vacuum secondaries vs mechanical

Discussion in 'Classic Mustang Specific Tech' started by allcarfan, Sep 14, 2004.


  1. allcarfan

    allcarfan The Answer Man Founding Member

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    My carb has vacuum secondaries on it. It is a performance street car that might see some track time. What advatages will a mechanical secondary carb have over this vacuum secondary carb? Can I convert my vacuum secondaries to mechanical ones somehow?
  2. Max Power

    Max Power New Member

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    Mechanical secondaries work better on deep geared cars with manual transmissions.

    For straight line performance, vacuums can be adjusted to work pretty well, but mechanicals have that extra accelerator pump that vacuums can't match.
  3. ratio411

    ratio411 Founding Member

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    Theoretically vacs are better.
    The carb only opens the secondary when needed.
    This is assuming you have it tuned perfectly.
    Mechanical secondarys open when you want them to.
    The purpose of the pump shot is to cover up the big air hole you make by opening the secondarys when the engine is not prepared.
    You are in effect shoving the fuel/air down the engine's throat if it wants it or not.
    If tuned perfectly, this can be a boon for upper rpm power.
    Vac tend to get better fuel economy, be more consistent, and make better torque.
    Mechs tend to use ALOT of fuel and make more hp.
    The general rule of thumb is vacs are happier on dual plane intakes and auto trannys are happier with vacs.
    Single planes have less vacuum signal, so they make for spongy vac operation. Larger cams can do the same without good tuning.
    Mechs like single planes and manual trannys, as a general rule... tuning can overcome alot though.
    Dave

    Edit: Do not attempt to make your vac carb mechanical.
    You do not have the ability to run a secondary accellerator pump, this will cause a giant bog. If your secondarys open without the pump shot, you have a big vacuum leak and the engine falls on it's face.
    I have seen guys make their linkage mechanical and then rave that they can now feel the back barrels kick in. What they are really feeling is the car slow down, then speed up when the secondary main circuit finally starts flowing.
  4. mustang70

    mustang70 Founding Member

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    I ran a mech secondary with an automatic and never had a problem with the response. This is just my experience though. It had excellent throttle response. I'm not sure about the mileage though since I never checked. Personally, I prefer mechanical secondaries.
  5. Vinyl66

    Vinyl66 Member

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    This is what my car does when I floor it from a standstill. I hear a big gulping vacuum sound and then my engine will just bog really bad or even die. It does not do it at speed though. If I floor it more gradually it does not do it.
    How can I check that my secondaries are working correctly on my Edelbog?
  6. BlueMonster65

    BlueMonster65 Founding Member

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    very good info ratio411. another thing to consider is vehicle weight. on a light vehicle mechanical secondaries usually provide better performance same logic as a steeper geared vehicle. so on a 65-66 mustang with 3:55 or numerically higher gears and either a properly stalled auto or manual trns connected to a mild to wild small block, i'd go with the mechanical. which is wierd becasue thats what i hve but i have a road demon with vacume and its awesome. but that will change when boosted becasue vac sec cannot be used. you can convert some holley 4160 to mechanical secondarys with the appropriate kit. but with the work involved it is better to just buy a new carb. or pick up a used 4150 and rebulid it.
  7. allcarfan

    allcarfan The Answer Man Founding Member

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    So, it boils down to a matter of preference almost? I have a fairly radical combination, a c4, vehicle weight around 2600 pounds give or take, and should make close to 475hp. I have a vacuum secondary carb, but was thinking of selling it for a mechanical secondary carb. I have a speed demon 650
  8. 88 Fox GT

    88 Fox GT New Member

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    I had an Edelbrock on my 302 with my Stealth intake, the mechanical secondaries worked well. :shrug:
  9. ratio411

    ratio411 Founding Member

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    Yep, that's why I used the word generally alot...
    Personally I prefer a mechanical secondary.
    They take alot of tuning.
    I have run them on all sorts of applications with good results, C4 with small cam included.
    The lighter the car and lower the gears, the less I would be concerned with anything else.

    allcar:
    I think you have a perfect candidate for a 750 dp.
    Possibly a 700. The good thing about 600,700,800 dps is that they come with 50cc pumps, unlike dps that end with "*50".
    Dave
  10. BlueMonster65

    BlueMonster65 Founding Member

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    i say go with either a mech speed demon or better yet a mighty demon. that is what i plan on swithing to once i install the supercharger.
  11. allcarfan

    allcarfan The Answer Man Founding Member

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    Well, now that I JUST found out that I am moving to Atlanta and my rent is going to double....

    I am stuck with my Speed Demon 650 vacuum secondaries. I am going to rejet it. Remember, it flows 753cfm.

    Thanks for all the replies
  12. BlueMonster65

    BlueMonster65 Founding Member

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    if you want to save some money and still go mech sec. you should see if you can buy a mech metering block and accel pump w/ linkage from BG. if its like my road demon it already has all the holes drilled and screws tapped for the sec. accel pump. as a matter of fact i considered briefly converting it to mech sec. with all the 4150 parts i have lying around. if your 650 is flow rated at 753cfm why is it a 650cfm carb and not a 750?
  13. allcarfan

    allcarfan The Answer Man Founding Member

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    Blue,

    Demon Carbs are flowed differently. I think they are wet flowed where as most other carbs are dry flowed and then rated.

    Thats why you see a lot of demon 650 carbs on 400+hp engines

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