Jets and Power Valves 750 Holley 4150

Discussion in 'Fuel and Air Systems' started by 70DusterBob, Nov 24, 2018.

  1. 70DusterBob

    70DusterBob Well-Known Member

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    Hey,

    I will be going with the Blue Print Engine's 408. I want a good set up for it. I already have a 750 Holley willing to go on it, but was wondering what would happen if I used both power valves, and if I do what size they should be, I also wondered what side jets to use on it.

    Any help is appreciated...
    Thanks in advance!
     
  2. mbaird

    mbaird mbaird FABO Gold Member

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    PV size will be dependant on Vacuum @ idle.
    Run both power valves.
     
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    • Fisher

      Fisher Old Guy with a Cool car.

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      71 jets on the primary side with a power valve, and 83 jets on the secondary side with a plug, no power valve will be a good starting point.
       
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      • AJ/FormS

        AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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        What would be the point of PVs in the secondaries? When they open, it's usually because your foot is in the carb. The secondaries will transition just fine without PVs.
        As to the front side, YellowRose says IIRC to run a PV a tad under your cruise vacuum regardless of idle vacuum, and I concur with that. If you have your transfer slot properly synced at idle, it will not activate the PV, at idle.
        My PV is a 10.5 which is .5 less than my idle vacuum. I have a manualtrans, and can suck the idle down to 550 rpm/4 mph in first gear. I don't recall the engine vacuum down there, but it must be pretty low. The PV stays closed just fine.
        How did I choose the 10.5 ?
        This has to be done AFTER the primary MJ had been determined, together with the PV. I disconnected the secondaries.
        I installed a PV plug and took her for a ride. Got her up to 30mph in second gear,~2500rpm, and rolled into the throttle paying attention to the vacuum gauge and engine response. It is very easy to detect when the engine goes lean. I noted the vacuum, at the stumble, and installed the 10.5. This gave me a nice transition from mains to power. I had been running the idle-vacuum divide by two PV, but I could tell that was not right. I suspected a faulty PV, so ordered up a box of them plus the Morosso checking tool. I calibrated all my PVs and boxed them up in marked compartments. I started with a 5.5 IIRC, and worked my way up to the 10.5, with improvements at every step. The PV plug routine was right on.
        When you get it right, the engine will be very snappy with great response. No part-throttle sluggishness.No tip-in hesitations. When you roll into the throttle it will deliver power smoothly like a diesel/electric freight-train.
         
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        • mbaird

          mbaird mbaird FABO Gold Member

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          Agree... but he is asking about his initial setup. So the 1/2 idle vacuum is a easy starting point.
          I am up to a 9.5 and my idle vacuum is 15.
          I need to check my vacuum at cruise again and adjust accordingly
           
        • 70DusterBob

          70DusterBob Well-Known Member

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          Thanks you guys,

          I am wondering, I have a Holley manual, a few of them actually, from back in the day. I recently relocated and don't know where they are right now, but I do need to re-read them on PV's, but what is the secondary PV for? Someone told me it was for Oval Track. Does it open the pathways for the fuel to be delivered through the jets? If so, would I need a smaller jet set up to run with a PV 0.5 less than my idle vac? It seems to me that from what I remember, the PV opens the pathway for the fuel to flow through the jets, is that right or do I not remember correctly???

          Thanks
           
        • 70aarcuda

          70aarcuda Master Hoader of SBM FABO Gold Member

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          the fuel that goes thru the power valve restriction channel does not go thru the jet....the channel enrich the fuel mixture....plug the rear as said above...
           
        • mbaird

          mbaird mbaird FABO Gold Member

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          what is the reasoning behind plugging the secondary PV ?
           
        • crackedback

          crackedback FABO Gold Member FABO Gold Member

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          How often are you at part throttle with the secondaries open? PV leans out the amount of fuel under cruise conditions.

          IMO, having a PV in the secondaries is pretty useless 99% of the time.
           
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          • mbaird

            mbaird mbaird FABO Gold Member

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            I am starting to realize that with todays high speed limits (80mph ) more often than I would have guessed... I have to rethink gears as well since I find myself running 4k ish to keep up with traffic .

            Not to disagree... just sayin.
             
          • Mattax

            Mattax Just the facts, ma'am FABO Gold Member

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            As far as the fuel to the engine goes, its for driving at high speed with moderate load on the engine, if the engine doesn't need enrichment early. Maybe circle track, more like road racing, or high speed time-speed events like Silver State Classic. It also also means he use of smaller secondary jets for slightly better fuel control under hard deceleration.
            Simplicity.
            There's a truth! In fact, secondaries are rarely open to begin with. Passing on the interstate and drag strip is about it for most.
             
          • Mattax

            Mattax Just the facts, ma'am FABO Gold Member

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            Mostly that would be on engines that are inefficient at part throttle.
            Seems reasonable.
            The situation where it gets people in trouble is engines with very poor idle characteristics. An engine that idles with 5 "Hg, very well might need a PV that opens at 6 or 8"Hg or there will be a flat spot in part throttle acceleration.
             
          • AJ/FormS

            AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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            You are probably right, I just haven't tuned any that big. The biggest I have tuned is the Mopar 292/292/108. At 11.3Scr, it was fairly easy. On that combo,I did quite a bit of experimenting. I tried 4 different carbs and 3 different intakes, and 3 different Ica's. It kept me busy for awhile. I finally cut a hole in the hood and gave her some fresh air.In the end it didn't fit my combo and driving style, so I pulled and sold it. Great cam! at WOT. Not so good in a streeter with a stick and 3.55s. She wanted a lot more rear gear and an overdrive. Smaller cam was waaay cheaper
             
          • 70DusterBob

            70DusterBob Well-Known Member

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            Most of the cruising around here is at 65, 70 once in a while. Out in West Tx it is yeah 85. I would be hittin' 4,500 probably. lol

            But at 65 I can adjust my secondaries to open just after that and it keeps them closed so no problem there. I hardly ever hit 70 crusin'. Unless I'm not paying attention, but then when the secondaries open it's like, "Oh, I'm over the speed limit." These new cars you can't tell how fast your going by engine noise because there isn't any. I got 3 speeding tickets in 6 months when I went to a rice burnin' toy for a daily driver. 27.4mpg but damn. I have to constantly look at the speedo or set the cruise to keep from getting another ticket...
             
          • 70DusterBob

            70DusterBob Well-Known Member

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            Yeah, I will pull the PV and put the plug back in the secondary hole.
             
          • Mattax

            Mattax Just the facts, ma'am FABO Gold Member

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            What's the list number
             
          • 70DusterBob

            70DusterBob Well-Known Member

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            I will get it for you tomorrow...