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Mechanical (VS) Vacuum secondary Carb

25K views 14 replies 11 participants last post by  shane918 
#1 ·
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
 
#2 ·
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-
 
#3 ·
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.
 
#4 ·
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.
 
#7 ·
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
 
#8 ·
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
 
#12 ·
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...:thumbsup: 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
 
#10 ·
What rear 3.90? Any stall? With that much gear you may like the DP.

I say try both.
 
#11 · (Edited)
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
 
#13 ·
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.
 
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