Bad Gas Mileage - Carb Tuning?

Discussion in 'High Performance Modifications' started by luda, Feb 19, 2013.

  1. luda

    luda Veteran Member

    Oct 1, 2002
    Eastern NC
    Hey guys, we recently made some changes to my car hoping to get better gas mileage and went from 10 to 12mpg. I realize my carb is too big for my setup. I have a few questions with regards to this.

    1) Can this Holley be tuned to give me better gas mileage?
    2) If not, how hard is an Edelbrock to tune?
    3) Could I expect a significantly better mpg with an Edelbrock 650?
    4) How much performance can I expect to lose if I drop to a 650? I'm guessing none but figured it was worth asking.

    If I can't tune this one for better mpg, something has to give...I can't afford it with the way gas is going with this being my daily driver. o_O

    75 Block 350
    Bored 60 over
    Forged Flat Top Pistons 8.5-9 to 1 compression
    Vortec heads with new springs for higher lift
    Edelbrock Performer EPS Intake
    Seegers Cam 292/288 advertised duration at .005
    224/224 @ .050
    .300 @ .450 intake
    .307 @ .460 exhaust
    Hooker Competition Painted Headers
    High Flow Cats
    Dynomax Super Turbo Mufflers
    2.5" true dual exhaust
    Holley 750 Vacuum Secondary
    4.11 Richmond Posi
    2600-2800 PTC Stall w/ Lockup
    700r4 Transmission Swap
    Last edited: Feb 19, 2013
  2. markw

    markw Veteran Member

    Jun 6, 2009
    4.11 gears + gas mileage = no
  3. jb451

    jb451 Veteran Member

    Jan 17, 2011
    Mesquite, Texas
    I was gonna say, 12 mpg is pretty dang good for 4.11's (if you have a th350 that is). I don't get much better with the stock 2.41's in mine, i get somewhere in the 15 mpg range. OD trans i think is going to be your next best step!

    I DD mine as well :bowtie:
  4. luda

    luda Veteran Member

    Oct 1, 2002
    Eastern NC
    OH...will edit, forgot to change that. We swapped it for a I do have overdrive and lockup. :cool:
  5. dale68z

    dale68z Veteran Member

    May 14, 2009
    Glendale Arizona
    Learn how to drive for fuel economy. Look ahead,lift early, take off slowly.
    Optimize your ignition timing and keep it in overdrive.

    My engine is larger, carb bigger, and cam more aggressive, but has 3.73 gears and OD. It gets better fuel economy than you are getting.
    When I want, I use my power. But because I daily drive my 71, now that I'm repainting the 68. It's not often.
  6. Todd80Z28

    Todd80Z28 Moderator Staff Member Lifetime Gold Member

    Jun 11, 2002
    Northern VA
    Are you using vac advance on the distributor?

    That cam is wrong, IMO, for best mileage also.
  7. ZS10

    ZS10 Moderator Staff Member Lifetime Gold Member

    Jan 18, 2003
    BC, Canada
    Gotta look at what you're doing and how you drive.

    Low rpm is key, so 14* less @.05 cam, set up the converter lock so it comes on as soon as the engine can handle it, 3.42 gears would help.

    After that, it's all how heavy your foot is.
  8. luda

    luda Veteran Member

    Oct 1, 2002
    Eastern NC

    Well that's fair enough. I didn't pick the cam though, shop did. Gotta make the best of it for a while, probably won't change that until I tear it down to rebuild which hopefully won't be for a while.

    As far as vacuum advance, yes I am.

    Also, I'm keeping my foot out of it as much as possible. Right now I need to work on the shift points and lockup. My lockup is hitting AS SOON as it hits OD so I either have to keep it in 3rd longer OR let it hit OD but barely tap hold the break so lockup doesn't occur until I want it to. You guys are thinking my secondaries could be opening up even at normal acceleration? If so...I might take the time to spring it as stiff as possible until I get my shift points worked out. Currently I have 68 jets in it.

    A side note, we went 4.11 because I mainly do city driving and it allows me to lock up as low as 50mph. I really don't know that I feel comfortable doing it much less than that...what do you guys think? Would it build up too much heat or lug the engine too badly at speeds lower than that?

    Thanks for all the help, I really appreciate y'alls responses...trying to not go broke while I finish my last year of my masters.
  9. Blown Camaro

    Blown Camaro Veteran Member

    Dec 13, 2009
    Deltona, FL
    You should be getting a lot better than that! I get 11mpg with two carbs on a 6-71, 3500 stall converter, 4.11 gears and a TH350.
  10. hhott71

    hhott71 Veteran Member Lifetime Gold Member

    Mar 30, 2001
    Joplin Mo. 64801

    The 3310 Holley is NOT too big, It is the same size as a 2 bbl if you keep your foot out of hearing those secondaries roar!
    Flat top pistons and 64cc chambers in 10.25:1 compression.
    You should use 91 or 93 octane and full advance.$0.10 more per gallon is only $1.50 more for a fill up.
    4.10 x .7 OD = 2.87 rear gears.
    So that is not hurting you much.
    3.42 x .7 = 2.39 so the 2.87 ain't bad with the 4.10's.
    Lock up stall is excellent choice.

    The 224* camshaft is not an econo cam

    10-12 mpg sucks with OD and a lock up.

    A similar engine but with 221* cam. 041 heads 10:1 comp and 2400 stall 350TH 4.10 gears got 12 mpg everywhere.

    SO carb tuning, stock 72 jets are fine.
    A 8.5 or 6.5 PV should do it, stock 10.5 is too high and may open at idle.

    Ignition, 12* initial, 36* at 2500 rpm then add in the vacuum advance get an adjustable one so you can set it up to where it is just under the detonation threshold.
    You should see 15-17 with some tuning of what you have.
    Last edited: Feb 20, 2013

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