My dad has sent me on a mission to figure out which type of secondary we would need on our carb (he's looking to replace the Rochester, because the new engine & intake we're going to be putting in is a square bore, not a spread bore).

The engine's going to be a 383 from T&L, probably pushing around 420-440 HP (haven't really decided which motor we're going with just yet), with a manual 4 spd tranny, and we're probably going to be putting on a vacuum pump to keep the power brakes, headlights, etc.

I've been looking at the Holley carbs, and I keep seeing a bunch of different carbs that have either vacuum or mechanical secondaries, and I'm not sure which one we would want...I know the Rochester that's currently on there is a mechanical secondary; I just don't know what the advantages/disadvantages of the mechanical vs. secondary are...

Can anyone give me any suggestions? Thanks!

 

A rochester quadrajet has vacuum secondaries not mechanical.

 

What will you & your Dad be doing with the car?

Unless you're going racing, I would stick with vac secondaries on the street. But opinions may and will vary.

Cheers



BB NJ Ken

 

You did it now.........

 

Nope. Q-Jets are mechanical secondaries. The only carbs that offer vacuum secondaries are Holley and BG. The Q-Jet is straight mechanical with a secondary airvalve.

 

With 400+ hp a 3310 Holley would be a good choice with it`s vacuum secondaries or one of comperable size.... very good with a solid lifter cam.

 

From the Delco Carb Service Manual



Both the vacuum Holleys' and the qjet are "demand" type carbs and can easily handle the hp you need.
The qjet may be a little more economical.

A lot of power discriminate users like to use a double pumper holley mechanical with a manual tranny.

 

Well, we might take it every once in a while to the 1/8th mi here to race it, but that's going to be once in a blue moon. Mostly, it'll be street driving, to car cruises & Sunday drives.

 

carter also made in years past manufactured both afb and wcfb carbs with vacuum secondarys that were opened by a venturi vacuum controlled secondary diaphram. Autolite/Ford also had a vacuum (venturi vacuum) secondary carburetor from 1957 thru 1966. in 67 autolite came out with a 4bbl with mechanical secondary throttle valve with a upper secondary air valve that is somewhat like a q-jet.

a q-jet is a more of a hybrid carb than a true mechanical secondary carburetor because it combines mechanical secondary throttle valves with the upper secondary air valves that are closed by a adjustable spring. A q-jet usesthe choke pull off/break to dampen how quick the upper air valves open in responce to the demands of the engine.
the carter AVS/edelbrock thunder carb have a similar upper secondary air valve that has a spring to control the opening of the air valve but they do not use a choke break to dampen the opening rate of the secondary air valve. most carter AFB/edelbrock performer(square bore) and WCFB carbs use a offset weight on the secondary air valves to controll how quick the secondary air valve will open.

a true mechanical secondary carburetor should have a accelerator pump system for the secondarys thus the term double pumper one exception is the drag race center squirter carbs such as a #4224 holley 660cfm center squirter.

henry @ oles

 

I've run both, also have run both on the same engine. Noticed better street manners/economy with the vacuum, but faster response with mechanical when going to 3/4 throttle or more. I prefer mechanical with a 4 speed but it's all in what you want. The mechanicals are also a little more work to get tuned/keep tuned as the vacuum's are more forgiving.

 

I just went through this same decision, now i can say i have driven with the vacum and the mech secondaries on the same engine. What i was told is this, the vacume sec. is more constant at the drag strip. the mechanical sec. are going to be more fun to drivewith a 4 speed. I opted for the mechanical sec. 750 double pumper it is a lot more responsive than my old 600 vacume sec. and is a blast to drive!

 

Use a vacuum carb you will be much happier. Lars is right about the Q-jet the throttle blades are mech. but the air valve that controlls the air flow through the sec side is vacuum controlled.

 

I would either run a Q-Jet or the mechanical Holley. I can't make a VS Holley do what I want (perform like a Q-Jet).