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Old 12-31-2002, 06:47 PM   #1
Corvettelvr73
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Default Vacuum Vs. Mechanical Secondaries

I was wondering is there such a thing as a kit you can put on a carb with vacuum secondaries to make them mechanical? Also what are the advantages and disadvantages to mechanical secondaries for the street?
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Old 12-31-2002, 07:33 PM   #2
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Default Re: Vacuum Vs. Mechanical Secondaries (Corvettelvr73)

Mechanical just dumps fuel into the engine. Vacuum opens the secondaries only when the motor needs or creates the vacumm to open them.

If the engine isn't setup correctly with mechanical it can cause the engine to bog down or not run quite as well because there isnt' enough oxygen to feed the fire.

Others will chime in with more info. Thats about all that I know. :cheers:
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Old 12-31-2002, 08:36 PM   #3
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Default Re: Vacuum Vs. Mechanical Secondaries (flood)

Mech. secondaries carbs don't make too much sense for street only cars.. For performance applications and street/strip cars.. there's nothing like mech. secondaries carbs.. With a proper adjusted mech. secondaries carb you can be sure that primaries and secondaries are fully open with WOT... Vacuum secondaries carbs often don't open 100%.. This actually happened to me.. That's why I switched to a mech. secondaries carb.. However - I Drag Race my Vette frequently.... For a street application - I would use a vacuum secondaries carb...

:cheers:

Olivier


[Modified by GrandSportC3, 7:38 PM 12/31/2002]
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Old 01-01-2003, 12:13 AM   #4
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Default Re: Vacuum Vs. Mechanical Secondaries (Corvettelvr73)

Years ago, kits were available to convert vac secondary Holley's to mech but they were no more than a bracket and a bolt to lock the linkages together. Due to product liability (the linkage could get stuck and the throttles wouldn't close), those kits haven't been available for years. While it's possible to accomplish the same thing inexpensively yourself, there would be no performance benefit and the risk of stuck throttles would remain.

Factory mechanical secondary carbs also have accellerator pumps on the secondary side while vacuum secondary carbs only have them on the primary side, and for good reason. When you mash the thottle, a vacuum secondary carb will delay the secondaries opening until it senses enough positive pressure to maintain a balanced fuel/air mixture. Mechanical secondary carbs rely on the skill of the driver to determine when the secondaries open and they have an accellerator pump on the secondary side (hence the term "double pumper") to help balance the air/fuel mixture during transition.

A properly set up vacuum secondary carb can perform as well as a comparable mechanical secondary carb in most situations. However, a properly sized and set up mechanical secondary carb will provide better throttle response and faster accelleration than a vacuum carb if the driver knows how to use it properly.

Whether a carb has mechanical or vacuum secondaries has no affect on fuel economy. DP carbs are usually jetted richer due to being intended for higher performance use but, if jetted the same and are otherwise the same, there isn't likely to be any difference between them. The secondary pumps on the DP do add more gas but, unless your constantly engaging the secondaries, it's not enough to impact fuel economy. Keep in mind that most aftermarket carbs (vac or mech) are jetter richer than stock. This is another product liablilty issue. Mfgs's don't know the ultimate application of their product and, while a too rich carb might foul your plugs, too lean for the application might destroy the engine.


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Old 01-01-2003, 01:18 AM   #5
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Default Re: Vacuum Vs. Mechanical Secondaries (Corvettelvr73)

Guessing it all depends on your application, but I've been working with a carb specialist tonight looking for what will work with my set-up. I'll be the first to say that I can rip down & put back together any 350/454, but I am one lost puppy when it comes to carbs and trannys.

Anyway... based on the engine specs(355/427HP... I gave them alot more info) that will be going in to this vette and the tranny/rear end here is their answer. I think you will find most of the above answers as very good information based upon what I was given.

The answer:

I would recommend we stay with the 750 vacuum secondary carb vs the 650 DP........The vacuum secondary carbs will let the secondarys come in just a little later then a DP(double pumper) and not give us a bog off the start......If ya had a 4 speed or loose 3500 type converter the DP would be ok.

My application by the way is weekend usage & occasional drag racing.



[Modified by bgrice, 5:21 AM 1/1/2003]
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Old 01-01-2003, 02:37 AM   #6
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Default Re: Vacuum Vs. Mechanical Secondaries (Corvettelvr73)

I personally like the vacuum sensing secondaries myself, but I seld play at the strip anymore. Just to expensive to break goodies.

