Vacuum VS Mechanical carb secondaries?
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Vacuum VS Mechanical carb secondaries?
Hey there, after sifting through pages and pages of information, I cannot find anything that helps out my situation. Any help would be greatly appreciated!
I am in the finishing steps of a 92' 302 F150 overhaul, but I can't figure out which type of carb is better for a rig like this. I'm a bit of an Edelbrock fanatic, and i'm thinking I don't need anything bigger than the 500, but i've heard so much good and bad about the differences between the vacuum and mechanical secondaries, I'm lost as to which one will suit me.
If anyone has any opinion changing advice, i'd love to hear it, because i've gotten no where. - Thanks ahead of time for any info! - Braden
I am in the finishing steps of a 92' 302 F150 overhaul, but I can't figure out which type of carb is better for a rig like this. I'm a bit of an Edelbrock fanatic, and i'm thinking I don't need anything bigger than the 500, but i've heard so much good and bad about the differences between the vacuum and mechanical secondaries, I'm lost as to which one will suit me.
If anyone has any opinion changing advice, i'd love to hear it, because i've gotten no where. - Thanks ahead of time for any info! - Braden
Laughing Gas
Joined: Aug 2010
Posts: 979
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From: Colorado Springs CO
Actually, the Edelbrock carb is neither-it's an air valve secondary. The only true mechanical secondary carbs are Holley double pumpers-they have an accelerator pump on the secondary side to cover up the "bog" created when you open up the secondaries. These are mostly used on drag race and high performance street cars. Vacuum secondary carbs (most Holleys and also the old Autolite "shoebox" 4100) use a vacuum diaphragm to open the secondary throttle plates by engine demand. This can be tuned by changing the springs on the diaphragm.
Carbs like the Edelbrock/Carter AFB and the Rochester Quadrajet use mechanically operated secondary throttle plates, but have an air valve (either counterweighted or spring loaded) that control the air/fuel flow on the secondary side. This has the same basic effect as the vacuum secondaries on the Holleys-it regulates the secondarys to engine demand.
IMO the Edelbrock carb is a great choice for a street engine.
Carbs like the Edelbrock/Carter AFB and the Rochester Quadrajet use mechanically operated secondary throttle plates, but have an air valve (either counterweighted or spring loaded) that control the air/fuel flow on the secondary side. This has the same basic effect as the vacuum secondaries on the Holleys-it regulates the secondarys to engine demand.
IMO the Edelbrock carb is a great choice for a street engine.
Lead Driver
Joined: Feb 2002
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From: Greater Austin, Texas
IMO the Edelbrock carb is a great choice for a street engine.
Mountain Pass
Joined: Dec 2006
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From: Osburn, ID
The Q-jet may not be perfect but I feels it's about as close as any carb has come to meeting a balance of economy using the small primaries, performance with the huge secondaries, and emission control. If rebuilt right, they are a very good carburator. Anthony
Senior User
Joined: Nov 2010
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Unless it is a race engine Mechanical Secondaries are not a good idea generally.
As others have said tuning a vacuum sec or air-valve is much easier.
a 500cfm on a 302 is a good choice it will give you good low end torque and in a DD truck you will not be high enough into the RPM's to "starve" for air.
Personally I would run a Holley VS Carb or an Edelbrock Carb...
As others have said tuning a vacuum sec or air-valve is much easier.
a 500cfm on a 302 is a good choice it will give you good low end torque and in a DD truck you will not be high enough into the RPM's to "starve" for air.
Personally I would run a Holley VS Carb or an Edelbrock Carb...
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I see, so an Edelbrock AVS 500 should be just fine?
I've been recommended the Q-Jets before but I can't find any under 750CFM, and not one new one, at least from Edelbrock. However, if I happen to find a suitable Q-Jet, what sets them apart from the vacuum secondary carbs? I understand how and what they do, but why would edelbrock stop making them if they're such a good carb?
- Thanks again!
I've been recommended the Q-Jets before but I can't find any under 750CFM, and not one new one, at least from Edelbrock. However, if I happen to find a suitable Q-Jet, what sets them apart from the vacuum secondary carbs? I understand how and what they do, but why would edelbrock stop making them if they're such a good carb?
- Thanks again!
Elder User
Joined: Nov 2010
Posts: 957
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From: Graniteville
The thing to keep in mind on a Q-Jet is they are spreadbore carbs and will need either a spreadbore intake or adapter plate to mount to a square bore. Personally, I like the Holley Street Avenger series carbs. Out of the box they are some of the best carbs holley has brought out. Put it on, adjust the idle and mix, and good to go.
Holley 0-80570 - Holley Street Avenger Carburetors - Overview - SummitRacing.com
Holley 0-80570 - Holley Street Avenger Carburetors - Overview - SummitRacing.com
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Postmaster
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Like CW said , with a stock motor vacuum secondaries will work better , i also like the holley street avenger , i guess thats just a matter preference....
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OK, well it's not exactly stock, it has shorty headers, true dual exhaust, higher than stock RPM cam, went from auto to stick tranny, bored .30 over and all the usual "modern" modifications like ported heads and 3 angle valves, but it's still a street truck. I'm assuming I still won't benefit from mechanical secondaries? Mechanical are more for real power machines, correct?
Postmaster
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If you have a light truck with like 400HP , with low gears and don't care about fuel mileage , go with a mechanical double pump , i like them , otherwise i would use the vacuum , the performance difference is not that much ....Lew
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