School Me A Bit
Thread Starter
Safety Car
Joined: Mar 2009
Posts: 3,558
Likes: 4
From: Oakland California
School Me A Bit
Hi guys, I'm looking for a carburetor for my Vette. I basically just want to know mechanical secondaries v. Vacuum secondaries. I've heard that mechanical secondaries are best for manual cars, but explain to me why please? My Vette is a 4 speed manual car, and am in need of this information as I am thinking about buying a new carburetor, and please don't ask "what's wrong with the Quadrajet." It's not the original carb, so I don't have a reason for sticking with it. I just want to know the differences between mechanical and vacuum secondaries, and why mechanical secondaries are best for a manual car. Thank you guys
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Safety Car
Joined: Sep 2007
Posts: 4,454
Likes: 328
From: Indy Indiana
This will tell you Holley's recomendation..
Does Holley even make a mechanical secondary spreadbore? presuming you are not going to put on some funky carb adapter.
http://holley.com/applications/Carbu...bSelection.asp
Does Holley even make a mechanical secondary spreadbore? presuming you are not going to put on some funky carb adapter.
http://holley.com/applications/Carbu...bSelection.asp
Drifting
Joined: Aug 2005
Posts: 1,682
Likes: 3
From: League City TX
Mechanical secondary carbs open the rear barrels via a mechanical link with the throttle linkage.
They also have accelerator pump squirters on the rear secondary barrels like all carbs do on the primaries.
Vacuum secondary carbs open the rear barrels with a vacuum diaphragm that gets operated by vacuum
created in one of the primary barrels.
How fast they open can be tuned by changing the springs in the diaphragm.
They also have accelerator pump squirters on the rear secondary barrels like all carbs do on the primaries.
Vacuum secondary carbs open the rear barrels with a vacuum diaphragm that gets operated by vacuum
created in one of the primary barrels.
How fast they open can be tuned by changing the springs in the diaphragm.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,114
From: Crossville TN
A Quadrajet carb is neither a mechanical nor a vacuum operated secondary system. I guess it is mechanical, in that there is a mechanical release system that fully opens the secondary throttle plates and will allow the secondary air valve to open at appropriate times (tied in to the choke system so the secondaries won't open when engine is cold). {Note: If the upper air valve is NOT open, it doesn't matter that the lower throttle plates are open.} But, the secondaries open proportionally with incoming airflow, after the lower throttle plates open from the throttle linkage. The higher the rpm, the more the upper air valve opens, so it is self-regulating, regardless of how much displacement the engine has. This is why a Q-jet can be placed on about any engine that requires less than 750 cfm (some units have 800 cfm rating).
Most [straight] mechanical secondaries have to be tuned to match the engine's flow requirements. The vacuum operated secondaries, also have to have the vacuum rate adjusted to match engine needs. There is also a simple (one lock screw) adjustment on the Q-Jet's upper air valve so that the engine doesn't experience a "bog" from opening too quickly or to slowly.
Most [straight] mechanical secondaries have to be tuned to match the engine's flow requirements. The vacuum operated secondaries, also have to have the vacuum rate adjusted to match engine needs. There is also a simple (one lock screw) adjustment on the Q-Jet's upper air valve so that the engine doesn't experience a "bog" from opening too quickly or to slowly.
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This will tell you Holley's recomendation..
Does Holley even make a mechanical secondary spreadbore? presuming you are not going to put on some funky carb adapter.
http://holley.com/applications/Carbu...bSelection.asp
Does Holley even make a mechanical secondary spreadbore? presuming you are not going to put on some funky carb adapter.
http://holley.com/applications/Carbu...bSelection.asp
I understand the theory that the carb may dump too much fuel (prematurely) versus a vacuum activated secondary but it has never bogged on me. You put your right foot down and it goes. I have mechanical secondary carbs on my older 4 speed cars as well and love them.
Melting Slicks
Joined: Feb 2006
Posts: 3,428
Likes: 5
From: Japan
Simply put, a vacuum secondary carb responds to engine demand (vacuum) whereas a mechanical secondary carb responds to driver demand. Since an automatic trans essentially does its own shifting it is more economical to have vacuum secondaries since the carburetor will essentially respond automatically based upon vacuum changes determined by which gear the transmission puts the car in. This response is generally somewhat delayed, whereas a mechanical secondary setup responds instantaneously to throttle inputs. The problem with using a mechanical secondary carb on an auto trans car is that the driver often tends to apply more throttle input than what the car can 'digest' given the gear that it is in so it goes out the tailpipe as wasted fuel and lost acceleration.
Thread Starter
Safety Car
Joined: Mar 2009
Posts: 3,558
Likes: 4
From: Oakland California
These are all great explanations guys. Now I see why a mechanical secondary carb is good for a manual car. I have the Performer RPM intake for my Vette and a spread bore Holley carb with mechanical secondaries. I was thinking about rebuilding this carb and using it, but that would require an adapter. Now I'm thinking about going with Summit Racing's 750 CFM mechanical secondary carb since that is a square bore design, so no need for an adapter. What do you guys think would be the best thing to do?
. Now I see why a mechanical secondary carb is good for a manual car. I have the Performer RPM intake for my Vette and a spread bore Holley carb with mechanical secondaries. I was thinking about rebuilding this carb and using it, but that would require an adapter. Now I'm thinking about going with Summit Racing's 750 CFM mechanical secondary carb since that is a square bore design, so no need for an adapter. What do you guys think would be the best thing to do?
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Race Director
Joined: Apr 1999
Posts: 11,520
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From: CORVETTE 77 385 C.I. TEXAS
Do not use an adapter & there is not enough clearance either. For a mild 350, you will be happier w/ 600 DP square bore or an intake for the 650 spread bore.
A DP is not for everyone & can be used on an auto. as capevettes said.
A DP is not for everyone & can be used on an auto. as capevettes said.
Safety Car
Joined: Jan 2009
Posts: 3,736
Likes: 56
From: anchorage ak
Basically the way I understand it they mechanical secondaries are best for cars under 3100 or manual trans, the reason for the manual trans is that when you shift coming from wide open throttle & back to it it gives you that extra squirt & less delay. With an auto trany your foot stays in it while you shift.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,114
From: Crossville TN
If you have a stock or just lightly upgraded 350 cu. in. engine, the most carb you will need is a 650 cfm unit. If you have mechanical secondaries and go larger, you will have problems.
Thread Starter
Safety Car
Joined: Mar 2009
Posts: 3,558
Likes: 4
From: Oakland California