My experience with carb tuning on a 350 with the sp3

Discussion in 'Small Block Tech' started by Reidk, Dec 19, 2021.

  1. Reidk

    Reidk Well-Known Member

    UPDATED WITH NEW VIDEO BELOW.

    Many of you have followed my stall converter thread that morphed into my build thread. Check video below to see how my car runs with a 600 cfm double pumper. Anyone not familiar with my story can keep reading. I feel like this new thread will be helpful to anyone else running the sp3 on a mild 350. I'm considering my setup mild. (284-88h cam, .020 shaved off heads, steel shim gaskets, valve job, sp3 intake, headers, stock bottom end, stock converter, stock 256 gears. I originally bought a 650 cfm edelbrock for this combo. The car was a turd. I spent a decent amount of time trying different jets and accelerator pump mods. nothing worked. I then bought a 750 cfm annular booster quickfuel double pumper. The quickfuels Perfomance was night and day in comparison to the edelbrock. But it was super rich in every circuit requiring 8 jet sizes leaner to get an acceptable cruise afr. it also had an off ifde hesitation that I couldn't tune out. The idle was pig rich and any changes to the bleeds and idle jets seemed to make the off idle stumble worse. I have david vizards book and the internet I spent days and multiple weekends trying to tune that carb. I drove the car all summer like that and occasionally would tweak the bleeds a little to see if anything helped with low speed drivability. I tried different transfer slot settings. I finally gave up and a few weeks ago I decided to buy a cheap summit brand 600cfm max performance double pumper carburetor. YES A 600 CFM. gs Johnny, and Ta performance reccomend that I try a smaller carb along with david vizards book, carb size calculators, and I read 2 build threads on this website that helped convince me. Jim weise built a 350hp 350 that in his words wasn't enough to tax a 750 cfm carb. The Motor trend article on this website features a 320hp 350 using a 600cfm carb. Both of these motors had dual plane intakes that are nowhere near as efficient as the single plane. The sp3 is extremely efficient. And the 600 cfm feeds it well in my experience. Check my video below. This isn't a perfect example of how it's really running. Its 30 degrees and the car had only been running 10 minutes when I started filming. In the video you will see zero to 100 mph in 17-18 seconds. My 750 cfm quickfuel did zero to 100 mph in the exact same time. But with the 600cfm the low speed drivability is consistently great. I dont think i lost much with the smaller carb. I have a 670 cfm vacuum secondary that i can experiment with too... Maybe next spring. In the meantime I will keep saving for better gearing, a better transmission setup, and a stall converter.

    About the annular boosters...they are supposed to be a high gain booster...problem with my quickfuel is I believe that they are positioned too high in the throttle bores. I believe this caused my stumble. (Low speed stumbles that idle and transition slot would normally cover). Im not talking about accelerator pump actuation here. Smaller air bleeds would allow the mains to come on quicker during transition from idle but would then be way too rich requiring the main jets to be reduced crazy amounts... that would then create stumbles and other issues all over again. Maybe a 650 annular with a smaller venturi would have been the ticket...not sure.



    Burnout video:
    20211223_151143.jpg 20211223_151122.jpg
     
    Last edited: Dec 23, 2021
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  2. gsjohnny1

    gsjohnny1 Well-Known Member

    all i can say is.......:D:D:D
     
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  3. Mart

    Mart Gold level member

    IDK....I think a 600 would choke my flow right out. :eek::eek:
     
  4. Reidk

    Reidk Well-Known Member

    You rev quite a bit higher dont you? If your setup works great I wouldn't mess with it. I mainly did this to cure my low speed drivability issues. But was shocked that my zero to 100mph time was exactly the same as my 750. Carb calculator wasn't real accurate when considering the dual plane with it's long runners but the single plane intake changes that equation. Look at this screen shot. I doubt I'm any where close to 110% volumetric efficiency. And My car will rarely see 5500 rpm. I haven't noticed a huge performance difference up top... Low end is better for sure. 600 cfm carb is enjoyable and reliable. Screenshot_20211220-225235_Samsung Internet.jpg
     
