Mechanical or Vacuum Secondaries?
 

Hi guys just want some opinions here, I'm upgrading my engine with 363 Stroker from Craft Performance. My question is vacuum or mechanical secondaries? Lance at Craft usually uses a vacuum secondary carb with his 363 build.

I just removed a Edelbrock 750cfm and installed a custom built Holley 750 double pumper mechanical secondaries no choke. Works very well,very responsive.

I have a Quickfuel with vacuum secondary's works great also adjusts easy with a screw driver big plus in my book .

Generally speaking, vacuum secondaries are controlled by the driver's right foot but moderated by sensed engine vacuum. This prevents the secondary throttle plates from opening fully until the engine vacuum signals it is ready for all that air. ... Mechanical secondaries are controlled entirely by the driver's right foot. If the driver shoves the throttle to the floor, the primary and secondary throttles open fully immediately. That means it is up to the driver to modulate the application of throtlle so he can achieve wide open throttle as quickly as possible without creating an imbalance in the air/fuel ratio. For all these reasons, vacuum secondaries are generally preferred for street use while mechanical secondaries are preferred by seasoned racers.

Mechanical every day for a light car like a Cobra.

It would take a lot of tuning to get the vacuum secondaries to work half as good as mechanical.

Save the vacuum secondaries for your Galaxie or F100.

John

Quickfuel HR series has two different mechanical secondary links available. One opens at 40% for racing, the other 60% for street.

Even the Holley web site recommends a mechanical secondary carb for our cars because they are so light. My roush engine came with a vacuum secondary holley carb. Swapped it for a mechanical four barrel after a year of driving. Woke that engine right up.

thanks for the replies it's the one issue I hear both sides on, not sure what to do. I have vacuum secondaries on my 347 now and it runs very well

IMHO, vacuum secondaries are more driver friendly and mechanical secondaries more performance friendly. In street driving I doubt most could tell the difference unless you treat the throttle like an on/off switch.

The only disadvantage of vacuum secondaries is that it can be a real PITA to get the opening "just right" because it involves switching out the spring in the vacuum canister. If you change your air filter housing to be less restrictive, you have to readjust everything or your car will actually run slower because the increased volume of air reduces the venturi vacuum that opens the secondaries. About eight years ago I installed a Quick Fuel Adjustable Vacuum Secondary Housing. Being able to adjust your secondaries' opening with a turn of the screw, along with being able to mark their opening with a zip tie, makes getting them right a piece of cake. BUT, it's still dangerous to do so on the street with a big block Cobra. It's really pretty simple, you just accelerate as hard and as fast as you can, and keep turning the screw so the secondaries open quicker, and repeating the process until you get a bog while accelerating, and then you close the screw back up a bit. To be honest with you, you really need a spot where there is nothing to hit, because any time you have your Cobra at WOT you really don't have full control over it. Anybody that tells you they have control over their big block Cobra at WOT on the street is lying to you. I've gotten old now, having had my Cobra for 15+ years, and I drive like a little old lady now. I might open the secondaries once or twice year. But I will tell you this, properly adjusted vacuum secondaries are absolutely the best way to go unless you do nothing but live on the race track.:cool:

Here's my old thread on the QF Adjustable Vacuum Secondary Housing. You might find it interesting: http://www.clubcobra.com/forums/holl...-question.html

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I changed a 428 stroker from a 750 vac secondary to 850 double pumper and you couldn't tell it was the same car.

The throttle response was way better and overall power seemed to be more.

John

Did you check to see that the secondaries were opening under load properly before you replaced the carb?:confused:

Yes - Used a zip tie loosely on the shaft to show when it rotated.

Secondaries fully opened and I made several adjustments. The car ran better, but the double pumper woke it up like nothing I have seen.

The new car (new engine has a double pumper).

John

Caesar -- you're not going to get a consensus here on what to do.:LOL: But the nice thing is that whatever you choose, a carb change-out is only a few hundred bucks and an hour's worth of wrenching. And you can always change it back if you're not happy. That's half the fun of owning these cars.:cool:

Well maybe not a consensus but I agree with what patrickt said other than the dangerous part.:3DSMILE: