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post #1 of 15 (permalink) Old Nov 9th, 08, 3:50 PM Thread Starter
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Al
 
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Mechanical (VS) Vacuum secondary Carb

I know this is a rookie question but I’m at the point now where I need to purchase a carb for my 350 being built. The guy building the engine said to get a 750 cfm carb. What are the advantages/disadvantages of (Mechanical (VS) Vacuum secondary) just trying to educate myself here.
Thanks
Al

72 Chevelle SS Clone.
Engine, 350
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Eaton Posi 3.90 gears
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post #2 of 15 (permalink) Old Nov 9th, 08, 9:45 PM
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Bobby
 
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Re: Mechanical (VS) Vacuum secondary Carb

If your looking for a driver that runs decent the VS carb is the way to go, now if you are looking for full performance and like all out power the mechanical secondaries are the way to go.

It can also depend on the build, you want to make sure there is enough motor there to handle the mechanical secondaries.

-Bobby-
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post #3 of 15 (permalink) Old Nov 9th, 08, 10:03 PM
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Re: Mechanical (VS) Vacuum secondary Carb

No idea at what level your 350 is being built at,but a 750 cfm carb will support a mild 454 all day long.if your builder insists on 750 cfm,start off with the vac secondary.At least it will size itself to your engines needs.The double pump mechanical shows no mercy atop a small cube engine at low rpm.

George

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post #4 of 15 (permalink) Old Nov 9th, 08, 10:16 PM
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Steve
 
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Re: Mechanical (VS) Vacuum secondary Carb

Assuming your engine is a mild "street" engine 350, a 600 cfm to 650 is good size for a street car. Vacuum secondaries are the way to go.
I usually recommend a 650 cfm edelbrock AVS carb. The secondaries are tunable as well with an air door (kind of like a Q-jet).

Mechanical secondaries are only wanted on a full on race car, IMO.
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post #5 of 15 (permalink) Old Nov 9th, 08, 11:30 PM Thread Starter
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Al
 
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Re: Mechanical (VS) Vacuum secondary Carb

I know the shop rates this engine at 410 hp and 428 lbs torque. I will get the specs in the morning. Thanks for the input.
Al

72 Chevelle SS Clone.
Engine, 350
12 Bolt Rear End
Eaton Posi 3.90 gears
Transmission 700R4 3000 stall
Work still in progress
Pics

Restoration website
http://72malibu.com
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post #6 of 15 (permalink) Old Nov 10th, 08, 12:28 AM
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Tom
 
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Re: Mechanical (VS) Vacuum secondary Carb

If you plan on driving it on the street then vacuum secondaries are the way to go. Mechanical secondaries work best at wide open throttle such as on the track.

Tom
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post #7 of 15 (permalink) Old Nov 10th, 08, 6:16 AM
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Bill
 
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Re: Mechanical (VS) Vacuum secondary Carb

Don't make the same mistake I did, go with the DP if you are going to see any track time at all.
I thought my edelbrock carb would be fine, great for the street, sucks for the track!!! Now looking for a Holley 750 DP.

You can compensate the DP on the street more then at the track

Bill
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post #8 of 15 (permalink) Old Nov 10th, 08, 6:24 AM
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Sean
 
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Re: Mechanical (VS) Vacuum secondary Carb

dunno why everyone hates mechanical secondaries on the street. i have always driven them... 600dp on my 400hp 350 worked well. now 750HP (also dp) on my 500hp stroker. you just gotta be sure to jet it right and have the right pump cam and power valve. i would rather tune those items that trying to get the right spring and never really being sure if my secondaries are even opening all the way or not. can't really verify that on a vs carb except on a dyno. just my .02

Sean
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post #9 of 15 (permalink) Old Nov 10th, 08, 7:01 AM
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Re: Mechanical (VS) Vacuum secondary Carb

Best carb for a performance 350 is a 750 Holley 3310 IMHO and experience...can't go wrong with the stuff the factory used (70-72 LT-1's came with 750-780 cfm vacuum secondary Holley)
If you want a mix of performance and economy,use a Qjet
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post #10 of 15 (permalink) Old Nov 10th, 08, 7:35 AM
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Vinny Vince
 
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Re: Mechanical (VS) Vacuum secondary Carb

What rear 3.90? Any stall? With that much gear you may like the DP.

I say try both.

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post #11 of 15 (permalink) Old Nov 10th, 08, 11:08 AM Thread Starter
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Al
 
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Re: Mechanical (VS) Vacuum secondary Carb

Car will be a weekend driver. And yea, she'll have to go to the track once or twice.
Here are the specs to what the shop is building.
Cam- 510
Duration- 318
Lift .0050
Pulls hard between 3000 - 6500 rpm?
Piston- Dome type
Compression 10.7
Heads- 202
Tranny- 700r
Stall 2500 minimum
Al

72 Chevelle SS Clone.
Engine, 350
12 Bolt Rear End
Eaton Posi 3.90 gears
Transmission 700R4 3000 stall
Work still in progress
Pics

Restoration website
http://72malibu.com

Last edited by shane918; Nov 10th, 08 at 1:44 PM.
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post #12 of 15 (permalink) Old Nov 10th, 08, 2:47 PM
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Tommy
 
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Re: Mechanical (VS) Vacuum secondary Carb

Quote:
Originally Posted by mr 4 speed View Post
Best carb for a performance 350 is a 750 Holley 3310 IMHO and experience...can't go wrong with the stuff the factory used (70-72 LT-1's came with 750-780 cfm vacuum secondary Holley)
So did the LS6... Back in the "old days", the 780 vac sec was considered to be one of the best choices for a performance car.

In general, a vac sec is better suited for a heavier, numerically lower geared car & the double pumper is better suited for a light car or higher gearing.

Tommy
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post #13 of 15 (permalink) Old Nov 10th, 08, 3:13 PM
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Vinny Vince
 
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Re: Mechanical (VS) Vacuum secondary Carb

There should be a sticky - DP vs. Vac sec.

If you do a search, you'll have enough material to read for about an hour.

*69 CHEVELLE SS EFI 502/T56 Mag/3.90
*06 C6 LS2/T56
*96 Stang LQ9 6.0 turbo - everybody should have one!
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post #14 of 15 (permalink) Old Nov 10th, 08, 3:30 PM
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Re: Mechanical (VS) Vacuum secondary Carb

I agree with the 3310. Great carb for most of the street engines out there.

Chris.

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post #15 of 15 (permalink) Old Nov 10th, 08, 10:19 PM Thread Starter
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Al
 
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Re: Mechanical (VS) Vacuum secondary Carb

I just came across this article about Demon carbs. Has some tech info on
Vacuum and Mechanical secondaries

http://www.carcraft.com/techarticles...arb/index.html

Al

72 Chevelle SS Clone.
Engine, 350
12 Bolt Rear End
Eaton Posi 3.90 gears
Transmission 700R4 3000 stall
Work still in progress
Pics

Restoration website
http://72malibu.com
shane918 is offline  
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