27, 2009
    Posts: 3,110

    dreracecar
    Member
    from so-cal

    Hmm, I'm calculating more like $110 savings on a 1,000 mile trip, about the length I'm contemplating. Still, that's steaks instead of burgers for the whole trip.


    Which makes it worse if you have to buy a new carb.
     
  19. mustang6147
    Joined: Feb 26, 2010
    Posts: 1,847

    mustang6147
    Member
    from Kent, Ohio

    Why not lean out the primaries on that carb...

    I am thinkin that carb is 84 sq with a 6.5 PV. You can drop the primaries down to 80 before you loose to much fuel atomization. Plus keeping your foot off the floorboards with the peddle sandwiched between the 2 will also help. :eek:

    Make sure your distributer advance is working and right. It amaizing how many MPG that will rob
     
  20. hoop98
    Joined: Jan 23, 2013
    Posts: 1,363

    hoop98
    Member
    from Texas

    Ported/Manifold doesn't affect mileage.

    Call Quick Fuel and tell them what you are trying to do, they can calibrate a carb that will make more power and help you get 11-13 MPG. it's about a better mixture formation at part throttle.

    [​IMG]

    In the mean time get a new power valve, 5''.

    Get an adjustable vacuum advance and increase advance until you get ping going up hill at part throttle.

    Before that make sure your mechanical advance is working and you have a total of initial plus mechanical in the 32-36 range.

    Get a vacuum gauge and draw a red line at 6.

    Good luck, 12 is out there maybe better.
     
  21. Jalopy Joker
    Joined: Sep 3, 2006
    Posts: 26,015

    Jalopy Joker
    Member

    Quick Fuel good company. but, you should step up and put a five speed in it.
     
  22. carbking
    Joined: Dec 20, 2008
    Posts: 2,960

    carbking
    Member

    A properly selected spread-bore WILL give you better mileage IF you also have a spread-bore intake.

    A spread-bore carb with an adapter on a square-bore intake will give you WORSE mileage.

    Do the math on a conversion as others have. It would take a lot of miles to break even if you have to do both a different carb and a different intake.

    As others have suggested, try to improve the efficiency of what you have. And don't overlook either a dragging brake piston or tire pressure. You would be shocked at how much mileage low tire pressure can steal.

    Jon.
     
  23. scrappybunch
    Joined: Nov 16, 2011
    Posts: 338

    scrappybunch
    Member
    from nj

    Plus 1

    And a wide band A/F ratio meter to see the fat and lean spots. Most likely your fat while cruising.
     
  24. blowby
    Joined: Dec 27, 2012
    Posts: 7,265

    blowby
    Member
    from Nicasio Ca

    It has an old Holley open plenum single plane manifold which I believe has both bolt patterns and a trapezoid shaped opening, if memory serves. I wouldn't buy a new carb, thought I might go sniffing in friend's garages for an old Q-jet. But I'll try the jets, timing and power valve first.

    Good point on the tire pressures. And vacuum gauge. And A/F gauge.
     
    Last edited: Aug 2, 2013
  25. 68vette
    Joined: Jul 28, 2009
    Posts: 306

    68vette

    78 chevy pickup with a fairly low milegae target master 350.....8-10 mpg no matter what.

    76 ford van with a 351.....10 mpg no matter how or what or where

    51 ford shoebox with 305 built by me, turbo 350, 2.79 rear gear, quadrajet tuned by me....24.4 and 24.6 mpg running 70 mph on the interstate.

    My 327-350hp on the highway....about 14mpg running 70 or better and playing a little...cruzing 45 mph on the parkway on sunday pm...about 20-22 mpg or better with a quadrajet.
     
  26. hoop98
    Joined: Jan 23, 2013
    Posts: 1,363

    hoop98
    Member
    from Texas

    You can use this 6" switch

    STANDARD MOTOR PRODUCTS SWITCH VOES 6" HG

    [​IMG]



    to turn on a warning light after you install the 5 inch PV. This will warn you your mileage is about to drop 35 pct.

    As far as drag and measurement go here...to learn how to do coastdown measurements yourself

    [​IMG]

    http://www.instructables.com/id/Measure-the-drag-coefficient-of-your-car/
     
    Last edited: Aug 2, 2013

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