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Thread: Merlin Hardcore 870 CFM?

  1. #1

    Default Merlin Hardcore 870 CFM?

    Hey guys, I acquired a Holley 0-4781C Model 4150 Double pumper. It's a World Parts Merlin Hardcore 870 cfm. I actually spoke with a guy at Bill Mitchell racing today, super nice guy. Heck, might have been ole Bill himself, I don't know. He said that it is a 750 body on a 850 base with upgrade goodies such as the custom float bowls. He said that combo worked better and flowed better than a stock 850dp which is how/where he came up with the 870 cfm.

    It's been many years since I've played with and tuned a Holley, since I've been out of the hot rod game for 32 years. I'm back in it now with a '68 Camaro, 454, 4 speed, 3:55 gears. This carb has #74 jets front and rear, is this typical to have the same jets front & rear? #45 power valves and 30cc plungers. It'll be sitting on a Edelbrock RPM Air-Gap intake. My cam setup will be a hydraulic roller 288/300 .050" 229/241 Lift 600/570 with 1.7 rockers 109 Lobe Sep with a max rpm of 5700-6000

    Does the jetting and power valve set up sound about right for a good place to start or what would you recommend? This is a pro tour style build, no racing except from stop light to stop light, and a little open road.
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  2. #2
    Join Date
    Jan 2010
    Location
    Brisbane, Australia
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    1,334

    Default

    There are many better carbs out there now.
    Your old school Holley does not have the metering blocks or air bleed adjustability of the late stuff.
    Power valves both side allows square jetting, but the secondary can be uncovered from high acceleration G-force.
    If the carb has been recently rebuilt, by all means run it as is, but I feel you might like something more modern.

    Your 74s sound a bit small considering most 4781s come out with 78-80 mains, but we don't know if the PVCRs have been opened up. The 4.5s are later opening than the factory 6.5s.

    Power valve opening point should be based on camshaft, and observing manifold vacuum while driving.

    Gary
    Last edited by Gaz64; 12-14-2018 at 05:55 PM.
    Regards, Gary

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