Holley 4150 750...recommendation to go 650

Discussion in 'Troubleshooting & Diagnosis' started by JD Z, Oct 27, 2017.

  1. G72Zed

    G72Zed Veteran Member

    376
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    Sep 8, 2015
    Canada
    1- JD Z, the "tech" might be basing his opinion on the "Holley" chart along with his view points, experience and what he see's based on your engine and set-up. As a guide, Holley does state a 600 cfm for 350ci up to 6500 rpm, 650 cfm up to 7,000 rpm and a 700 cfm for 7,500 rpm. You may get them to check the carb to see if it's been re jetted. The 4779-9 should have a 71 jet primary with a 6.5 Power valve, 81 jet in the back, no Power Valve.

    2- You will not be losing anything with the 650, all else being equal. You may in fact gain throttle response at most rpm's, and drive-ability if the carb is tuned right by a pro.

    PS : There are great carb companies out there that could help you figure out what carb is best based on engine size,RPM, and components being used and expected results (Pro Systems/AED/Quick Fuel to name a few).

    Based on my experience, (SBC, single carb) the quick rule of thumb I use, and have observed on many occasions, is you need approx. 1.3 to 1.4 cfm per HP. That would mean a 650 CFM would deliver well on a 465 to 500HP engine. FWIW, I have done the test on my engine, going from a 750 Holley to a 650 Holley, all else equal, I only lost only 6 tq and 8 hp on the top of the rpm band. My engine used 700 CFM.

    Hope this helps out.
     
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  2. Lee45

    Lee45 New Member

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    3
    May 28, 2017
    IMO, and I apologize if this might sound rude, but I feel like you'd be wasting you're money buying anew carb. Most people who can't really tune a carb well will just decide it's too big, you can make a 283 snap and rock with a 950 if you're good. That being said a 750 is very at home on even the worst 350, a nice a 850 would run great too. You should really invest in learning to work on your own car and tune it yourself. Saves money and you'll always care more about it than anyone else. Get a A/F ratio gauge, best thing since efi, cost less than a new carb but you'll need a jet kit too. And don't get confused on how a carb works, yes they get "more fuel" but they are rated in cubic feet of air and you also get "more Air." 13 parts air to 1 part fuel is always the same, the TUNE decides your gas mileage not the size of the carb. X amount of air and fuel to turn an rpm is X amount of air and fuel regardless of the carb. And the hard starting is probably is related to both the carb and something else, weak spark possibly. My 350 fires up first hit with chokeless drage race 850 quickfuel double pumper and a 246@.050 cam when its 50 degrees outside.
     
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  3. JD Z

    JD Z Veteran Member

    G72, well said and thank you for your experience. With the car being new to me and not knowing how long, how much or why, I'm thinking a new 650 tuned by a professional would be a good thing. Drivability and throttle response are important. Drag strip though nice, not as important as overall drivability.
     
  4. G72Zed

    G72Zed Veteran Member

    376
    21
    Sep 8, 2015
    Canada
    My pleasure JD Z , you will not be disappointed in a properly tuned 650, and will enjoy the driveability more than a few less HP on the top rev range. Good luck to you.
     

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