  5. Mart

    Mart Gold level member

    I don't think that formula works.
    69 Z28 302's had single 780's, and the dual cross-ram option had dual 585's. That's 1170 cfm!:eek: on a 302.:D
     
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  6. gsjohnny1

    gsjohnny1 Well-Known Member

    hey....we don't do no stinkin' chebbies here.....:D
     
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  7. Dadrider

    Dadrider Silver Level contributor

     
    Last edited: Dec 21, 2021
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  8. Dadrider

    Dadrider Silver Level contributor

    Reidk, I didn’t mean to hijack your thread or take it off topic. I read and kept up with your other post and what you went through. It is Great to hear your happier with the driveability.
     
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  9. Mart

    Mart Gold level member

    Soon as Reid gets his convertor, gears & 2004r, he'll start all over again...:)

    He's done more trial & error than most of us playing with carb combos.
     
  10. Reidk

    Reidk Well-Known Member

    I wish I would have bought your carb back when you offered it. I think a 750 with downleg boosters would work okay on my combo. But I wanted annular. I was so obsessed with the annular boosters and the low speed torque they are known for. I think its possible that the annular boosters in my carb have too big of holes or maybe too small of holes and or not enough of them. Lol crazy huh... Custom annular carb boosters have as many as 18 small holes. The amount of holes in the booster and where its positioned in the throttle bore will determine how responsive it is. Also, my metering blocks are whack. I put the primary metering block off of my quickfuel onto my 600 cfm and it ran like dog crap so its actually crossed my mind that maybe it was my metering blocks causing some of the issue....one thing is for sure...the car runs good with the 600 cfm.
     
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  11. Fox's Den

    Fox's Den 27 years of racing the same 355 Buick motor

    The big carb gets you the HP and the small carb makes the off idle transition quicker and faster. No one has really done much dyno their are some old Ken Bell tests but you have to do all the other stuff to get the same results.

    I bet their is not a big difference on a 350 But then I got to say at the same time their was a 30 HP difference between a 750 Carter AFB and a Holley 750 dbl pump... Holley won.
     
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  12. partsrparts

    partsrparts Silver Level contributor

    My combo is alot different from yours,( compression, cam, stall) and I put a 650cfm DP Brawler on mine here about a month ago straight out of the box, the only thing I have adjusted is air/fuel mix. I have not had time to dial it in, it has adj. air and fuel bleeds, but it is a lot more responsive and runs leaner than the Holley 750 DP. I could never get the Holley to lean out enough for the street without it being to lean at WOT. The Holley did not have adj. bleeds.
     
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  13. Reidk

    Reidk Well-Known Member

    Are you running the sp3 intake?
     
  14. dr

    dr Well-Known Member

    I will follow the thread for reference. Thank you
     
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  15. Buick#455

    Buick#455 Well-Known Member

    Yes he is. Also he's running ta aluminum heads too, I believe.....
     
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  16. partsrparts

    partsrparts Silver Level contributor

     
  17. Reidk

    Reidk Well-Known Member

    I think the sp3 is the game changer here. The efficient intake doesn't need so much carb...the power difference between a dialed in 650 and an 850 are probably less than 10 HP on a 350. Is cylinder wash and crappy throttle response worth it? Not for me.
     
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  18. Fox's Den

    Fox's Den 27 years of racing the same 355 Buick motor

    I would say we need to go back to a dyno and do these tests with a 350 and different carbs. someday this engine will be tested like the 455 then we will know. Until then people like Reidk and myself along with others here working this 350 will test it.

    I would like to add the 2 inch taper spacer but need a different hood to get my big air cleaner to fit.
     
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  19. Mark Demko

    Mark Demko Well-Known Member

    A 950 DP would be just about right:p
     
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  20. Reidk

    Reidk Well-Known Member

    There's a chassis dyno 2 hours from me in des moines. Theres another member here on the board that has used them. I would like to try both carbs on the dyno. Maybe this spring.
     

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