Holley 650 DP Mec Sec or 750 Vac Sec - Hot Rod Forum : Hotrodders Bulletin Board
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Old 12-28-2003, 09:33 PM
84 Z28 Convertible
 
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Holley 650 DP Mec Sec or 750 Vac Sec

I talk to my engine builder and he thinks I should run a Holley 650 DP Mec Sec rather than my plan to use the 750 Vac Sec. He dont think that I will have enough vacuum to operate the vac sec of the Holley 750. However I think my combo is calling for the 750 Vac Sec. UDHarold said he did not have a problem with this cam running power brakes in a 406.
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Old 12-28-2003, 09:35 PM
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Vacuum depends a lot on your cam specs....what are they?
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Old 12-28-2003, 09:36 PM
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Cam: Grind: Solid Lunati/ (Recommended by Harold Brookshire)
280/284F10 .525/.530 Lift, 247/251@.050, 159/162 at .200, .560/.565 valve lift(1.6), Pro-Street power with an extra 5* at .200" 2500-7000 RPM Range, Advance Cam 4* Degrees as Per Harold, .026" Valve Lash Hot, Adjustment Range: .018" to .030" Hot. The SB280/284F10 has 62 of overlap. The valve timing is: IVO 33 BTDC IVC 67 ABDC EVO 75 BBDC EVC 29 ATDC. The int CL is 106 ATDC.
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Old 12-28-2003, 09:55 PM
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Manifold vacuum doesn't open the secondaries. Air velocity in the front half of the carb creates the vacuum needed to open the secondaries on a Holley carb.

You can see the hole air passes over, and even use an air hose to check the operation of the secondaries. If you look in the right front bore, from the top, there is a small hole on the back rear corner of the venturi. It's small, so look close.

If you can pass enough air past that port for it to over come the spring pressure in the can, the secondaries will open. Dan
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Old 12-28-2003, 09:57 PM
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Something I didn't know, makes sense.
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Old 12-28-2003, 10:37 PM
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Ohio Dan is right on the money. I am shocked your engine builder didn't know this (or maybe you mis-understood him).

Judging by your cam, it looks like you are more concerned with performance. Basic rule of thumb, heavy cars work well with a vacuum secondaries, auto cars typically work well with vacuum secondaries, light cars or stick cars work best with mechanical secondaries. This basic rule applies to street only. If you want all out performance and don't care about mileage I say go with a double pumper (mechanical secondary). What size engine are we working with? Why would you compare a 750 VS to a 650 DP? True with vacuum secondaries they are more forgiving if you over carb but, if this is a 406 you are working with you need at least a 750 (maybe more depending on the planned use of the car).
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Old 12-28-2003, 11:28 PM
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I would run the 750 vac. for now. The 650 DP is too small for your combo. That is a bigger cube motor with a cam that is good sized. I would go with a 750 DP or a 750 vac. sec.. If you want better performance use the DP. If you want better mileage and streetability use the vacuum carb.
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Old 12-29-2003, 04:25 PM
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DTL, how much RPM you plan to run this 406 too ?
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