A lot of your experimentation comes from trial and error and knowing how your carb works. I recommend that all folks that want to play with they're carbs get Holleys. They are by far easier to play with wether it be jetting, power valve changes, or metering plate experimentation. With the small block chevys, I truly love the 780 dual lines. Many feel that they are obsolete today but I still have a ball with them. My second choice is the old Carter AFB.

Have fun....and experiment, and never forget to keep a good CO2 fire extinguisher on hand. It's a sin to burn a vette...and yourself for that matter. I install kill switches in all my engine compartments just in case I have a emergency that requires my immediate remedy.

Happ New Year :thumbs: :thumbs: :thumbs: :thumbs: :thumbs:
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Old 01-01-2003, 03:09 AM   #7
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Default Re: Vacuum Vs. Mechanical Secondaries (mengbar)

I only can talk from personal experience and dyno results... Mech. secondaries carbs will give you more HP then a comparable vacuum secondaries carb.. even if it's only a little bit... I had a way better dyno test with the mech. sec. carb than with a compareable vacuum secondaries..
I was told that - especially with single plane intakes and a spacer - the vacuum signal for the vacuum secondaries carb isn't too accurate... and causes the secondaries to open late or not open completely...

:cheers:

Olivier
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Old 01-01-2003, 06:29 AM   #8
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Default Re: Vacuum Vs. Mechanical Secondaries (GrandSportC3)

I have always thought that if your running with a Auto tranny,your best off using Vac Sec.And if your running with A man Tarnny,you should use the Mec Sec...Any logic in this? :smash: :smash: :cheers:
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Old 01-01-2003, 10:32 AM   #9
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Default Re: Vacuum Vs. Mechanical Secondaries (BB wowbagger)

The answer is in the setup. If your engine, trans, and rear gear will let the motor breath when you stomp on it and build RPM's fast than a mechinical secondary is the way to go. If you have a small gear an it takes awhile to get really moving then you will need to stay with the vacuum sec.
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Old 01-01-2003, 12:23 PM   #10
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Default Re: Vacuum Vs. Mechanical Secondaries (redwingvette)

I had a 650dp on a street/strip 455 olds. Even with that big motor if I floored the pedal from a dead stop it would bog a little. If I gave it half throttle for the first 20 feet then punched it it would not bog. So driving technique is definatly important when using dp's. The cool thing was you could feel the resistance in pedal effort right when the secondaries were about to be opened. I guess its the stronger spring on the secondary side that does this. It was nice to know by pedal feel where the begining of the secondaries were.

On my 350 vette I opted for a 600 Holley vac sec. Right out of the box I could feel the delay in the secondary opening. I put a quick change diaphram housing on it and went with the lightest spring. Now it has no delay and opens on time with no bog whatsoever. I also converted the secondary metering plate to a metering block with another kit from Holley. Then I jetted up the secondaries 2 sizes and got some more grunt from it.
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Old 01-01-2003, 01:23 PM   #11
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Default Re: Vacuum Vs. Mechanical Secondaries (Corvettelvr73)

I've had both and would go mech. all the way. I went against all rules and had a big double pumper carb with an automatic transmission for street use. Granted I had 4.11 gears at the time and a 3000 stall, but it was a wonderful combonation that worked very very well on the street.
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Old 01-01-2003, 01:28 PM   #12
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Default Re: Vacuum Vs. Mechanical Secondaries (redwingvette)

Quote:
The answer is in the setup. If your engine, trans, and rear gear will let the motor breath when you stomp on it and build RPM's fast than a mechinical secondary is the way to go. If you have a small gear an it takes awhile to get really moving then you will need to stay with the vacuum sec.
For street applications - this is true..however - I've never seen a vacuum secondaries carb on serious performance applications.... Another thing that I was told is that vacuum secondaries carbs work better with dual plane intakes....

:cheers:

Olivier
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Old 01-01-2003, 07:16 PM   #13
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Default Re: Vacuum Vs. Mechanical Secondaries (GrandSportC3)

Either will work, but it can take some tuning to get mechanical secondaries to work on auto trannies. I used to run a 650 vacuum, but I'm currently running a 750 DP. Out of the box I would get a bog accelerating from a stop, but I tried a few different shooter sizes and a few different pump cam shapes and got it working great w/ no bog off the line - even WOT starts are smooth. I'm running a 383, dual plane manifold w/ spacer, auto tranny w/ a 2500 stall, and 3.55 gears.

Deciding which to use though all depends on your usage - if your mainly a street driver a vacuum will be easier to tune and drive. Otherwise, you might have luck with a double pumper.
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Old 01-01-2003, 07:16 PM
 
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