Holly 870 street avenger??

Discussion in 'Holley' started by Steve Yahnke, May 8, 2007.

  1. Steve Yahnke

    Steve Yahnke Well-Known Member

    Has anyon e on the board used this carb? If so how streetable is it And if you realized any big performance gains over a quadrajet? at present i have a john osborn q-jet on a 455 that runs quite well,but would like more....thanks steve
     
  2. gsxbuildernut

    gsxbuildernut Well-Known Member


    If you have a 800 q-jet that john did I doubt you will get much gain from the Holley.

    Steve
     
  3. thefenderbender

    thefenderbender Well-Known Member

    I don't have on in my Buick, but my other half had one in his '57 Chevy Truck. It kept having issues with not getting enough fuel and jumping - I guess that'as the best word for it. It's a lag you can definatly feel when you put your foot all the way down. His buddy had one too that did ther same thing. So now I'm looking for a differnt model for my Skylark - I don't have to have emission where I live.
     
  4. Stage1 Jeff

    Stage1 Jeff Guest

    Re: Holley 870 street avenger??

    I have the 770 street avenger, and it has been a pain in the butt from day one
    (I have had it since nov '03) my main issue was mainly the choke, it would never work right, so I have it blocked wide open, and my rich condition has went away. I plan on having the air horn milled off next.

    I too had a stumble problem, took several tries, but I finally found the right accel pump squirter,and jets and that went away also.
     
  5. Andy Brinksman

    Andy Brinksman Gold Level Contributor

    I'm running the 870 on my 455. I'm still tying to sort out a lean condition with this carb. I increased the primary and secondary jets by 6 sizes and it still needs more fuel. I have only seen a one tenth gain in ET over my Q Jet. I will be trying a well prepared 850 DP in a few weeks to see the difference.
     
  6. Stage1 Jeff

    Stage1 Jeff Guest

    gee, I had the opposite problem, a rich condition. took many,many tries to get it usable. so much for their claim of "bolt it on and go, street ready out of the box" HA! after 3 years of screwing with mine, I may have it where I can finally be happy with it
     
  7. thefenderbender

    thefenderbender Well-Known Member

    Did you end up jetting it down a few nothces or chanign out the secondary meetering plate? The problem my guy had was it caused hesitation under wide open throttle. After messing with a few different ones for about a year with a few different Avenger models they traced it back to the secondary metering plate - It didn't have enough vacuum to switch from one circuit to another.

    For mine I'm going off the formula for carb cfm size as given to me:

    Engine Size x Max RPM / 3456

    300 x 5000 / 3456 = 434.02 (for my new engine)

    Sine it's going to be street for me running no more than 5000 RPM and a bit more than a basic stock engine multiply the total by 1.1 for 110% efficiency.

    300 x 5000 / 3456 x 1.1 = 477

    That's my magical number not to go over or it'll be too much for the engine. I'm looking at a 600 Holley vacuum secondaries with electric choke. I know summit caries them if you are open to looking at it more.

    Hope that helps. lol. Took me a long time to start to finally understand carbs and I don't think I'll ever fully get it!
     
  8. Stage1 Jeff

    Stage1 Jeff Guest

    I'm running 72 front,and 75 jets now, I had to go up from the stock sizes. I also had to go up on the accelerator pump squirter too, I forget what size I went up to. I did no changes to the metering blocks, since I have the choke full open all the time now, it seems to act better, I'll know more when I take it on the road in 2 weeks

    I'd happily trade my carb for a good 850 DP :)
     
    Last edited by a moderator: Jun 6, 2007
  9. Schurkey

    Schurkey Silver Level contributor

    1. If there are problems with a Holley choke--and we're talking about the style of choke where there's a choke coil on the side of the carb--the problem is usually the choke pulloff piston adjustment. NOBODY seems to adjust the choke pulloff piston, and I've never seen one adjusted right by the factory. In many cases, the vacuum supply to the piston is faulty because of a very tiny gasket that is often out-of-position or not used at all.

    The typical symptom is that the engine runs terrible when first started--either it stalls; or it gargles on gasoline, smokes, and misses. As the engine warms up, the symptoms go away.

    Too many people compensate for an out-of-adjustment choke pulloff piston by turning the choke coil too far lean. That just makes for poor cold starting.

    2. A better guide to CFM needed by a given engine would include compensation for expected volumetric efficiency, as well as compensation for the style/efficiency of the intake manifold.

    You have the basics correct: Engine size X RPM / 3456 (where 3456 is a number that takes into consideration the number of cubic inches in a cubic foot; and the fact that these engines use a 4-stroke cycle.)

    So your 300 cubic inch engine running 5,000 RPM would need 434 cfm; just as you said.

    From there; I'd adjust for a projected 80% volumetric efficiency.

    434 X .80 = 347

    Then you multiply by a correction factor depending on your intake manifold. A single-plane manifold would use a factor of 1.1 to 1.3, with higher performance (race-type) manifolds using a LOWER number. A dual-plane manifold would use a correction factor of 1.3 to 1.5; again, higher-performance manifolds use a LOWER number.

    Since you're using a basically stock engine, I'll guess at 1.4 for a correction factor.

    347 X 1.4 = 486 CFM carb size.

    Using a larger carb is NOT a problem IF it will still meter fuel well enough at lower speeds. Special booster venturis; and/or fierce carb tuning can yield performance gains and still retain streetability.
     
  10. thefenderbender

    thefenderbender Well-Known Member

    Okay, I followed everything but the 80%. I thought 80% was for street use on the stock engine, but with the machine work, and replcement cam, heads, and intake that you use the larger number. What determines if it is 80 or 110%. There I am confused.
     
  11. Schurkey

    Schurkey Silver Level contributor

    For normally-aspirated engines, you'd typically be using 70--90%, and, basically, take your best guess. A ram-tuned engine could go over 100%, but only for a narrow part of the RPM band.
     
  12. Steve Yahnke

    Steve Yahnke Well-Known Member

    Thanks guys...I think I'll stay with my Q-jet...since John rebuilt it I've had no problems..I was hoping for a few more tenths,but for that much hassel I'll just leave well enough alone.
     
  13. sean Buick 76

    sean Buick 76 Buick Nut


    I suggest you use a Q jet on your 300. You'd use an adapter but it would work great. The q-jet is variable cfm based onthe need of the motor. it is imposible for a Q-jet to overcarb where as a holly style carb will cause a HUGE bog if its too big.

    People have reported big time bogging issues using 600 cfm hollys on the 300.
     

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