Mechanical (VS) Vacuum secondary Carb - Chevelle Tech
Chevelle Tech join team chevelle as a supporting member  
Chevelle Parts at SS396.com      
GROUND UP & SS396.com         
Official Sponsor of Team Chevelle
     

Auto Insurance

Chevelles.com is the premier Chevrolet Chevelle Forum on the internet. Registered Users do not see the above ads.
Chevelle Tech General Tech Area
Please try and post in specific forums

Reply
 
Thread Tools Search this Thread Display Modes
  #1  
Old Nov 9th, 08, 4:50 PM
shane918 shane918 is offline
Tech Team
Al
 
Join Date: Mar 2007
Location: Phx, Az
Posts: 331
Default Mechanical (VS) Vacuum secondary Carb

I know this is a rookie question but Iím 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
12 Bolt Rear End
Eaton Posi 3.90 gears
Transmission 700R4 3000 stall
Work still in progress
Pics

Restoration website
http://72malibu.com
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2  
Old Nov 9th, 08, 10:45 PM
Lilracr Lilracr is offline
Senior Tech Team
Bobby
 
Join Date: Nov 2008
Location: Va.
Posts: 1,711
Default 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-
Reply With Quote Quick reply to this message
  #3  
Old Nov 9th, 08, 11:03 PM
ironhead ironhead is offline
Senior Tech Team
 
Join Date: Oct 2006
Location: Detroit
Posts: 1,230
Default 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

8-71 blown 461 Rat on E-85
Reply With Quote Quick reply to this message
  #4  
Old Nov 9th, 08, 11:16 PM
steve da wrench steve da wrench is offline
Tech Team
Steve
 
Join Date: Nov 2008
Location: Oregon
Posts: 63
Default 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.
Reply With Quote Quick reply to this message
  #5  
Old Nov 10th, 08, 12:30 AM
shane918 shane918 is offline
Tech Team
Al
 
Join Date: Mar 2007
Location: Phx, Az
Posts: 331
Default 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
Reply With Quote Quick reply to this message
  #6  
Old Nov 10th, 08, 1:28 AM
figbash figbash is offline
Senior Tech Team
Tom
 
Join Date: Sep 2000
Location: Farmington Hills MI
Posts: 1,505
Default 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
__________________
1972 CHEVELLE SS
Reply With Quote Quick reply to this message
  #7  
Old Nov 10th, 08, 7:16 AM
MrBill66Malibu MrBill66Malibu is offline
Senior Tech Team
Bill
 
Join Date: Aug 2005
Location: King NC via Long Island NY
Posts: 3,783
Send a message via AIM to MrBill66Malibu
Default 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
385
4 speed M20
3:42 posi
1/8 mile 8.6 @ 84mph not too impressive LOL
http://s71.photobucket.com/albums/i1...libu/Chevelle/
Reply With Quote Quick reply to this message
  #8  
Old Nov 10th, 08, 7:24 AM
troposcuba troposcuba is offline
Senior Tech Team
Sean
 
Join Date: Sep 2003
Location: Tucson, AZ
Posts: 1,818
Default 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
'65SS, 385ci, AFR 195's, Comp XR 288HR, Forged bottom end, TKO600, 3.73/12 Bolt
Now in our new home in Tucson AZ
http://spotted.stripes.com/galleries....php?id=337573
"Aint no fun 'till somethin gets broke!"
Reply With Quote Quick reply to this message
  #9  
Old Nov 10th, 08, 8:01 AM
mr 4 speed mr 4 speed is offline
Lifetime Premium Member
 
Join Date: May 1999
Location: Under the hood of some musclecar
Posts: 22,955
Default 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
Reply With Quote Quick reply to this message
  #10  
Old Nov 10th, 08, 8:35 AM
69-CHVL 69-CHVL is online now
Senior Tech Team
Vinny Vince
 
Join Date: Jun 2003
Location: Thornton, Pa
Posts: 12,358
Default 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.
__________________
*69 CHEVELLE SS EFI 502/T56/3.90
*06 C6 LS2/T56
*96 Stang LS/6.0 turbo - everyone should have one!
Reply With Quote Quick reply to this message
  #11  
Old Nov 10th, 08, 12:08 PM
shane918 shane918 is offline
Tech Team
Al
 
Join Date: Mar 2007
Location: Phx, Az
Posts: 331
Default 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 2:44 PM.
Reply With Quote Quick reply to this message
  #12  
Old Nov 10th, 08, 3:47 PM
LS6 Tommy LS6 Tommy is offline
Tech Team
Tommy
 
Join Date: Oct 2008
Location: Denville, NJ
Posts: 98
Default 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
Reply With Quote Quick reply to this message
  #13  
Old Nov 10th, 08, 4:13 PM
69-CHVL 69-CHVL is online now
Senior Tech Team
Vinny Vince
 
Join Date: Jun 2003
Location: Thornton, Pa
Posts: 12,358
Default 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/T56/3.90
*06 C6 LS2/T56
*96 Stang LS/6.0 turbo - everyone should have one!
Reply With Quote Quick reply to this message
  #14  
Old Nov 10th, 08, 4:30 PM
Chris R Chris R is online now
Lifetime Premium Member
 
Join Date: Sep 1998
Location: Brooklyn Park, MN, USA
Posts: 17,636
Default Re: Mechanical (VS) Vacuum secondary Carb

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

When the Boogeyman goes to bed, he checks the closet for Chuck Norris.

If it aint broke, just give me a minute.
Reply With Quote Quick reply to this message
  #15  
Old Nov 10th, 08, 11:19 PM
shane918 shane918 is offline
Tech Team
Al
 
Join Date: Mar 2007
Location: Phx, Az
Posts: 331
Default 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
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
Reply
Chevelle Tech > Information Exchange > Chevelle Tech      Current Topic: Mechanical (VS) Vacuum secondary Carb
Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the Chevelle Tech forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address. Note, you will be sent a confirmation request to this address.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.



Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -4. The time now is 10:45 PM.


Is there a site like this for?    El Caminos Camaros Novas Impalas GTO Chevy Punch All Chevys

© 2009 Team Chevelle - AutoForums